Objective C ceil returns wrong value

Question

NSLog(@"CEIL %f",ceil(2/3));

should return 1. However, it shows:

CEIL 0.000000

Why and how to fix that problem? I use ceil([myNSArray count]/3) and it returns 0 when array count is 2.

Answer 1

The same rules as C apply: 2 and 3 are ints, so 2/3 is an integer divide. Integer division truncates so 2/3 produces the integer 0. That integer 0 will then be cast to a double precision float for the call to ceil, but ceil(0) is 0.

Changing the code to:

NSLog(@"CEIL %f",ceil(2.0/3.0));

Will display the result you're expecting. Adding the decimal point causes the constants to be recognised as double precision floating point numbers (and 2.0f is how you'd type a single precision floating point number).

Maudicus' solution works because (float)2/3 casts the integer 2 to a float and C's promotion rules mean that it'll promote the denominator to floating point in order to divide a floating point number by an integer, giving a floating point result.

So, your current statement ceil([myNSArray count]/3) should be changed to either:

([myNSArray count] + 2)/3           // no floating point involved

Or:

ceil((float)[myNSArray count]/3)    // arguably more explicit

Answer 2

2/3 evaluates to 0 unless you cast it to a float. So, you have to be careful with your values being turned to int's before you want.

float decValue = (float) 2/3;
NSLog(@"CEIL %f",ceil(decValue));

==>

 CEIL 1.000000

For you array example

float decValue = (float) [myNSArray count]/3;
NSLog(@"CEIL %f",ceil(decValue));