Round NSNumber to a specific number of significant digits

Question

Let's say I have an NSNumber object that holds a value 0.12345. All I want is an NSNumber object with a same value, but rounded to a specified number of significant digits, say 2, so value looks like this: 0.12.

I don't want to convert it to NSString and back, which is what some of the answers suggest
I would prefer not to cast/convert and/or do multiplication/division manually like in this answer

Is there an obvious way to do it that I am missing?

`NSNumber` doesn't have any built-in rounding functions. You could use `NSDecimalNumber` which does. — dan, Aug 15 '17 at 21:08
Explain your goal a bit more. Why do you need to round the `NSNumber`? — rmaddy, Aug 15 '17 at 21:27
"I would prefer not to ..." - explain why you don't want to solve a math problem with math. There are plenty of answers in your linked question, ranging from bad to good (the scores will help with that). — CRD, Aug 16 '17 at 08:13

score 3 · Accepted Answer · answered Aug 15 '17 at 23:12

It's pretty much impossible to do it and maintain all your constraints. NSNumber has no rounding methods, so you're going to have to convert to something else. The obvious route is to convert to a double and multiply, round and divide, but you say you would prefer not to do that. So that leaves this route:

NSNumber *round(NSNumber *number, NSInteger places, NSRoundingMode mode) {
    NSDecimal d = number.decimalValue;
    NSDecimal rd;
    NSDecimalRound(&rd, &d, places, mode);
    return [NSDecimalNumber decimalNumberWithDecimal:rd];
}

this is the closest to what I had it mind, thank you – C0D3LIC1OU5 Aug 17 '17 at 14:33 — C0D3LIC1OU5, Aug 17 '17 at 14:33

score -1 · Answer 2 · answered Aug 15 '17 at 23:48

Another hacky way is:

Convert your number to round after multiplied it by 100.

int yourNum = 0.12345 * 100; yourNum now is 12.

Then whenever you need the number just divide by 100.

float yourNumF = yourNum / 100.0;

Not a fan of this method myself but it fits your requirement. :D

Round NSNumber to a specific number of significant digits

2 Answers2