CODEWITHSUNDEEP
objective-c
Hassy
Hassy

Reputation: 5228

Adding Float Causes wrong answer

I am adding two floats like:

NSString *val1=@"355.00";
NSString *val2=@"55555550.00";

NSString *result=[NSString stringWithFormat:@"%.2f",[val1 floatValue]+[val2 floatValue]];

The answer i am getting is 55555908.00 while on calculator it is 55555905.00

What i am doing wrong?

Upvotes: 1

Views: 968

Answers (2)

Martin R
Martin R

Reputation: 540125

As already stated in the comments, this is a floating point precision problem. A float has a precision of about 7 decimal digits, a double about 15 decimal digits.

The Foundation framework also provides the NSDecimalNumber class for doing base-10 arithmetic, with a precision of 38 decimal digits:

NSString *val1 = @"355.00";
NSString *val2 = @"55555550.00";
NSDecimalNumber *dec1 = [NSDecimalNumber decimalNumberWithString:val1];
NSDecimalNumber *dec2 = [NSDecimalNumber decimalNumberWithString:val2];
NSDecimalNumber *decsum = [dec1 decimalNumberByAdding:dec2];
NSString *result = [decsum stringValue];
// --> 55555905

Upvotes: 5

Oliver Atkinson
Oliver Atkinson

Reputation: 8029

See: float vs. double precision

Floating point numbers in C use IEEE 754 encoding.

This type of encoding uses a sign, a significand, and an exponent.

Because of this encoding, you can never guarantee that you will not have a change in your value.

Also, the number of significant digits can change slightly since it is a binary representation, not a decimal one.

Single precision (float) gives you 23 bits of significand, 8 bits of exponent, and 1 sign bit.

Double precision (double) gives you 52 bits of significand, 11 bits of exponent, and 1 sign bit.

Upvotes: 2

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