Objective-C If Statement Comparing Floats not working

Question

This is literally the weirdest thing I have ever encountered. So, I have a float called ratingDecimal, and I use some conditions to compare it. Here is what the conditions look like:

    if (ratingDecimal >= 0.0) {

    if (ratingDecimal < 0.3 && ratingDecimal != 0.3) {

        ratingDecimal = 0.0;

        NSLog(@"bigger or equal to 0 and smaller than 0.3");
    }
}

if (ratingDecimal >= 0.3) {

    if (ratingDecimal < 0.8 && ratingDecimal != 0.8) {

        ratingDecimal = 0.5;

        NSLog(@"middle");
    }
}

if (ratingDecimal >= 0.8) {

    NSLog(@"bigger or equal to 0.8");

    ratingDecimal = 1.0;
}

Here is a weird problem. I have set the ratingDecimal to 0.8, the console logs ratingDecimal as:

0.8

but calls:

middle

Which should only be called if ratingDecimal is bigger or equal to 0.3 but smaller than 0.8 and DOES NOT equal to 0.8.

The statement that should be getting called is:

if (ratingDecimal >= 0.8) {

NSLog(@"bigger or equal to 0.8"); 

ratingDecimal = 1.0;

}

But I do not see

bigger or equal to 0.8

You are probably wondering why my conditions are so tedious and complex, they can be as simple as:

if (ratingDecimal >= 0.3  < 0.8)

They used to be like that but since this has never worked I kept on breaking the statement down and it is STILL acting weird.

Why is this happening!?

Here is an image example:

PROBLEM

If ratingDecimal < 0.3 then there is no point in checking whether ratingDecimal != 0.3. It must be different from 0.3. — gnasher729
– gnasher729, Commented Apr 15, 2014 at 21:14

Greg Hewgill · Accepted Answer · 2012-10-15 01:17:01Z

7

Your ratingDecimal value is probably something like 0.7999999999999. When printed to the console, something is probably rounding it to 0.8, but the comparison still sees it as less than 0.8.

This is due to the binary nature of floating point numbers. See What Every Computer Scientist Should Know About Floating-Point Arithmetic for extensive information on this.

answered Oct 15, 2012 at 1:17

Greg Hewgill

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1 Comment

David Schwartz Over a year ago

It's the same reason it's very hard to get 3 * (1/3) to equal 1 in decimal. There is no fixed-length decimal value that gives 1 when multiplied by 3. In most fixed-length decimal systems 3 * (1/3) will be printed as 1 but is internally .999999.. and so will also test as less than 1.

rmaddy · Accepted Answer · 2012-10-15 01:38:15Z

1

@Greg gave a good answer. I'd like to point out that you can write that code in a much clearer and simpler fashion:

if (ratingDecimal >= 0.0) {
    if (ratingDecimal < 0.3) {
        ratingDecimal = 0.0;

        NSLog(@"bigger or equal to 0 and smaller than 0.3");
    } else if (ratingDecimal < 0.8) {
        ratingDecimal = 0.5;

        NSLog(@"middle");
    } else {
        ratingDecimal = 1.0;

        NSLog(@"bigger or equal to 0.8");
    }
} else {
    // negative
}

As an FYI - a line such as:

if (ratingDecimal < 0.3 && ratingDecimal != 0.3) {

doesn't make such sense. If a number is less than 0.3 then it must be unequal to 0.3 so the second check doesn't do anything useful.

answered Oct 15, 2012 at 1:38

rmaddy

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1 Comment

MCKapur Over a year ago

I know that, in my question I say that the only reason I put that was to break the if statement and make 100 percent sure I was doing it right

Animesh · Accepted Answer · 2012-10-15 08:06:08Z

0

you can use functions like roundf() according to your need.

answered Oct 15, 2012 at 8:06

Animesh

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