I see lots of programmers write code like this one:
pair<int, int> p;
vector<int> v;
// ...
p = make_pair(3, 4);
v.push_back(4); v.push_back(5);
while you can just do this:
pair<int, int> p;
vector<int> v;
// ...
p = {3, 4};
v = {4, 5};
1. Assign value by a pair of {} to a container
I see lots of programmers write code like this one:
pair<int, int> p;
// ...
p = make_pair(3, 4);
while you can just do this:
pair<int, int> p;
// ...
p = {3, 4};
even a more complex pair
pair<int, pair<char, long long> > p;
// ...
p = {3, {'a', 8ll}};
What about vector
, deque
, set
and other containers?
vector<int> v;
v = {1, 2, 5, 2};
for (auto i: v)
cout << i << ' ';
cout << '\n';
// prints "1 2 5 2"
deque<vector<pair<int, int>>> d;
d = {{{3, 4}, {5, 6}}, {{1, 2}, {3, 4}}};
for (auto i: d) {
for (auto j: i)
cout << j.first << ' ' << j.second << '\n';
cout << "-\n";
}
// prints "3 4
// 5 6
// -
// 1 2
// 3 4
// -"
set<int> s;
s = {4, 6, 2, 7, 4};
for (auto i: s)
cout << i << ' ';
cout << '\n';
// prints "2 4 6 7"
list<int> l;
l = {5, 6, 9, 1};
for (auto i: l)
cout << i << ' ';
cout << '\n';
// prints "5 6 9 1"
array<int, 4> a;
a = {5, 8, 9, 2};
for (auto i: a)
cout << i << ' ';
cout << '\n';
// prints "5 8 9 2"
tuple<int, int, char> t;
t = {3, 4, 'f'};
cout << get<2>(t) << '\n';
Note that it doesn't work for stack
and queue
.
2. Name of argument in macros
You can use '#' sign to get exact name of an argument passed to a macro:
#define what_is(x) cerr << #x << " is " << x << endl;
// ...
int a_variable = 376;
what_is(a_variable);
// prints "a_variable is 376"
what_is(a_variable * 2 + 1)
// prints "a_variable * 2 + 1 is 753"
3. Get rid of those includes!
Simply use
#include <bits/stdc++.h>
This library includes many of libraries we do need in contest like algorithm
, iostream
, vector
and many more. Believe me you don't need to include anything else!
4. Hidden function (not really hidden but not used often)
one)
__gcd(value1, value2)
You don't need to code Euclidean Algorithm for a gcd function, from now on we can use. This function returns gcd of two numbers.
e.g. __gcd(18, 27) = 9.
two)
__builtin_ffs(x)
This function returns 1 + least significant 1-bit of x. If x == 0, returns 0. Here x is int
, this function with suffix 'l' gets a long
argument and with suffix 'll' gets a long long
argument.
e.g. __builtin_ffs(10) = 2 because 10 is '...10 1 0' in base 2 and first 1-bit from right is at index 1 (0-based) and function returns 1 + index.
three)
__builtin_clz(x)
This function returns number of leading 0-bits of x which starts from most significant bit position. x is unsigned int
and like previous function this function with suffix 'l gets a unsigned long
argument and with suffix 'll' gets a unsigned long long
argument. If x == 0, returns an undefined value.
e.g. __builtin_clz(16) = 27 because 16 is ' ... 10000'. Number of bits in a unsigned int
is 32. so function returns 32 — 5 = 27.
four)
__builtin_ctz(x)
This function returns number of trailing 0-bits of x which starts from least significant bit position. x is unsigned int
and like previous function this function with suffix 'l' gets a unsigned long
argument and with suffix 'll' gets a unsigned long long
argument. If x == 0, returns an undefined value.
e.g. __builtin_ctz(16) = 4 because 16 is '...1 0000 '. Number of trailing 0-bits is 4.
five)
__builtin_popcount(x)
This function returns number of 1-bits of x. x is unsigned int
and like previous function this function with suffix 'l' gets a unsigned long
argument and with suffix 'll' gets a unsigned long long
argument. If x == 0, returns an undefined value.
e.g. __builtin_popcount(14) = 3 because 14 is '... 111 0' and has three 1-bits.
Note: There are other __builtin
functions too, but they are not as useful as these ones.
Note: Other functions are not unknown to bring them here but if you are interested to work with them, I suggest this website.
