Stop Float value rounding off in objective c - ios

Question

I have to prevent the value from rounding off after the decimal.
Here is the code I use :
NSNumberFormatter* nf = [[[NSNumberFormatter alloc] init] autorelease];
nf.positiveFormat = #"0.###";
NSString* trimmedValue = [nf stringFromNumber: [NSNumber numberWithFloat:[exRateLabel doubleValue]*amount]];
trimmedValue = [trimmedValue substringToIndex:[trimmedValue length]-1];
In this case, if I multiply 1000 * 50.1234 I'm getting 50123.3984, but it should be 50123.4.
NSLog(#".2f",50.1234*1000);
For this case it's showing the correct value but for
NSLog(#".2f",50.1234*123);
it is rounding off the actual value, which is 6165.1782, to 6165.18.

Answer 1

Just use double instead of float and a proper rounding rule:
NSNumberFormatter* nf = [[NSNumberFormatter alloc] init];
nf.positiveFormat = #"0.###";
nf.roundingMode = NSNumberFormatterRoundFloor;
NSString* trimmedValue = [nf stringFromNumber: [NSNumber numberWithDouble:50.1234*123]];
NSLog(#"trimmedValue: %#", trimmedValue);
And the result is:
trimmedValue: 6165.178

Answer 2

If the mathematics is required to be precise, I'd suggest using an NSDecimalNumber.
NSDecimalNumber *myNumber = [[NSDecimalNumber alloc] initWithMantissa:501234 exponent:-4 isNegative:NO]];
NSDecimalNumber *answer = [self multiplyDecimalNumber:myNumber with:[NSDecimalNumber decimalNumberWithMantissa:1000 exponent:0 isNegative:NO];
NSLog(#"Answer: %g", floor([answer doubleValue]));
I made a quick wrapper for the multiplication, I never wanted an exception, your needs may be different:
-(NSDecimalNumber *) multiplyDecimalNumber:(NSDecimalNumber *) lhs with:(NSDecimalNumber *) rhs {
NSDecimalNumberHandler *handler = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundPlain scale:NSDecimalNoScale raiseOnExactness:NO raiseOnOverflow:NO raiseOnUnderflow:NO raiseOnDivideByZero:NO];
return [lhs decimalNumberByMultiplyingBy:rhs withBehavior:handler];
}

Answer 3

They are both correct.
First, I believe you have a typo - you're missing the %.
NSLog(#"%.2f", 50.1234*1000); // same as #"%.2f", 50,123.4 = #"50123.4"
the math leaves you with a clean 1/10th, even though you are asking for 2 decimal places to be printed.
NSLog(#"%.2f",50.1234*123); // same as #"%.2f", 5,161.1782 = #"5161.18"
you are asking for two decimal numbers and rounding up is the default behavior.
It sounds like the formatting you actually want to use is:
NSLog(#"%.1f", number);
or if you want a forced, zero-padded two digits, use
NSLog(#"%.02", number); // first case would come out #"50123.40"
this will force all trailing zeros to be printed

Answer 4

you are probably using a 'double' representation, cast the number to a float or you can use your own policy for truncation using functions like:
float roundUp (float value, int digits) {
int mod = pow(10, digits);
float roundedUp = value;
if (mod != 0) {
roundedUp = ceilf(value * mod) / mod;
}
return roundedUp;
}
float roundDown (float value, int digits) {
int mod = pow(10, digits);
float roundedDown = value;
if (mod != 0) {
roundedDown = floorf(value * mod) / mod;
}
return roundedDown;
}
float nearestf (float value, int digits) {
int mod = pow(10, digits);
float nearest = value;
if (mod != 0) {
nearest = floorf(value*mod + 0.5) / mod;
}
return nearest;
}
'roundUp' should be the one you need, or try with the more generic 'nearestf'

Answer 5

For me it was requirement to show atleast two decimal and maximum decimal number.
Following code did worked for me.
NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
[numberFormatter setNumberStyle:NSNumberFormatterDecimalStyle];
[numberFormatter setMaximumFractionDigits:2];
[numberFormatter setMinimumFractionDigits:2];
[numberFormatter setRoundingMode:NSNumberFormatterRoundDown];
NSString *num = #"123456.019";
NSString *stringFromNumber = [numberFormatter stringFromNumber:[numberFormatter numberFromString:num]];
NSLog(#"check this %#", stringFromNumber);