Question

I have a string with formatting tags in it, such as There are {adults} adults, and {children} children. I have a dictionary which has “adults” and “children” as keys, and I need to look up the value and replace the macros with that value. This is fully dynamic; the keys could be anything (so I can’t hardcode a stringByReplacingString).

In the past, I’ve done similar things before just by looping through a mutable string, and searching for the characters; removing what I’ve already searched for from the source string as I go. It seems like this is exactly the type of thing NSScanner is designed for, so I tried this:

NSScanner *scanner = [NSScanner scannerWithString:format];
NSString *foundString;
scanner.charactersToBeSkipped = nil;

NSMutableString *formatedResponse = [NSMutableString string];

while ([scanner scanUpToString:@"{" intoString:&foundString]) {
    [formatedResponse appendString:[foundString stringByReplacingOccurrencesOfString:@"{" withString:@""]]; //Formatted string contains everything up to the {

    [scanner scanUpToString:@"}" intoString:&foundString];

    NSString *key = [foundString stringByReplacingOccurrencesOfString:@"}" withString:@""];

    [formatedResponse appendString:[data objectForKey:key]];

}

NSRange range = [format rangeOfString:@"}" options:NSBackwardsSearch];

if (range.location != NSNotFound) {
    [formatedResponse appendString:[format substringFromIndex:range.location + 1]];
}

The problem with this is that when my string starts with “{“, then the scanner returns NO, instead of YES. (Which is what the documentation says should happen). So am I misusing NSScanner? The fact that scanUpToString doesn’t include the string that was being searched for as part of its output seems to make it almost useless…

Can this be easily changed to do what I want, or do I need to re-write using a mutable string and searching for the characters manually?

Answer 1

Use isAtEnd to determine when to stop. Also, the { and } are not included in the result of scanUpToString:, so they will be at the beginning of the next string, but the append after the loop is not necessary since the scanner will return scanned content even if the search string is not found.

// Prevent scanner from ignoring whitespace between formats. For example, without this, "{a} {b}" and "{a}{b}" and "{a}     
//{b}" are all equivalent
[scanner setCharactersToBeSkipped:[NSCharacterSet characterSetWithCharactersInString:@""]];
while(![scanner isAtEnd]) {
    if([scanner scanUpToString:@"{" intoString:&foundString]) {
        [formattedResponse appendString:foundString];
    }
    if(![scanner isAtEnd]) {
        [scanner scanString:@"{" intoString:nil];
        foundString = @""; // scanUpToString doesn't modify foundString if no characters are scanned
        [scanner scanUpToString:@"}" intoString:&foundString];
        [formattedResponse appendString:[data objectForKey:foundString];
        [scanner scanString:@"}"];
    }
}