fast I/O in C/C++

There many ways to do input/output in C/C++. Some are slow, some are fast and some can be very fast. Here I’ll be discussing some of these methods.

Many a times during competetive programming you come across the warning “Careful – large input/output“. Now what exactly does this means?
This basically means that you have to use an optimized code for your reading/writing to the standard input/output to stay inside the tighter time constraints of the problem. So let’s get going and explore the different methods to optimize your I/O in C/C++.

All the code can be found here: GitHub.

First one is the basic method that you guys must already be using. This is based on cin/cout methods of the iostream library. It is the most basic method and does not require you to specify the type of input you are expecting, but, is extremely slow.

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

int main() {
	/* integer or any integer like */
	int integer;

	/* character string */
	char charstring[100];
		cin>>charstring;	// stops input after a "space"

		cin.getline(charstring, 100);	// stops after 100 character or eof whichever comes first

	string strstring;
		cin>>strstring;		// stops input after a "space"

		getline(cin, strstring);	// stops after eof or '\n', whichever comes first

	/* safest way to get an integer (but very slow) - taken from */
	int number = 0;
	string input;
	while(true) {
		getline(cin, input);

		stringstream myStream(input);

	/* Safest way to get a single character */
	char singlechar = {0};
	while(true) {
		getline(cin, input);

		if(input.length() == 1){
			singlechar = input[0];

The second method is based on the simple printf/scanf functions of stdio.h library for C. These are much faster than the above method and can be easily used in competetive programming for a decent time limit. These are multi-thread safe as they lock the file before writing.
Detailed description of the stdio.h library you can refer to here.

#include <cstdio>
#include <string>

int main() {
	/* integer I/O */
	int a;
	scanf("%d", &a);
	printf("%d\n", a);

        Other format specifiers.
        %d, %i = signed integer
        %u = unsigned integer
        %l = prefix for long
        %f = signed floating point
        %e = signed scientific
        %c = single character

	/* sting I/O */
	char charstring[100];
	scanf("%s", charstring);    // only till the first white space is stored
	printf("%s\n", charstring);

	scanf("%[^\n]s", charstring);   // sets th delimeter to be "new line"
	printf("%s\n", charstring); // thus whole line is read until a \n is observed
								// does not eliminate \n from the input stream

	printf("%s\n", charstring);

This method is very fast than the last method and used unlocked versions of the above functions used. I have written the code to scan integer and a string. For others you can easily write your function referring the below code. These are NOT multi-thread safe. Should be used in caution. But, in competetive programming plateforms, one program is already separated from others, so can easilt be used for better I/O in competetive programming which have even tighter time constraints.

#include <cstdio>

inline void fastRead_int(int &x) {
    register int c = getchar_unlocked();
    x = 0;
    int neg = 0;

    for(; ((c<48 || c>57) && c != '-'); c = getchar_unlocked());

    if(c=='-') {
    	neg = 1;
    	c = getchar_unlocked();

    for(; c>47 && c<58 ; c = getchar_unlocked()) {
    	x = (x<<1) + (x<<3) + c - 48;

    	x = -x;

inline void fastRead_string(char *str)
    register char c = 0;
    register int i = 0;

    while (c < 33)
        c = getchar_unlocked();

    while (c != '\n') {
        str[i] = c;
        c = getchar_unlocked();
        i = i + 1;

    str[i] = '\0';

int main()

  int n;
  char s[100];

  	printf("%d\n", n);

  	printf("%s\n", s);
  return 0;

There is another method I came across a solution in one of the problems on codechef. I haven’t tested it’s performance, but, his solution had the lowest timing with the same logic for the actual problem. So, I am assuming this gives a better performance for I/O. You can download the code here.
The solution referrenced is this.

I’ll come up with more updates on the same topic.

8 responses to “fast I/O in C/C++

    1. x=x*2^1+x*2^3+c-48…
      Now you can understand what it does.
      It multiplies the current value with 10 and then add the incoming character.
      lets say x=0,c=’5′;
      now c=’6′;

  1. Hey Chirag,

    Great site you got here. I’m not too familiar with C++ but thank you for providing the source to this fix. I also check out GitHub when I’m stuck.

    Thanks for the resource.


    1. The required header files are given in the code itself.
      For the first one you need: iostream , string and sstream
      second part: cstdio and string
      For the third code you only need: cstdio

Leave a Reply

Your email address will not be published. Required fields are marked *