CODEWITHSUNDEEP
cfloating-point
rkachowski
rkachowski

Reputation: 41

Floating point variable won't decrement in C - infinite while loop

The following code is terrible, but was encountered in a production situation. It was solved by doing something less insane - but I can't work out why the value remains constant. FWIW this arbitrary large value was taken from a timestamp.

#import <stdio.h>

int main(void)
{
    float wtf =  466056.468750;

    while(wtf > .01)
    {
        wtf -= .01;

        /* other operations here */

        printf("wtf = %f\n", wtf);
    }

    return 0;
}

When the program is run the output produced is

wtf = 466056.468750

wtf = 466056.468750

wtf = 466056.468750

wtf = 466056.468750

wtf = 466056.468750

wtf = 466056.468750

wtf = 466056.468750

wtf = 466056.468750

wtf = 466056.468750

wtf = 466056.468750

When debugging with, I can see that an appropriate value is returned for the expression wtf - .01 but it just doesn't seem to persist.

My question is, why isn't the decremented value stored in the variable?

In gdb the value of the operation is printed out as follows

10          printf("wtf = %f\n", wtf);
(gdb) p wtf
$1 = 466056.469
(gdb) p wtf - .01
$2 = 466056.45874999999
(gdb) n

Whilst there is a clear change in precision, the value 466056.45874999999 is neither 466056.469 nor 466056.468750 (the value printed to the console)

Upvotes: 0

Views: 140

Answers (3)

ouah
ouah

Reputation: 145839

466056.468750f and 466056.468750f - 0.01f both have the same representation as a float.

Floating point numbers have a finite representation. Multiple real numbers have the same floating point representation. Floating points are roughly logarithmically spaced and when the floating point number is bigger, there are more and more real numbers that have its same representation.

Upvotes: 5

Marco10
Marco10

Reputation: 87

Change the type of wtf from float to double. The float type has no precision for that number (read somewhere the web to understand the differences) Hope I could help!

Upvotes: 4

piokuc
piokuc

Reputation: 26184

What's happening with your program is caused by floating point inaccuracy. The program will work as you expect if you replace float with double.

Upvotes: 2

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