Multiplying float with 100 sometimes gives a slightly unexpected result in Objective C

Question

So I'm trying to convert the value of UISlider to an integer so I can use it as an index to get a value from an NSArray but multiplying the slider value sometimes gives an unexpected result..

int index = self.moveHistoryLabelSlider.value * 100;

The result's fine for the first few but when it reaches 0.15, it starts to give a different result. Here's an NSLog of the slider.value and slider.value*100:

slider value = 0.150000 slider * 100 = 14.999999

It happens in other float numbers too like 0.19 * 100 gives 19.999998. How do I fix this?

Answer 1

This problem is the nature of Floating Point Numbers (https://floating-point-gui.de/basic/). You can use two simple approaches to minimize rounding error:

1. Calculate integers instead of floats with needed precision:

// Float calculation
float value = 0.15; // 0.150000006
float multiplier = 100; // 100
NSLog(@"%f", value * multiplier); // 15.000001

// Convert float to integers and back
int precision = 100;
int intValue = value * precision; // 15
float result = (intValue * (int)multiplier) / (float)precision;
NSLog(@"%f", result); // 15.000000

2. Use NSDecimalNumber for calculation

NSDecimalNumber* decimalValue = [[NSDecimalNumber alloc] initWithDecimal:@(value).decimalValue]; // 15 x 10^-2
NSDecimalNumber* decimalMultiplier = [[NSDecimalNumber alloc] initWithDecimal: @(multiplier).decimalValue]; // 10 x 10^2
NSDecimalNumber* decimalResult = [decimalValue decimalNumberByMultiplyingBy:decimalMultiplier];
NSLog(@"%f", decimalResult.floatValue); // 15.000000