Hello, Codeforces!

In this article, I would like to share with you some general tips'n'tricks you could've used in your cpp sources. Some of them require advanced cpp knowledge, others might not.

bits/stdc++.h

This is a simple enough tip to reduce amount of includes. However, it might not work on your machine: that happens because of bits/stdc++.h is feature of GCC, not the entire cpp standard. However, there're few ways to workaround it.

Content

Well, before we start to put stdc++.h to the proper location, there's one thing to determine: content to put in. You either can determine what default headers you might need, or just google "gcc bits/stdc++.h source" and copy/paste it [if you've decided to do so, you might need to remove some gcc-specific headers: just try to compile it and remove one by one non-compiling includes].

Add it to default location

Well, the easiest one is to find one of include paths, like /usr/local/include (might add to /usr/include, but it's system directory, do it on your own risk) on Mac, or somewhere else for Windows. All you need to do after this is to create folder bits and file stdc++.h inside it.

Add to user-defined header search paths

If you compile/run you code manually, there's the option to add proper location as compiler flag isystem directly:

$ g++ -std=c++2077 a.cpp -o run_a -isystem /path/to/your/custom/includes

Note that inside folder /path/to/your/custom/includes must be folder bits with file stdc++.h.

Add to you build system

If you're using CMake based build system (what CLion actually does), then you could edit your CMakeLists.txt directly:

include_subdirectory(/path/to/your/custom/includes)

Prewritten code

According to CodeForces' official rules, you might use some predefined code. But it might be annoying to search over your sources to find the code you need. Well, you could put in a separate file like z.hpp with all content you might need to use on a contest and simply include it to use all functionality you need without rewriting it.

For example with this file you could write smth like:

#include <bits/stdc++.h>
#include <z.hpp>

using namespace std;

constexpr int mod = 1000 * 1000 * 1000 + 7;
using Mint = Z<mod>;

template <class T>
T bin_pow(T base, int64_t exponent) {
  if (exponent == 0) return T(1);
  if (exponent % 2 == 1)
    return base * bin_pow(base * base, exponent / 2);
  return bin_pow(base * base, exponent / 2);
}

int main() {
  Mint a, b;
  cin >> a >> b;

  // note that following addition performed by modulo
  cout << a + b << '\n';

  // note even pure bin_pow uses modular arithmetics
  cout << bin_pow(a, b) << '\n';
}

Don't forget to copy/paste actual source (if you need those) to your file before submitting: there's no z.hpp accessible by codeforces's compiler. It may be enhanced by custom script doing it instead of you, but there's no such script yet.

stdin, stdout, stderr

Your program using at least three streams to communicate with OS: stdin for input, stdout for output, and stderr for errors. Moreover, by default there's sync for cin and cout which may slow down you solution on huge inputs because of reading. You could discover more here. If you don't like it appear in main every time, you could've use the following trick: if you need to run some side effects even before main starts (like ios::sync_with_stdio(false)), you could do it while global variables instantiate:

int main() {
  // some code
}

// empty namespace to not conflict with any other possible names
namespace {

// global variable assigns to result of the lambda function.
// Since your variable cannot be void, put `int` as result:
auto fast_io = [] {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  return 0;
}();

}  // namespace

Since any checker/grader reads only stdout, I strongly recommend using stderr for debug information (cerr instead of cout) — that way once you find out a bug and solved it, you won't need to remove all debugging stuff before submit. However, comparing to remove all code this one could affect performance a bit.

Use some tooling

Since you're writing some code and test it against few testcases it may be annoying to write all of them on your own every time you run the program. To get rid of copy/paste it from a webpage, or trying to copy with spaces from pdf, you could've put it into files at once and run input from file. There's an amazing tool from xalanq. It ignores all your debug output while checking, but still prints it above testing verdicts.

Run debug code only in debug mode

You may need to run some extra code for debugging purposes only. For example, print graph, perform additional checks or compare with the naive solution. It may affect resulting performance and put your from AC zone to TL one. You may delete this code, but if you do so and the solution didn't succeed, you might add some fixes before realizing that you still need that debug code.