5. Variadic Functions and Macros
We can have a variadic function. I want to write a sum function which gets a number of ints, and returns sum of them. Look at the code below:
int sum() { return 0; }
template<typename... Args>
int sum(int a, Args... args) { return a + sum(args...); }
int main() { cout << sum(5, 7, 2, 2) + sum(3, 4); /* prints "23" */ }
In the code above I used a template. sum(5, 7, 2, 2) becomes 5 + sum(7, 2, 2) then sum(7, 2, 2), itself, becomes 7 + sum(2, 2) and so on... I also declare another sum function which gets 0 arguments and returns 0.
I can even define a any-type sum function:
int sum() { return 0; }
template<typename T, typename... Args>
T sum(T a, Args... args) { return a + sum(args...); }
int main() { cout << sum(5, 7, 2, 2) + sum(3.14, 4.89); /* prints "24.03" */ }
Here, I just changed int
to T
and added typename T
to my template.
In C++14 you can also use auto sum(T a, Args... args)
in order to get sum of mixed types. (Thanks to slycelote and Corei13)
We can also use variadic macros:
#define a_macro(args...) sum(args)
int sum() { return 0; }
template<typename T, typename... Args>
auto sum(T a, Args... args) { return a + sum(args...); }
int main() { cout << a_macro(5, 7, 2, 2) + a_macro(3.14, 4.89); /* prints "24.03" */ }
Using these 2, we can have a great debugging function: (thanks to Igorjan94) — Updated!
#include <bits/stdc++.h>
using namespace std;
#define error(args...) { string _s = #args; replace(_s.begin(), _s.end(), ',', ' '); stringstream _ss(_s); istream_iterator<string> _it(_ss); err(_it, args); }
void err(istream_iterator<string> it) {}
template<typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
cerr << *it << " = " << a << endl;
err(++it, args...);
}
int main() {
int a = 4, b = 8, c = 9;
error(a, b, c);
}
Output:
a = 4
b = 8
c = 9
This function helps a lot in debugging.
6. Here is C++0x in CF, why still C++?
Variadic functions also belong to C++11 or C++0x, In this section I want to show you some great features of C++11.
one) Range-based For-loop
Here is a piece of an old code:
set<int> s = {8, 2, 3, 1};
for (set<int>::iterator it = s.begin(); it != s.end(); ++it)
cout << *it << ' ';
// prints "1 2 3 8"
Trust me, that's a lot of code for that, just use this:
set<int> s = {8, 2, 3, 1};
for (auto it: s)
cout << it << ' ';
// prints "1 2 3 8"
We can also change the values just change auto
with auto &
:
vector<int> v = {8, 2, 3, 1};
for (auto &it: v)
it *= 2;
for (auto it: v)
cout << it << ' ';
// prints "16 4 6 2"
two) The Power of auto
You don't need to name the type you want to use, C++11 can infer it for you. If you need to loop over iterators of a set<pair<int, pair<int, int> > > from begin to end, you need to type set<pair<int, pair<int, int> > >::iterator
for me it's so suffering! just use auto it = s.begin()
also x.begin() and x.end() now are accessible using begin(x) and end(x).
There are more things. I think I said useful features. Maybe I add somethings else to post. If you know anything useful please share with Codeforces community :)
From Ximera's comment:
this code:
for(i = 1; i <= n; i++) {
for(j = 1; j <= m; j++)
cout << a[i][j] << " ";
cout << "\n";
}
is equivalent to this:
for(i = 1; i <= n; i++)
for(j = 1; j <= m; j++)
cout << a[i][j] << " \n"[j == m];
And here is the reason: " \n"
is a char*
, " \n"[0]
is ' '
and " \n"[1]
is '\n'
.
From tubo28's comment:
Usage of tie
and emplace_back
:
#define mt make_tuple
#define eb emplace_back
typedef tuple<int,int,int> State; // operator< defined
int main(){
int a,b,c;
tie(a,b,c) = mt(1,2,3); // assign
tie(a,b) = mt(b,a); // swap(a,b)
vector<pair<int,int>> v;
v.eb(a,b); // shorter and faster than pb(mp(a,b))
// Dijkstra
priority_queue<State> q;
q.emplace(0,src,-1);
while(q.size()){
int dist, node, prev;
tie(dist, ode, prev) = q.top(); q.pop();
dist = -dist;
// ~~ find next state ~~
q.emplace(-new_dist, new_node, node);
}
}
And that's why emplace_back
faster: emplace_back
is faster than push_back
'cause it just construct value at the end of vector but push_back
construct it somewhere else and then move it to the vector.
