calculation of wall clock

Question

How to modify this code to do sequential program to calculate the wall clock time (time_t) needed to calculate the dot product of the complex numbers.

#include "stdafx.h"
#include <stdlib.h>
#include<stdio.h>

typedef struct complex{
double real;
double img;
}complex;

complex add(complex a, complex b);
complex multiply(complex *a, complex *b);


int _tmain(int argc, _TCHAR* argv[])
{
int choice, temp1, temp2;
complex a, b, c;

while (1)
{
    printf("Press 1 to add two complex numbers.\n");
    printf("Press 2 to multiply two complex numbers.\n");
    printf("Press 3 to exit.\n");
    printf("Enter your choice\n");
    scanf_s("%d", &choice);

    if (choice == 3)
        exit(0);

    if (choice >= 1 && choice <= 2)
    {
        printf("Enter a and b where a + ib is the first complex number.");
        printf("\na = ");
        scanf_s("%d", &a.real);
        printf("b = ");
        scanf_s("%d", &a.img);
        printf("Enter c and d where c + id is the second complex number.");
        printf("\nc = ");
        scanf_s("%d", &b.real);
        printf("d = ");
        scanf_s("%d", &b.img);
    }
    if (choice == 1)
    {
        c.real = a.real + b.real;
        c.img = a.img + b.img;

        if (c.img >= 0)
        printf("Sum of two complex numbers = %d + %di", c.real, c.img);
        else
        printf("Sum of two complex numbers = %d %di", c.real, c.img);
    }

    else if (choice == 2)
    {
        c.real = a.real*b.real - a.img*b.img;
        c.img = a.img*b.real + a.real*b.img;

    if (c.img >= 0)
    printf("Multiplication of two complex numbers = %d + %di", c.real, c.img);
        else
    printf("Multiplication of two complex numbers = %d %di", c.real, c.img);
    }


    else
    printf("Invalid choice.");

    printf("\nPress any key to enter choice again...\n");
}
    }

Answer 1

A typical approach is to record time() twice and run the code many times to get a 1st order approximation.

time_t t0,t1;
time(&t0);
int N = 1000000;
for (int i=0; i< N; i++) {
  DoCodeUnderTest();
  // c.real = a.real + b.real;
  // c.img = a.img + b.img;
}
time(&t1);
printf("Time %e\n", (double) (t1-t0) / N);

Suggest using profiling tools for a more accurate answer.

---

@Jonathan Leffler suggestion to use clock() is also an improvement.

clock_t c1,c12;
c1 = clock();
...  // repeat runs of the code
c2 = clock();
printf("Time %e\n", (double) (c1-c0) / CLOCKS_PER_SEC / N);

Given 1) @Jonathan Leffler 2nd suggestion about the legitimacy of repeating since a compiler may out-think the code and 2) cache issues hints that any brute force methods as suggested here are at best illustrative and not definitive time measurements.