Why is 100 * 4.2 coming out as 419 and not 420?

Question

Can't figure out error in my code when 4.2 is input as change will be then 420 but instead is coming out to be 419. What is wrong in my code?

#include<stdio.h>
#include<cs50.h>

int evaluate(int change){

    int quarter,dime,nickel,penny;
    int coins = 0;

    quarter = change/25;
    change = change%25;
    printf("quarters:%d\n" ,quarter);
    printf("change left: %d\n",change);
    dime = change/10;
    change = change%10;

    printf("dimes:%d\n" ,dime);
    printf("change left: %d\n",change);
    nickel = change/5;
    change = change%5;

    printf("nickels:%d\n" ,nickel);
    printf("change left: %d\n",change);
    penny = change;
    coins = quarter+dime+nickel+penny;

    printf("pennies:%d\n" ,penny);
    printf("change left: %d\n",change);
    return coins;
}


int main()
{
    printf("How much:");
    float input = GetFloat();
    int coin = 0;

    while(true)
    {
        if(input>0)
        {
            int change = (100*input);
                printf("change: %d\n",change);

            coin = evaluate(change);

            printf("%d\n",coin);
            return 0;
        }    

        else
        {
            printf("are you serious? retry:");
            input = GetFloat();
    }    }
}

The program is to evaluate the least number of coins a cashier has to give to customer. Problem is that on getting input 4.2, the output is coming out wrong; also change=100*input is coming out to be 419 for input to be 4.2. Please help me figure out problem.

Code on left side and output on right.

_Questions seeking debugging help ("why isn't this code working?") must include the desired behavior, a specific problem or error and the shortest code necessary to reproduce it in the question itself. Questions without a clear problem statement are not useful to other readers. See: How to create a Minimal, Complete, and Verifiable example._ — Sourav Ghosh, Dec 07 '16 at 12:41
Post your code as code, not an image. The folks you want to have help will not waste time with this question. — Neo, Dec 07 '16 at 12:42
Have you tried rounding 100.0 times input? I am guessing right now your code is truncating 419.99 something. — Jeremy Kahan, Dec 07 '16 at 12:53
Teaching you the best way to ask your questions ("don't post code as an image", "use a relevant title") *is* helpful, and the people you sneer at for "complaining" *are* attempting to help you. — Jongware, Dec 07 '16 at 14:33

score 2 · Answer 1 · edited May 23 '17 at 12:24

4.2 cannot be expressed exactly with the binary floating point format used on you platform. When you multiply it by 100, it gives a number that is just slightly smaller than 420. Converting it to an integer yields 419.

You can prevent this by rounding the number prior to conversion with:

int n = round(number);

Or by adding a rounding adjustment:

int n = (int)(number + 0.5);

Do not use float for this computation, use a properly rounded amount of cents as explained on this answer: Integer overflow in greedy coin counting.

Why is 100 * 4.2 coming out as 419 and not 420?

1 Answers1