Also in the code above you can see how tie(args...)
works. You can also use ignore
keyword in tie
to ignore a value:
tuple<int, int, int, char> t (3, 4, 5, 'g');
int a, b;
tie(b, ignore, a, ignore) = t;
cout << a << ' ' << b << '\n';
Output: 5 3
I use this macro and I love it:
#define rep(i, begin, end) for (__typeof(end) i = (begin) - ((begin) > (end)); i != (end) - ((begin) > (end)); i += 1 - 2 * ((begin) > (end)))
First of all, you don't need to name the type you want to use. Second of all it goes forwards and backwards based on (begin > end) condition. e.g. rep(i, 1, 10)
is 1, 2, ..., 8, 9 and rep(i, 10, 1)
is 9, 8, ..., 2, 1
It works well with different types e.g.
vector<int> v = {4, 5, 6, 4, 8};
rep(it, end(v), begin(v))
cout << *it << ' ';
// prints "8 4 6 5 4"
Also there is another great feature of C++11, lambda functions!
Lambdas are like other languages' closure. It defines like this:
[capture list](parameters) -> return value { body }
one) Capture List: simple! We don't need it here, so just put []
two) parameters: simple! e.g. int x, string s
three) return value: simple again! e.g. pair<int, int> which can be omitted most of the times (thanks to Jacob)
four) body: contains function bodies, and returns return value.
e.g.
auto f = [] (int a, int b) -> int { return a + b; };
cout << f(1, 2); // prints "3"
You can use lambdas in for_each
, sort
and many more STL functions:
vector<int> v = {3, 1, 2, 1, 8};
sort(begin(v), end(v), [] (int a, int b) { return a > b; });
for (auto i: v) cout << i << ' ';
Output:
8 3 2 1 1
From Igorjan94's comment:
Usage of move
:
When you work with STL containers like vector
, you can use move
function to just move container, not to copy it all.
vector<int> v = {1, 2, 3, 4};
vector<int> w = move(v);
cout << "v: ";
for (auto i: v)
cout << i << ' ';
cout << "\nw: ";
for (auto i: w)
cout << i << ' ';
Output:
v:
w: 1 2 3 4
As you can see v
moved to w
and not copied.
7. C++0x Strings
one) Raw Strings (From IvayloS's comment)
You can have UTF-8 strings, Raw strings and more. Here I want to show raw strings. We define a raw string as below:
string s = R"(Hello, World!)"; // Stored: "Hello, World!"
A raw string skips all escape characters like \n
or \"
. e.g.
string str = "Hello\tWorld\n";
string r_str = R"(Hello\tWorld\n)";
cout << str << r_str;
Output:
Hello World
Hello\tWorld\n
You can also have multiple line raw string:
string r_str =
R"(Dear Programmers,
I'm using C++11
Regards, Swift!)";
cout << r_str;
Output:
Dear Programmer,
I'm using C++11
Regards, Swift!
two) Regular Expressions (regex)
Regular expressions are useful tools in programming, we can define a regular expression by regex
e.g. regex r = "[a-z]+";
. We will use raw string for them because sometimes they have \
and other characters. Look at the example:
regex email_pattern(R"(^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$)"); // This email pattern is not totally correct! It's correct for most emails.
string
valid_email("[email protected]"),
invalid_email("hello world");
if (regex_match(valid_email, email_pattern))
cout << valid_email << " is valid\n";
else
cout << valid_email << " is invalid\n";
if (regex_match(invalid_email, email_pattern))
cout << invalid_email << " is valid\n";
else
cout << invalid_email << " is invalid\n";
Output:
[email protected] is valid
hello world is invalid
Note: You can learn Regex in this website.
three) User-defined literals
You already know literals from C++ like: 0xA
, 1000ll
, 3.14f
and so on...
Now you can have your own custom literals! Sounds great :) So let's see an example:
long long operator "" _m(unsigned long long literal) {
return literal;
}
long double operator "" _cm(unsigned long long literal) {
return literal / 100.0;
}
long long operator "" _km(unsigned long long literal) {
return literal * 1000;
}
int main() {
// See results in meter:
cout << 250_m << " meters \n"; // Prints 250 meters
cout << 12_km << " meters \n"; // Prints 12000 meters
cout << 421_cm << " meters \n"; // Prints 4.21 meters
}
Note that a literal should start with an underscore (_
). We declare a new literal by this pattern:
[returnType] operator "" _[name]([parameters]) { [body] }
note that parameters only can be one of these:
(const char *)
(unsigned long long int)
(long double)
(char)
(wchar_t)
(char16_t)
(char32_t)
(const char *, size_t)
(const wchar_t *, size_t)
(const char16_t *, size_t)
(const char32_t *, size_t)
Literals also can used with templates.
