For floating point number formatting you can use %f, and you have to specify the minimum field width and the precision in terms of characters. If you use %4.2f, means four characters wide minimum (if less than 4 are used, will be filled with blank spaces at left) and 2 characters for precision. If you want 10 characters of minimum wide and 0 of decimal precision, you use %10.0f .
In your particular case, you should use %.0f for no digits of decimal precision.
buttonScreen.text = [NSString stringWithFormat:@"%.0f",currentNumber];
You can find a quick reference here . Is not the objective-c documentation, but is equivalent.
For the sake of completeness, you can find the documentation of the specification that NSString formatting follows in:
and here:
http://pubs.opengroup.org/onlinepubs/009695399/functions/printf.html
EDIT for truncation:
buttonScreen.text = [NSString stringWithFormat:@"%d",(int)currentNumber];
But, you should read a little about floating representation, precision and casting to be aware of possible undesired behavior.