Faster Input Scan Method in C

Sometimes we need to scan input really fast like when we are participating in coding competitions like codechef or SPOJ where the time limit is very strict and inputs are large, we can save some time by adopting faster scanning methods.

We can achieve our gaol using getchar_unlocked function.

code below scans 100000 integers and has two different methods one is scan_normal() this will scan integers using standard scanf() function and the other one is scan_fast() this scan integers using getchar_unlocked function, we will execute the program twice one for each method mark the time taken for execution to evaluate the performance.

#include<stdio.h>
#define gc getchar_unlocked // gc for saving for getchar_unlocked
int scan_fast(){
	int n=0;
	char ch=gc();
	while(ch < '0' && ch >'9'){
		ch=gc(); // ignoring chars that are not digit
	}
	while(ch>='0' && ch<='9'){
		n=n*10+ch-'0'; // for each digit add it to n in proper place
		ch=gc();
	}
	return n; 
}

int scan_normal(){
	int n;
	scanf("%d",&n);
	return n;
}

int main(){
	int num=1000000; //we have 100000 integers to test
	while(num--){
	//	scan_normal(); // uncomment this for 1st execution
	//	scan_fast(); // uncomment this for 2nd execution
	}
	return 0;
}

for checking i am using linux time command to check the execution time and a file text-file.txt containing 100000 integers from 0 to 99999

command is as follows

time ./scan_methods < text-file.txt

output using scan_normal

real	0m0.112s
user	0m0.101s
sys	0m0.012s

output using scan_fast

real	0m0.040s
user	0m0.033s
sys	0m0.008s