Its awesome. Thanks Swift :)
who is swift?
taylor swift
why is she here?
switching from singing to engineering XD
swift is Hossein's old handle (blog's author)
Thank you for clarification. IDK, why people mislead others though.
Your
sum
function returns an incorrect result forsum(1, 1.5)
. To fix, declare the return type asauto
.My sum function designed to sum numbers from one type. I mean integers, doubles, ... not mix of these types. BTW, How should I use auto in that function?
I mean you can't have a
auto
return type for any function as far as I know.http://ideone.com/6l4Wc7
Interesting! my Xcode can't compile that code. I'll edit blog post.
Thank you.
my xcode doesn't allow __gcd or any function that start with __builtin. and doesn't allow bits/stdc++.h
how do you do it in your Xcode ?
and thanks for the entry.
Why not? http://pastie.org/9817864
Your code has
decltype
(actually because of->
). Xcode won't compile code without it. However IDEONE compiles it. So I edited my post.Isn't
decltype
C++14?I suppose not.
It's supported since c++14. :)
It's better to use auto& in range-based loop when the object is not primitive (e.g pair, vector). UPD: I realized that you mention it at the end, but there are some code written poorly because of that in the first part.
actually, compiler optimizations will get rid of the extra copy operations if you are not modifying the element. so I don't think it will be any slower in runtime compared to auto&.
You can use auto& if you are too suspicious, but I don't think that the first part is categorized as 'written poorly'. it is just OK.
const auto&
is even better if you want to be really strict about it."these things are belong to C++11" — https://www.youtube.com/watch?v=8fvTxv46ano :)
LMAO =))
c++ is a very useless language
mukel already has written nice "C++11 for dummies" tutorial http://codeforces.net/blog/entry/10124 . I think it's a good idea to provide that link directly in entry.
Excellent tutorial, I'll add it at top of blog.
Could you give link to compiler that you use? Because I get CE on my GNU 4.7.1:)
In CF, use
GNU C++0x 4
instead ofGNU C++ 4.7
.Get latest GCC, and from your terminal/cmd use one of these flags
-std=gnu++11
or-std=c++11
You can download it for your computer: Windows —Anyone knows how to include <bits/stdc++.h> on OS X? I am already using gcc but it cannot found that header...
Go to:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1
Create a folder named
bits
Add a file into that named
stdc++.h
Edit it and include libraries
yeah, that works, I did the same :)
What is the content of the file (stdc++.h)?
Here: https://gist.github.com/eduarc/6022859
Ah, forgot to say. Thank you! It worked :)
there is another way: install GCC using brew terminal package manager!
The second sum function (with
auto
) isC++14
standard, notC++11
.C++11
doesn't allow function without a return type.Thanks for sharing your knowledge to us! That's why Xcode couldn't compile that. Now I tested it with C++14 and everything is OK. So let's make it clear in blog.
And it is still possible to write sum (or other) functions for mixed type using
std::common_type
As for
__gcd()
, it may be a little tricky at some compilers.The best thing is that you can write like this (C++11 vs C++) :D
instead of this
why u downvoted me ?
c++ is bullshit
If C++ is that bad, why all of your codes are in this language?
give a kiss baby :)
Here you are:
tanx
Cause he don't do them...
(cheat)
Yep. I also do this in my post:
deque<vector<pair<int, int>>> d;
May be you can tell something more about this
Well, Great creativity :)
" \n"
is a char*, " \n"[0] is ' ' and " \n"[1] is '\n'.Also this is a correct one too:
It's because e.g. a[8] and 8[a] are the same thing both of them are (a + 8)* and (8 + a)*.
no
Actually
" \n"[j == m]
was correct, but that doesn't matter at all now :)Oops! You're right!
They aren't exactly equivalent to the original because in the original there is one extra space at the end of each line. I still like the idea.
For a while, I thought that this is Iverson's bracket :D
Do you know tie and emplace ?
Such a great feature.
emplace_back
is faster thanpush_back
'cause it just construct value at the end of vector butpush_back
construct it somewhere else and then move it to the vector.Can you get the previous element in an, let's say, vector using
auto
? Here is whyauto
is not the best option for dp-like tasks where you need information from the previous elements.Use this approach:
Output:
Use range-based for-loop only when you want exact element, when you need to access other elements use normal for-loop, but this doesn't mean that you can't use auto in that for-loop.
Hm, I didn't know it could be done. Still, it is easier with normal for loop.
