Question

I’m having trouble optimizing a C++ program for the fastest runtime possible.

The requirements of the code is to output the absolute value of the difference of 2 long integers, fed through a file into the program. ie:

./myprogram < unkownfilenamefullofdata

The file name is unknown, and has 2 numbers per line, separated by a space. There is an unknown amount of test data. I created 2 files of test data. One has the extreme cases and is 5 runs long. As for the other, I used a Java program to generate 2,000,000 random numbers, and output that to a timedrun file — 18.MB worth of tests.

The massive file runs at 3.4 seconds. I need to break that down to 1.1 seconds.

This is my code:

int main() {
long int a, b;
while (scanf("%li %li",&a,&b)>-1){
  if(b>=a)
    printf("%li/n",(b-a));
  else
    printf("%li/n",(a-b));
  } //endwhile
return 0;
}//end main

I ran Valgrind on my program, and it showed that a lot of hold-up was in the read and write portion. How would I rewrite print/scan to the most raw form of C++ if I know that I’m only going to be receiving a number? Is there a way that I can scan the number in as a binary number, and manipulate the data with logical operations to compute the difference? I was also told to consider writing a buffer, but after ~6 hours of searching the web, and attempting the code, I was unsuccessful.

Any help would be greatly appreciated.

Answer 1

What you need to do is load the whole string into memory, and then extract the numbers from there, rather than making repeated I/O calls. However, what you may well find is that it simply takes a lot of time to load 18MB off the hard drive.