Float not being converted correctly

Question

I am working on a program that will convert from dollars to cents. I take in user input as a float using a predefined function then i try to multiply this float by 100 but it gives me an incorrect value. Heres the code:

printf("Change in dollars: ");
change= GetFloat();
cents= round(change*100);
printf("Cents is %f",cents);

This is a logical error because the program runs fine but there is something wrong with the mathematics for example if i enter 1.50 when prompted, the return i get is 1004 which is clearly wrong. What i want to happen is 150 to be outputted.

Please show `GetFloat()`. _and beware of floating-point math_, at times. :-) — Sourav Ghosh, Jun 25 '15 at 11:44
Yeah please edit your question and add the function GetFloat() — Noober, Jun 25 '15 at 11:48
What locale settings, and really post `GetFloat()` it's almost surely the problem. — Iharob Al Asimi, Jun 25 '15 at 11:49
@iharob .^^..here goes your answer... `i am using ubuntu ` :-) — Sourav Ghosh, Jun 25 '15 at 11:49
@student1999 What is it that you want to do?Do you want to get the decimal part of the input or what? — Noober, Jun 25 '15 at 11:50
it sounds pretty straightforward but the answer provided by iharob seems a little too complex — Farid, Jun 25 '15 at 11:53
@student1999 simply output the value of `change` to see if the input is your problem - then think about handling invalid input like iharob suggests — BeyelerStudios, Jun 25 '15 at 11:55
@student1999 sorry, why is it complex? Error checking is one of the most important and often ignored things a programmer should do. — Iharob Al Asimi, Jun 25 '15 at 11:55
Dear @ student1999 , somehow did you miss the first part of my _very_ first comment? — Sourav Ghosh, Jun 25 '15 at 11:57
I can't be sure, because a lot of your code is missing, but it's likely that you have not properly declared everything as floating-point. Your variables `change` and `cents` must be declared as `double` (or maybe `float`). Your functions `GetFloat()` and `round()` must be declared (and defined) as returning `double` (or maybe `float`). Please check these things. — Steve Summit, Jun 25 '15 at 12:14
Thank you guys! I found the error, it was that i previously declared all my variables as ints when some of them were supposed to be floats — Farid, Jun 25 '15 at 12:24

Iharob Al Asimi · Accepted Answer · 2015-06-25T12:00:25.483

This is most likely because you have some locale where the decimal separator is , instead of ., and since you are very likely not checking the return value of scanf() "which is what most books do and almost every tutorial", then the variable is not being initialized and what you are printing is a consequence of the layout of your program instead of a value inputed by a user.

To verify what I say, I suggest compiling the same program without modification but with different compilation flags.

What you must do is ensure that the input is correct, try this

float GetFloat(int *ok)
 {
    float value = 0;
    if (scanf("%f", &value) != 1)
        *ok = 0;
    else
        *ok = 1;
    return value;
 }

which you would call like this

int   result = 0;
float change = GetFloat(&result);
if (result == 0)
    return -1;
float cents = round(change * 100);

printf("Cents is %f",cents);

If the decimal separator is an issue, the above program will not output anything, because scanf() will return 0, and hence ok will be 0 after you call the function.

One important consequence of ignoring the return value of non-void functions is that if the value was not initialized like in this case with scanf(), and you don't know that because you didn't check the return value, then undefined behavior will happen, meaning that your program will contain bugs that are very difficult to find and hence very hard to fix.

@student1999 I don't know, because you didn't post your code. — Iharob Al Asimi, Jun 25 '15 at 11:58

Float not being converted correctly

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