Rounding a float number in objective-c

Question

I want to know if there is a simple function that I can use such this sample. I have a

float value = 1.12345;

I want to round it with calling something like

float value2 = [roundFloat value:value decimal:3];
NSLog(@"value2 = %f", value2);

And I get "1.123" Is there any Library or default function for that or I should write a code block for this type of calculations?

thank for your help in advance

Answer 1

Using NSLog(@"%f", theFloat) always outputs six decimals, for example:

float theFloat = 1;
NSLog(@"%f",theFloat);

Output:

1.000000

In other words, you will never get 1.123 by using NSLog(@"%f", theFloat).

Cut-off after three decimals:

float theFloat = 1.23456;
float newFLoat = (int)(theFloat * 1000.0) / 1000.0;
NSLog(@"%f",newFLoat);

Output:

1.234000

Round to three decimals (using roundf() / lroundf() / ceil() / floor()):

float theFloat = 1.23456;
float newFLoat = (int)(roundf(theFloat * 1000.0)) / 1000.0;
NSLog(@"%f",newFLoat);

Output:

1.235000

Round to three decimals (dirty way):

float theFloat = 1.23456;
NSString *theString = [NSString stringWithFormat:@"%.3f", theFloat];
float newFloat = [theString floatValue];
NSLog(@"%@",theString);
NSLog(@"%f",newFloat);

Output:

1.235
1.235000

Answer 2

For printing the value use:

NSLog(@"value2 = %.3f", value2);

Rounding to 3 decimal digits before calculations doesn't really make sense because float is not a precise number. Even if you round it to 1.123, it will be something like 1.122999999998.

Rules:

1. Usually you round up only to print the result - string formatter can handle it (see above).
2. For precise calculations (e.g. currency), don't use floating point, use NSDecimalNumber or fixed point arithmetics.

Answer 3

Floating point numbers don't have decimal places, they have binary places. Decimal-radix numbers have decimal places. You can't round floating point numbers to specific numbers of decimal places unless you convert to a decimal radix. No routine, method, function etc., that returns a floating point value can possibly carry out this task.