Btw, using
auto
is just for inferring type you are working with. If your type isint
, it's better to use that ('cause it's just 3 characters) but if your type isstd::vector<std::pair<std::set<int>, bool>>::iterator
so I think usingauto
is a must :)XD yeah I agree about this one.
Just saying. Cumulative sum can be done only with this-
In 2, I use:
#define DB(x) cerr << __LINE__ << ": " << #x << " = " << (x) << endl
In this way I get the number of the line in which this instruction is executed. It's useful when we have more than one variable with the same name. Also, x needs to be enclosed in parenthesis due to operators precedence.
would you please tell me about vector ,i don't know anything about that !
vector
You say that "Variadic functions also belong to C++11", but that's not really correct. Even C had variadic functions. New feature in C++11 is variadic templates.
Yeah. You're right. Here I used variadic template so I said it's for C++11.
I thing you should consider defining short version of your blog post, now that it is on the main page.
OK. I'll do it.
In my country, at this time, we are not allowed to use C++11 in national contest.
Is C++11 being used in IOI? If this is the case, I guess it should not be hard to convince the judge committee to change.
if i have a vector < pair<int, pair<int, int> > > a;
could i use emplace_back to insert {1, {2, 3} }? i tries to emplace_back(1, 2, 3); but of course it's an error.
thanks in advance :-)
You could emplace_back(¹, mp(2,3))
thank you for replying. i was looking forward for a method like that above something like (1, 2, 3); as i don't like using macros, something that's faster to write.
thanks in advance :)
Don't use
pair<int, pair<int, int>>
! Code less and usetuple<int, int, int>
:Well, actually sometimes
pair<int, pair<int,int> > x;
may make more sense thantuple<int,int,int> x;
, for instance whenx.second
are coordinates of some point andx.first
is some property of this point.When working with tuples, you don't really use get(tuple) you do use tie:
And that makes sense.
then you probably have that point as a variable, not as two coordinates.
I often use
Yeah let's write ugly unreadable code with nested pairs and macros instead of class/struct.
I totally agree that classes/structs are more readable. I just wanted to point out that in some cases
tuple<int,int,int>
is less readable (at least for me) thanpair<int, pair<int,int> >
.The real solution to this would be something that lets us write
and then would supply a commonsense
bool operator< (..)
automatically.Thanks for this! I'm sure many of us would also be interested in a Java tricks article! :)
The advantage of Java is that there are no tricks.
I can also write an article about HosseinYousefi's tricks. But no one here, cares about that language :)
your debugging function doesn't work for
#args
with spacesso, I think it's better to rewrite split to more universal
(Note no copying because of move, another cpp trick)
and macro will be:
It also brings default space before arguments, e.g.
err(a, b)
outputs:but it's better for arguments like
a + b
so I'll replace it with my code.oh, yep, I forgot I changed your err to
if you are interested in it, I also have writeln and readln on variadic templates, which helps to write smth like this:
you can find it here 9388829(I deleted all spaces for more compact view)
if trailing space is unimportant, half of code can be deleted:)
it can be simply extended on user's types by overloading ostream and istream operators
this template is with cin/cout, and this->9316393 with scanf/printf
yes, looks awful, and for only prewritten use:)
Actually this use of
std::move
is superfluous. The compiler will move the return value automatically (search for: return value optimization).One can omit return type in lambda expression in most cases.
P.S. I have to say, 'tie' looks awesome, I need to start using it.
You haven't to specify return type in lambda functions if all return values are the same type.
see more about lambda functions
you can even write your own recursive functions inside the main in lambdas, that's really cool and useful for less code.
But here instead of using auto you should specify the return type and the parameters type of the lambda expression.
see my submission here
Thanks. Useful information.
Thank you so much :) I learned a lot :D
+669 for vain' blog !why?
You are GiveMinus! Both of you have a comment "give a kiss baby :)"
give a kiss baby :)
+726 for a lot of useful info, that's why.
could write:
The example which is now given for
move
(definew = move(v)
and then output contents ofv
) is actually undefined behaviour. What the compiler will actually do in this situation is just swap the contents of the two vectors (v
with the emptyw
); however, in theoryv
is now "junk" and should not be touched at all (it can not even be a vector with arbitrary contents, but just something referring to some arbitrary place in memory, which might, in theory, no longer correspond to any correct contents of a vector, and it can do basically anything when its methods (such as the range-based for loop) are called).http://cplusplus.com/reference/vector/vector/operator=
"The move assignment (2) moves the elements of x into the container (x is left in an unspecified but valid state)."
We'd better call
v.clear()
afterw = move(v)
to bringv
to a determinate (empty, actually) state. And then we can access it.Didn't know that. Thanks for the correction!
