Incorrect result when dividing an integer over a float

Question

I'm working on Objective C and I'm trying to deduce the proper index of 2 dimensions array.

To do that I've a few buttons where the tag is setted merging the two index in a single number. In example, a position of 1,1 in the array results on a tag = 11.

After that to unmerge the two components of the index, I'm using following code:

float tag = (float)[sender tag];

    float x = [[NSString stringWithFormat:@"%.2f", tag / 10.f] floatValue];
    //float x = (float)tag / 10.f;

    int y = floor (x);

    int z = (x - y) * 10;

The theory works fine, but I'm surprised when I'm obtaining these results:

tag = 23 x = 2.29999995 instead of expected result of 2.3 (23 / 10 = 2.3 and not 2.2999995)!

I've tried too with double and several operations without success.

Anyone knows what I'm doing wrong?

Thanks.

Answer 1

Use integer arithmetic. Floating point does not represent all numbers with complete accuracy, especially non-integer values.

Example where [sender tag] is 23:

int senderTag = 23;
int tagA = senderTag / 10;
int tagB = senderTag % 10;
NSLog(@"senderTag: %d, tagA: %d, tagB: %d", senderTag, tagA, tagB);

NSLog output:

senderTag: 23, tagA: 2, tagB: 3

Answer 2

Especially, fractions like 1/3, 1/5 can't be represented correctly by floating point arithmetics.