Question

I’m new to xcode and am stuck on this one action.
I’m trying to add a “Negative” sign or “-” to a field when a button is clicked.

The following code works when someone enters a number into the field… it will add or remove a negative sign to it.

However, if the field is blank and you click on the button it throws an error.

Here’s the code:

- (IBAction)fPosNeg:(id)sender {
    NSMutableString *str = [userFahrenheit.text mutableCopy];
    char iChar = [str characterAtIndex:0];
    if (iChar == '-') {
        userFahrenheit.text = [userFahrenheit.text substringFromIndex:1];
    } else if (iChar != '-') {
        [str insertString:@"-" atIndex:0];
        userFahrenheit.text = str;
    } else {
        userFahrenheit.text = [NSString stringWithFormat:@"-"];
    }
}

Here’s the error:

Terminating app due to uncaught exception ‘NSRangeException’, reason:
‘-[__NSCFString characterAtIndex:]: Range or index out of bounds’

Answer 1

You need to put a check before calling this -> [str characterAtIndex:0];

check will be

     if(![str isEqualToString:@""]) // so that if string is blank, you cant access its character at index 0