Variadic functions and macros are awesome. Now I've got unique functions for debug, input and output, no more gi2, gi3, ... !!!
I like the string literals fucntionality. Sometime it can make code much simpler, especially for competitions:
And the result on ideone can be seen here.
I didn't know about this! Thank you. Could you please write a tutorial about this, I'll move it to this post.
c++11 also introduces a set of new string literals. Some of them are really useful for professional programming, but not very helpful for competitions(like UTF-8, UTF-16 and UTF-32 literals) and thus they are not that much of an interest(you can read about them in the wiki article that I link to). However one type of string literal is particularly interesting — the raw string literal. To write a raw string literal you need to prefix the opening quotes with R and immediately after the quotes you should write some delimiter, the delimiter can be a string of up to 16 characters and should not contain whitespace or control characters, You should terminate the string with the same delimiter before the closing quote and also the string should be in brackets(after the delimiter). Here is an example usage:
And the output can be seen here.
Note that the string can span multiple lines and that you don't need to escape special characters in it. In this case I use END as my delimiter.
Following is also useful for GCC. Very fast ASM bit operations:
Note, that offset can be >=32, any valid offset will work. However, I didn't know if inline assembly allowed in CF. Should work.
Why do you need
volatile
everywhere?Just to make sure that value is actually changed. It gives information to the compiler that memory is changed indirectly (inside asm block), to avoid unexpected optimizations. Modern compilers have aggressive optimizations. If you used some value from memory, compiler probably saved it to intermediate register. Let's imagine, that you then called bitset on that memory and used value again. Compiler may decide: "Ok, he didn't even touched that mem variable, I'll use the old value". But it's wrong. You changed it inside asm block. Everything inside asm — direct instructions to processor, compiler doesn't know what you are doing there.
Yes, GCC does not know what is inside the asm block. However, GCC does know which variables are used and modified — you specified this yourself in the asm block input/output operands! In particular,
"+m"
should tell GCC that this variable/location in memory is read and modified.You can see that GCC indeed reloads the value as it should here: http://goo.gl/Jz8SYH. If GCC thought the variable was unmodified, it would do
instead (comment out the
btr()
call to see this).Bottom line:
volatile
is not needed in correct code. The only valid uses forvolatile
I can think of are signal handler flags and hardware registers that are mapped in memory.Well, it seems like
volatile
is indeed redundant in this case. Clobber "+m" should take care of all things. I put it there just in case. Because redundant information isn't a problem, but lack of information is.volatile
also comes in handy in multithreaded programs, when you are messing up with custom synchronization/locking technique. Actually anything that involves shared memory involves volatile somehow. In regular programs volatile rarely used, because everything is already written (like synchronization primitives/threadsafe data structures...) and program uses high-level functions for this.I'm sorry for being a nerd, but
volatile
can't be used to implement thread synchronization primitives too. Evenvolatile sig_atomic_t
won't do. You are confusingvolatile
with atomic operations, which are two different things.Please note that regex is part of the standard but it is not part of g++(at least prior to 4.9). Have a look here. I'm not 100% sure but I think code with regex will not compile on codeforces.
actually, regex's compile fine on g++4.6 or 4.7 (I don't remember) but they just worked incorrectly.
As is mentioned in the bug I relate to, some of the functionality is not working as expected and some of not implemented at all. As per the comments in the bug I think this is fixed in 4.9. However I think codeforces uses an earlier version.
I use some tricks too, for example:
Input in vector n elements:
for ( int i = 0 ; i < n ; cin >> vec [ i++ ] );
Or analog of:
//
I would call it not a C++ trick, but a creative way to use for in C++. It's indeed shorter (just a little), but the code is unreadable IMHO.
This is really priceless!
Just another two tricks that might help.
std::string to_string( int value ); // Converts a numeric value to std::string.
int stoi( const std::string& str, std::size_t* pos = 0, int base = 10 ); // Interprets a signed integer value in the string str.
For more information, review std::to_string and std::stoi.
Thanks, very interesting. Let's do blogs like this often!
Can someone tell what I am doing wrong with trick
__builtin_popcount
where it's writtenfunction with suffix 'l' gets a unsigned long argument and with suffix 'll' gets a unsigned long long argument
in this problem485C - Bits
Solution 9506498 gives WA because of overflow.
