Faster I/O in C

Question

I have a problem which will take 1000000 lines of inputs like below from console.

0 1 23 4 5
1 3 5 2 56
12 2 3 33 5
...
...

I have used scanf,

Answer 1

You can greatly reduce the time of execution by taking input using fread() or fread_unlocked() (if your program is single-threaded). Locking/Unlocking the input stream just once takes negligible time, so ignore that.

Here is the code:

#include <iostream>

int maxio=1000000;
char buf[maxio], *s = buf + maxio;

inline char getc1(void)
{
   if(s >= buf + maxio) { fread_unlocked(buf,sizeof(char),maxio,stdin); s = buf; }
   return *(s++);
}
inline int input()
{
   char t = getc1();
   int n=1,res=0;
   while(t!='-' && !isdigit(t)) t=getc1(); if(t=='-')
   {
      n=-1; t=getc1();
   }
   while(isdigit(t))
   {
     res = 10*res + (t&15);
     t=getc1();
   }
   return res*n;
}

This is implemented in C++. In C, you won't need to include iostream, function isdigit() is implicitly available.

You can take input as a stream of chars by calling getc1() and take integer input by calling input().

The whole idea behind using fread() is to take all the input at once. Calling scanf()/printf(), repeatedly takes up valuable time in locking and unlocking streams which is completely redundant in a single-threaded program.

Also make sure that the value of maxio is such that all input can be taken in a few "roundtrips" only (ideally one, in this case). Tweak it as necessary.

Hope this helps!

Answer 2

Out of curiosity, what generates that many lines that fast in a console ?