ios - How to round CGFloat

Question

I made this method

+ (CGFloat) round: (CGFloat)f {
    int a = f;
    CGFloat b = a;
    return b;
}

It works as expected but it only rounds down. And if it's a negative number it still rounds down.

This was just a quick method I made, it isn't very important that it rounds correctly, I just made it to round the camera's x and y values for my game.

Is this method okay? Is it fast? Or is there a better solution?

Answer 1

2018 Answer

The other answers here are either dated or don't give good examples. It is easy to round a CGFloat using Swift's built in rounded function.

let x: CGFloat = 3.5
let y = x.rounded() // 4.0

If you want to round the value in place you can use round:

var x: CGFloat = 3.5
x.round() // 4.0

Rounding Rules

If you want more precise control over how numbers are rounded, you can use a FloatingPointRoundingRule.

Away from zero

x.rounded(.awayFromZero)

Numbers above zero are rounded up and numbers below zero are rounded down.

3.000  ->  3.0
3.001  ->  4.0
3.499  ->  4.0
3.500  ->  4.0
3.999  ->  4.0

-3.000  ->  -3.0
-3.001  ->  -4.0
-3.499  ->  -4.0
-3.500  ->  -4.0
-3.999  ->  -4.0

Down

x.rounded(.down)

Rounds any number with a decimal value down to the next smaller whole number. This is the same as floor(x).

3.000  ->  3.0
3.001  ->  3.0
3.499  ->  3.0
3.500  ->  3.0
3.999  ->  3.0

-3.000  ->  -3.0
-3.001  ->  -4.0
-3.499  ->  -4.0
-3.500  ->  -4.0
-3.999  ->  -4.0

To nearest or away from zero

x.rounded(.toNearestOrAwayFromZero)   // same as x.rounded()

Decimal numbers get rounded to the nearest integer value. However, when the value is exactly in the middle (like 3.5 or -3.5) then positive numbers get rounded up and negative numbers get rounded down.

It may have a long complicated name, but this is normally how one learns rounding in school. It is also the rule used if you just do x.rounded().

3.000  ->  3.0
3.001  ->  3.0
3.499  ->  3.0
3.500  ->  4.0  ***
3.999  ->  4.0

-3.000  ->  -3.0
-3.001  ->  -3.0
-3.499  ->  -3.0
-3.500  ->  -4.0  ***
-3.999  ->  -4.0

To nearest or even

x.rounded(.toNearestOrEven)

This is similar to toNearestOrAwayFromZero, except now the .5 values get rounded to the even whole number.

3.000  ->  3.0
3.001  ->  3.0
3.499  ->  3.0
3.500  ->  4.0   ***
3.999  ->  4.0
4.500  ->  4.0   ***

-3.000  ->  -3.0
-3.001  ->  -3.0
-3.499  ->  -3.0
-3.500  ->  -4.0   ***
-3.999  ->  -4.0
-4.500  ->  -4.0   ***

Toward zero

x.rounded(.towardZero)

This just has the effect of cutting off any decimal values. If you needed an Int you could do the same thing with Int(x).

3.000  ->  3.0
3.001  ->  3.0
3.499  ->  3.0
3.500  ->  3.0
3.999  ->  3.0

-3.000  ->  -3.0
-3.001  ->  -3.0
-3.499  ->  -3.0
-3.500  ->  -3.0
-3.999  ->  -3.0

Up

x.rounded(.up)

This is the opposite of .down. All decimal numbers are rounded up. This is the same as ceil(x).

3.000  ->  3.0
3.001  ->  4.0
3.499  ->  4.0
3.500  ->  4.0
3.999  ->  4.0

-3.000  ->  -3.0
-3.001  ->  -3.0
-3.499  ->  -3.0
-3.500  ->  -3.0
-3.999  ->  -3.0

Notes

• Don't forget to take negative values into account.
• The results of round and rounded are still CGFloat. If you need an Int you have to convert it like Int(myCGFloat).
• There is no need to use the C math functions round(x), ceil(x) and floor(x) anymore. However, if you do use them, they handle both 64 and 32 bit architecture so any answers you may have seen with roundf, ceilf and floorf are now obsolete.