1ll<<i
Thanks man!! and after that contest I cursed
__builtin_popcount
for making me lose points :P .I wonder then what is the difference between
__builtin_popcount
and__builtin_popcountll
as both solution give AC. I thought__builtin_popcount
should give wrong result if I send long long as an argument.9506854 --> __builtin_popcountll
and 9506856 __builtin_popcount
One of the best quick C++/STL tutorials,I have ever read. Congratulations to people who helped for this tut.
It is not part of c++11(only one of this), but useful cpp functions
All functions have two iterators as input, some of them have outputIterators and init values. All operators, used in these functions can be user-defined or standard:
These functions are defined in <numeric>
HosseinYousefi ,I think you forgot a semicolon in your perfect tutorial,right here:
""""" auto f = [] (int a, int b) -> int { return a + b; } ..HERE.. cout << f(1, 2); // prints "3" """"
Thanks, now corrected.
Using
complex
,p.real() = x
orcin >> p.real()
don't work in C++11 but they do in C++98.You can use
p.real(x)
in C++11. I don't know any way tocin
real.Here is a trick that might interest you. In C++, a class can inherit from a template instantiation of itself. So you can write
class X: vector<X> {...};
for example. Class X inherits the members of vector and you can use this trick to implement multidimensional arrays, tries, and other useful data structure without using pointers. More here.C++11 Tricks or Traps?
One should not use this:
instead of this:
or, am I missing something?
should be
Oh I see, misunderstood that, thanks.
You trapped in your own mistake!
There is a tiny typo in the section 6, dijkstra's part:
tie(dist, ode, prev) = q.top(); q.pop();
should be:
tie(dist, node, prev) = q.top(); q.pop();
Here's another trick:
For max/min functions, these functions don't need to take two parameters, they can take more :)
Instead of writing,
You can just use "{ }" braces around your parameters and insert a list into the max function (works the same for min function) like below:
Here's a source code for reference: http://ideone.com/lllqIK
Hey is there a shortcut to Something like:
a = max(a , Something being computed);
I always wanted something like: a+=Something being computed for max too. Although a function with variable parameters can be defined in a template but I don't like working with templates! :)
What's wrong with templates? This would work just fine:
Probably I fear them! Can you suggest some source to read more about templates and classes and stuff!
How does this works? Why "&" only before l and not before r?
Since we are only changing l while we iterate and not r.
Here's another trick:
You can write
return 14 / 88
instead ofreturn 0
Can I write a void which like
and got the result
a=1, b=2, c=3, d=4
if I have input 4 numbers 1, 2, 3, 4 when runread(a,b,c,d)
?Yes. Why do you ask? You can simply test it by doing so!
I got this error
when I ran that code. How to fix it?
lol, change
to
I changed it, but when i ran with
1 2 3 4
the result was1 0 0 0
. How to fix it?p/s: haha, I learnt to code for a while but now I still get that mistake =)) so ashame =))
You probably need to pass the rest of the arguments by reference somehow, not only the first one.
Here's how.
The Dijkstra code that uses emplace_back + tie has a little typo: node is spelt as ode
Thanks a lot! I am beginning to love C++ <3
How do I define the "rep" macro if i want to include the end indexes too ?
Like -> rep(i,1,10) prints 1...10 rep(i,10,1) prints 10....1 .
An ugly way, but it works. link
The link you mentioned isn't working . Can you post it on ideone ?
I have use this code and try 1000 test cases to make sure that it is correct.
Here is 3 codes:
By
ftoa
By normal
for
Make test case
Note: to make the test cases you download these 3 codes and then run the third one. It will automatically run.
Thanks for the great tips; but are all of them usable without C++14?
Most of them are. Is there any reason why you would use C++11?
Why would you use
instead of
?
To use it as elements of vector for example.
vector<array<int, 4>> v
What are the advantages of
vector<array<int, 4>> v;
overvector<vector<int>> v;
?Memory will be allocated only once.
Because you can compare arrays, access elements with bound-checking or get iterators support.
I think biggest advantage is
map<long long int, array<int,4> > m
Why the downvotes I didn't say anything wrong did I ???
Here's a submission by me using what I described (the check function)23252012(I got WA because the idea is wrong not the implementation)
My life now is a lot easier...Thank you HosseinYousefi.
:)
I'm not sure if this is well known but in C++ you can give a default value to a function for example:
void DFS(int node, int par = -1){
...
}
int main(){
// input a graph
DFS(1);
// rest of the code
}
the DFS function works as a normal function but when you don't provide a second parameter it will take the default value you have given it as its value...hope this helps.
:)
Great Work Man
Old post, but one important mistake: there should be no
std::move()
call at the end of yoursplit()
function.std::move()
should never be used to move automatic objects out of functions.Source
Auto comment: topic has been updated by HosseinYousefi (previous revision, new revision, compare).
