calculating execution time in c++

Question

I have written a c++ program , I want to know how to calculate the time taken for execution so I won't exceed the time limit.

#include<iostream>  using namespace std;  int main () {     int st[10000],d[10000],p[10000],n,k,km,r,t,ym[10000];     k=0;     km=0;     r=0;     scanf("%d",&t);     for(int y=0;y<t;y++)     {     scanf("%d",&n);     for(int i=0;i<n;i++)     {             cin>>st[i] >>d[i] >>p[i];     }     for(int i=0;i<n;i++)     {             for(int j=i+1;j<n;j++)             {                     if((d[i]+st[i])<=st[j])                     {                               k=p[i]+p[j];                     }                     if(k>km)                     km=k;             }         if(km>r)         r=km;     }     ym[y]=r; }     for( int i=0;i<t;i++)     {          cout<<ym[i]<<endl;     }       //system("pause");     return 0; }      

this is my program and i want it to be within time limit 3 sec !! how to do it ? yeah sorry i meant execution time !!

Answer 1

If you have cygwin installed, from it's bash shell, run your executable, say MyProgram, using the time utility, like so:

/usr/bin/time ./MyProgram 

This will report how long the execution of your program took -- the output would look something like the following:

real    0m0.792s user    0m0.046s sys     0m0.218s 

You could also manually modify your C program to instrument it using the clock() library function, like so:

#include <time.h> int main(void) {     clock_t tStart = clock();     /* Do your stuff here */     printf("Time taken: %.2fs\n", (double)(clock() - tStart)/CLOCKS_PER_SEC);     return 0; } 

Answer 2

With C++11 for measuring the execution time of a piece of code, we can use the now() function:

auto start = chrono::steady_clock::now();  //  Insert the code that will be timed  auto end = chrono::steady_clock::now();  // Store the time difference between start and end auto diff = end - start; 

If you want to print the time difference between start and end in the above code, you could use:

cout << chrono::duration <double, milli> (diff).count() << " ms" << endl; 

If you prefer to use nanoseconds, you will use:

cout << chrono::duration <double, nano> (diff).count() << " ns" << endl; 

The value of the diff variable can be also truncated to an integer value, for example, if you want the result expressed as:

diff_sec = chrono::duration_cast<chrono::nanoseconds>(diff); cout << diff_sec.count() << endl; 

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