Question

I am trying to write a generic method for String conversion (purpose being I am writing a parser for a RESTful API).

The message aims to convert strings as follows

creationTSZ -> creation_tsz

userId -> user_id

The message handles converting userId -> user_id, currently inefficiently looping through the string and change parts.

It doesn’t yet handle creationTSZ -> creation_tsz, I think looping any further is very inefficient, and am wondering if there’s a better way to do this?

Possibly Regex?

-(NSString *)fieldsQueryString 
{

    NSArray *fieldNames = [self fieldList];

    /* Final composed string sent to Etsy */
    NSMutableString *fieldString = [[[NSMutableString alloc] init] autorelease];

    /* Characters that we replace with _lowerCase */
    NSArray *replaceableChars = [NSArray arrayWithObjects:
                                 @"Q", @"W", @"E", @"R", @"T", @"Y", @"U", @"I", @"O", @"P", 
                                 @"A", @"S", @"D", @"F", @"G", @"H", @"J", @"K", @"L",
                                 @"Z", @"X", @"C", @"V", @"B", @"N", @"M", nil];

    /* Reusable pointer for string replacements */
    NSMutableString *fieldNameString = nil;

    /* Loop through the array returned by the filter and change the names */
    for(NSString *fieldName in fieldNames) {
        /* Loop if the field is to be omited */
        if ([[self valueForKey:fieldName] boolValue] == NO) continue;
        /* Otherwise change the name to a field and add it */
        fieldNameString = [fieldName mutableCopy];
        for(NSString *replaceableChar in replaceableChars) {
            [fieldNameString replaceOccurrencesOfString:replaceableChar 
                                             withString:[NSString stringWithFormat:@"_%@", [replaceableChar lowercaseString]] 
                                                options:0 
                                                  range:NSMakeRange(0, [fieldNameString length])];
        }
        [fieldString appendFormat:@"%@,", fieldNameString];
        [fieldNameString release];
    }
    fieldNames = nil;

    /* Return the string without the last comma */
    return [fieldString substringToIndex:[fieldString length] - 1];
}

Answer 1

Assuming that your identifiers are structured like

<lowercase-prefix><Uppercase-char-and-remainder>

you could use:

NSScaner *scanner = [NSScanner scannerWithString:fieldName];
NSString *prefix = nil;
[scanner scanCharactersFromSet:[NSCharacterSet lowercaseLetterCharacterSet] intoString:&prefix];
NSString *suffix = nil;
[scanner scanCharactersFromSet:[NSCharacterSet letterCharacterSet] intoString:&suffix];
NSString *fieldNameString = [NSString stringWithFormat:@"%@_%@", prefix, [suffix lowercaseString]];

This would perform the conversion of field identifiers (but you should perform some error checking in case either prefix or suffix stay nil).

The easiest way to build the list of fieldNames would be add them to a NSMutableArray, and to then join them:

NSMutableArray *fields = [[NSMutableArray alloc] init];
for (NSString *fieldName in [self fieldList]) {
    // code as above
    [fields add:fieldNameString];
}
NSString *commaFields = [fields componentsJoinedByString:@","];
[fields release];
return commaFields;