Unless in Objective-C

In Objective-C, unless can be used as a keyword to specify a code block that should be executed only if a certain condition is not met. The syntax for using unless is similar to that of if, with the only difference being the keyword used.

Here are some examples:

1. unless to check if a number is negative:

int num = -5;
unless (num < 0) {
    NSLog(@"The number is positive or zero");
}

This will not execute because num is negative.

2. unless to check if an array is empty:

NSArray *myArray = @[];
unless ([myArray count] == 0) {
    NSLog(@"The array is not empty");
}

This will not execute because myArray is empty.

3. unless to check if a string is not nil:

NSString *myString = nil;
unless (myString != nil) {
    NSLog(@"The string is nil");
}

This will execute because myString is nil.

Note that in Objective-C, unless is not a built-in keyword like if. It is actually defined as a preprocessor macro:

#define unless(x) if(!(x))

This macro simply negates the expression inside the parentheses and provides it to the if statement.