I'm using NSScanner to restrict a text field's value to numbers only, but I want to allow "-"

Question

I have an NSScanner in a custom NSNumberFormatter that scans for non-int values but I want it to skip "-" (dash)

- (BOOL)isPartialStringValid:(NSString *)partialString newEditingString:(NSString **)newString errorDescription:(NSString **)error {
    if ([partialString length] == 0) {
        return YES;
    }
    
    NSScanner *scanner = [NSScanner scannerWithString:partialString];
    
    if (!([scanner scanInt:0] && [scanner isAtEnd])) {
        return NO;
    }
    
    return YES;
}

I thought I had to use [scanner charactersToBeSkipped] but I don't know how that works

But why? Is it because the string is valid only if it is an integer, and an integer can be negative? In that case, might it be that NSScanner is not the best way to do this? — matt, Oct 04 '20 at 23:50
Yeah that's why. What do you suggest I do instead of using NSScanner? — YaYPIXXO, Oct 05 '20 at 00:58
See [How to check is a string or number](https://stackoverflow.com/a/38481180) — Willeke, Oct 05 '20 at 07:28
What about using `NSNumberFormatter` to validate the text field's contents? — sbooth, Oct 05 '20 at 15:40

YaYPIXXO · Answer 1 · 2020-10-05T13:42:20.697

I came up with my own solution. Each time a character is typed, it checks if the first one is a dash. If it is, it skips the first character and checks if the rest of the string contains ints. If the first character is not a dash, it scans the whole string for ints.

- (BOOL)isPartialStringValid:(NSString *)partialString newEditingString:(NSString **)newString errorDescription:(NSString **)error {
    if ([partialString length] == 0) {
        return YES;
    }
    
    NSString *stringFromSecondChar;
    
    NSString *firstChar = [partialString substringToIndex:1];
    
    if ([partialString length] > 1 && [firstChar isEqualToString:@"-"]) {
        stringFromSecondChar = [partialString substringFromIndex:1];
    }
    else {
        stringFromSecondChar = partialString;
    }
    
    NSScanner *scanner = [NSScanner scannerWithString:stringFromSecondChar];
    
    if ([partialString length] == 1 && [firstChar isEqualToString:@"-"]) {
        return YES;
    }
    else if ([partialString length] >= 1 && [stringFromSecondChar containsString:@"-"]) {
        return NO;
    }
    
    if ([firstChar isEqualToString:@"-"]) {
        if (!([scanner scanInt:0] && [scanner isAtEnd])) {
            return NO;
        }
        else {
            return YES;
        }
    }
    
    if (!([scanner scanInt:0] && [scanner isAtEnd])) {
        return NO;
    }
    
    return YES;
}

Willeke · Answer 2 · 2020-10-08T08:57:44.427

Solution with regular expression:

- (BOOL)isPartialStringValid:(NSString *)partialString newEditingString:(NSString **)newString errorDescription:(NSString **)error {
    if (partialString.length == 0)
        return YES;
    NSRange range = [partialString rangeOfString:@"^[-]?\\d*$" options:NSRegularExpressionSearch];
    return (range.location != NSNotFound);
}

Solution with NSScanner:

- (BOOL)isPartialStringValid:(NSString *)partialString newEditingString:(NSString **)newString errorDescription:(NSString **)error {
    if (partialString.length == 0 || [partialString isEqualToString:@"-"])
        return YES;
    NSScanner *scanner = [NSScanner scannerWithString:partialString];
    return ([scanner scanInt:NULL] && [scanner isAtEnd]);
}

I'm using NSScanner to restrict a text field's value to numbers only, but I want to allow "-"

2 Answers2