Now that C++17 is here in CF, is there anything new and useful in the newer edition that we can use in competitive programming?
Gcd, structured bindings, clamp.
how do you write GCD function in c++17
std::gcd
Here are you
Also, one more cool thing C++(11?) has is the
throw
instruction andtry/catch
. You can get out of recursive call stacks and treat "No solution" / "Solution found" cases much more easily.Example:
Thanks a lot for the awesome tutorial, specially for the debug function. But it doesn't work perfectly if there is space in the macro parameter, e.g.
error(get<0> (tuple1), get<0> (tuple2));
Besides, replacing comma with spaces is also unnecessary, when we can tokenize based on comma:ShafinKhadem Could you provide some working of this debugger?
Thank you.
After some days, I realized that tokenizing on comma is a bad idea, as it fails in cases like bug(func(a,b),func(c,d)), but if we tokenize based on space, we can easily avoid and add some spaces to make it work. Now-a-days I use it like this:
Notes: After every token u must add both comma and space and there should not be space in single token (e.g. func(x,y), x*y). It won't compile in versions older than c++11.
use std::tie to write complex comparators:
range-for:
you can use it for input:
works with C-style arrays too:
actually you can use std::array instead of C-style arrays:
how to reference the global variable if there's local one with the same name:
It's awesome thanks for the blog!!
c++ 17 Better (for faster execution) used int instead short or bool or __int64
example: const int MAX = 1e4; vector< int > v(MAX); //instead vector< bool > v(MAX); int score;
//to use logical operations: for (int a=0;a<1e4;++a) for (int b=0;b<1e4;++b) score += (v[a] ^ v[b]);
you can be sure of solving the problem http://acmp.ru/index.asp?main=task&id_task=659
That (bits/stdc++.h) Library doesn't actually include everything like these two.
If you didn't know or you miss this information, because I searched for hours on the error for calling unordered_map in my code when including that bits only :D , so I suggest editing the post for these two.
If you use c++11 or later (which I think everyone should), using bits/stdc++.h includes them too.
Are variables in namespace initialised to 0 for c++? Thanks in advance.
For better clearification
C++ does not initialize most variables to a given value (such as zero) automatically. Thus when a variable is assigned a memory location by the compiler, the default value of that variable is whatever (garbage) value happens to already be in that memory location!
In c++17 we can do things like this:
That's something new I saw. Thanks :)
.
In C++17 there is also an inbuilt function for LCM e.g. cout<<lcm(3,4); //prints 12
for using this function directly, you need to include boost library and in codeforces you're not allowed to include external libraries
No you don't need to use boost library it is defined in header file numeric i.e.
#include<numeric>
or if are using#include<bits/stdc++.h>
no need to include numeric header fileyes , you are right but i don't know why it showing me compilation error on using this function I have G++17(Gcc 9.3) version on my PC,I tried it on Codechef IDE where this compiled well, but in other online IDE like https://www.jdoodle.com/online-compiler-c++/ it failed
By default GCC version>=5 compiles c++ code in c++14 If you are using command line e.g. cmd or powershell ( in windows ) or terminal ( in linux ) try adding
-std=c++17
flag while compiling If you are using any IDE on your PC try finding and enabling c++17 flag. For command line tryg++ -std=c++17 filename.cpp -o filename
replace filename with your current file name, Or if it does not fixed your error try attaching a screenshot of your error and code.Happy Coding
Thanks:), it is working now
Nice tricks.
Man!Why didn't I come across this earlier! Thanks for the blog!
I really liked that trick with the multi argument sum. I was wondering whether a similar trick exists for the **MAX and MIN ** methods as well ?
If you want template metaprogramming then
I prefer
I saw "array<int, 4> a;" first time. Is it same as int arr[4] or something different? It looks like an STL b/c the traversing is similar to other STL.
std::array
is an STL container for arrays of objects, with the size of the array that can be determined at compile time. It's essentially a wrapper for a normal array you make, for instance,std::array<int, 4> a;
andint a[4];
are the same, however, you get quite a few STL functions for free, and copying does a deep copy and not a pointer copy (basically you can return this by value).Is the __gcd(x,y) function of C++ faster than Euclidean Algorithm?
There is standard C++17 function std::gcd(x, y) in header . Don't use __gcd, since it's GCC-specified function
You can assume that both functions not slower than hand-coded Euclidean.
Any tricks for converting
string s="12345"
into an integer likeint a = 12345
easily?Here
Thanks a lot!!