There's the straightforward solution: use #ifdef preprocessor directive:

int main() {
  int a, b;
  cin >> a >> b;
#ifdef DEBUG
  cerr << "a = " << a << " b = " << b << '\n'
#endif // DEBUG
  cout << a + b << '\n';

To run this code in debug-mode, you need to pass -DDEBUG or -D DEBUG option to your compiler. If you don't know how to do it, or simply don't wanna to do it, codeforces defines ONLINE_JUDGE for you:

int main() {
  int a, b;
  cin >> a >> b;
#ifndef ONLINE_JUDGE
  cerr << "a = " << a << " b = " << b << '\n'
#endif // ONLINE_JUDGE
  cout << a + b << '\n';

Plenty of code just to output two variables, isn't it? Let's solve two problems:

• print all variables given to function (macro)
• write code (block, or just function) that runs in debug mode and event doesn't evaluate arguments in release mode

Printing all variables

Ok, let's provide macro LOG(x, y, z) that works like cerr << "x = " << x << " y = " << y << " z = " << z << "\n", but works with plenty of arguments. Since we can't take x as string from function argument, let's start from simple scratch:

#define LOG(...)  cerr << "[ "#__VA_ARGS__" ] = "; log(__VA_ARGS__);

Where log function prints all arguments given to it. You could start from scratch like mentioned in my previous article:

template <class ...Args>
auto &log(const Args &...args) {
  return ((cerr << " " << as), ...) << '\n';
}

Let's put it all together:

template <class ...Args>
auto &log(const Args &...args) {
  return ((cerr << " " << as), ...) << '\n';
}

#define LOG(...)  cerr << "[ "#__VA_ARGS__" ] = "; log(__VA_ARGS__);

int main() {
  int a, b;
  cin >> a >> b;

  LOG(a, b, a + b);
  // prints: [ a, b, a + b ] = 2 2 4

  cout << a + b << '\n';
}

Well, there's plenty things you could've improved, like mentioned in this discussion. However, it still doesn't work fine for things with comma in subexpressions like:

LOG(a, b, some_func(a, b))

Running code in debug mode only

How to run code in debug and even don't evaluate arguments in release mode? Well, here's trick you might seen here involving if:

#ifndef DEBUG

#define CERR if (false) cerr

#endif

// somewhere in code

CERR << "a = " << a << '\n';

// expands to

if (false) cerr <<  << "a = " << a << '\n';

But there's bug here:

if (a == 0)
  CERR << "b=" << b << '\n';
else {
  CERR << "c=" << c << '\n';
  // dance like no one's watching
  // with a != 0
}

// expands to

if (a == 0)
  if (false)
    cerr << "b=" << b << '\n';
  else {
    if (false)
      cerr << "c=" << c << '\n';
    // a is still zero here :(
  }

Well, there's alternative with while (false) which doesn't have such bugs. But what to do instead of while (false) to run any code? We may try to put nothing [don't know if there's bug], but I'd prefer one time running for. Let's put it all together:

template <class ...Args>
auto &log(const Args &...args) 
{ return ((cerr << " " << as), ...) << '\n'; }

#ifdef DEBUG
#  define LOG(...)  cerr << "[ "#__VA_ARGS__" ] = "; log(__VA_ARGS__)
#  define RUN       for (bool _flag = true; _flag; _flag = !_flag)
#else  // DEBUG
#  define LOG(...)  while (false) cerr
#  define RUN       while (false)
#endif // DEBUG

int main() {
  int n, m;
  cin >> n >> m;

  LOG(n, m);
  
  vector<vector<int>> g(n);
  for (int e = 0; e < m; ++e) {
    int u, v;
    cin >> u >> v;

    g[--u].push_back(--v);
    g[v].push_back(u);
  }
   
  RUN {
    for (int u = 0; u < n; ++i) {
      cerr << u << ":";
      for (int v : g[u]) 
        cerr << " " << v;
      cerr << "\n";
    }
  }
}