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cfloating-pointfloating-point-precision
Benjamin
Benjamin

Reputation: 29

Inconsistent summation in C

When I run the following program:

#include <stdio.h>
#include <math.h>

int main()
{
    double sum, increase;
    long amount, j;

    printf("sum = ");
    scanf("%lf", &sum);
    printf("increase = ");
    scanf("%lf", &increase);
    printf("amount = ");
    scanf("%ld", &amount);

    for (j = 1; j <= amount; j++)
    {
        sum += increase;
    }

    printf("%lf\n", sum);

    return 0;
}

I obtain the following response for these values:

MacBook:c benjamin$ ./test
sum = 234.4
increase = 0.000001
amount = 198038851
432.438851
MacBook:c benjamin$ ./test
sum = 234.4
increase = 0.000001
amount = 198038852
432.438851
MacBook:c benjamin$ ./test
sum = 234.4
increase = 0.000001
amount = 198038853
432.438852

where I have increased the variable 'amount' by 1 in each case.

Why does this happen?

Although the code doesn't seem to be very useful, I have just written the part in question. I actually wanted to use it in a larger program.

Thanks!

Upvotes: 3

Views: 199

Answers (1)

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726619

This is an issue with the formatting, not the number. If you change printf to

printf("%.12lf\n", sum);

the results look more along the lines of what you've expected:

sum = increase = amount = 432.438851000198
sum = increase = amount = 432.438852000198
sum = increase = amount = 432.438853000198

The "junk" at the end is due to the limited precision of floating point numbers.

Upvotes: 3

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