dynamic string format with NSString stringWithFormat

Question

I\'ve been trying to make a dynamic string format so that through user options precision or padding could be somewhat user defined.

An example includes, the padding of

Answer 1

Use two steps:

    NSString* hourString=nil;
    if(...){
         hourString=[NSString stringWithFormat:@"%i", hour];
    }else{
         hourString=[NSString stringWithFormat:@"%02i", hour];
    }
    NSString* result=[NSString stringWithFormat:@"Hour: %@", hourString];

The following is considered a bad practice:

    NSString* formatString=nil;
    if(...){
         formatString=@"Hour: %i";
    }else{
         formatString=@"Hour: %02i";
    }
    NSString* result=[NSString stringWithFormat:formatString, hour];

because it's very dangerous to have a variable formatString, which can be used in many types of cracker attacks.

Answer 2

I think you could build up your format in an entirely separate stage, then use that. Also, don't forget to escape one of the %'s to make it literal.

NSString *format = [NSString stringWithFormat:@"%%%@i", padding];
NSString *result = [NSString stringWithFormat:@"Hour: %@", format]; 

update: oops maddy's answer pointed out a problem with mine: to escape a %, you should use %%, not \%. answer updated.

Answer 3

There's a better way to do this.

... stringWithFormat:@"Hour: %0*i", length, hour]; // note the asterisk

where length is the number of digits you want. Use 1 to get no leading zeros, use 2 to get a length of 2 with leading zeros as needed.

FYI - to solve what you originally tried you need to do it in two steps:

NSString *dynFmt = [NSString stringWithFormat:@"Hour: %%%@i", padding];
NSString *res = [NSString stringWithFormat:dynFmt, hour];