Replace occurrences in NSString with those in an NSArray

Question

I'm looking for a quick way to debug my SQLite queries in an iOS app. I have something like this:

NSString *sql = [NSString stringWithString: @"SELECT * FROM table WHERE foo = ? AND bar = ?"]; 
NSArray *params = [NSArray arrayWithObjects: @"baz", @"bat", nil];
NSLog(@"%@ %@", sql, params);

I'd like to know if there is a one or two-liner to replace the question marks with the params to make the output more readable. I'm not looking for something that outputs valid sql but rather something easier to read than my current NSLog.

warrenm · Accepted Answer · 2012-07-23 17:49:26Z

1

@interface NSString (SQLQueryDebugging)
- (NSString *)stringBySubstitutingParameters:(NSArray *)params forInstancesOfPlaceholder:(NSString *)placeholder;
@end

@implementation NSString (SQLQueryDebugging)

- (NSString *)stringBySubstitutingParameters:(NSArray *)params forInstancesOfPlaceholder:(NSString *)placeholder
{
    NSString *composedQuery = self;
    NSRange substitutionRange = [composedQuery rangeOfString:placeholder];
    NSInteger parameterIndex = 0;
    while(substitutionRange.length != 0)
    {
        NSString *currentParam = [params objectAtIndex:parameterIndex];
        composedQuery = [composedQuery stringByReplacingCharactersInRange:substitutionRange withString:currentParam];
        ++parameterIndex;
        NSInteger lastSubstitutionIndex = substitutionRange.location + [currentParam length];
        NSRange searchRange = NSMakeRange(lastSubstitutionIndex, [composedQuery length] - lastSubstitutionIndex);
        substitutionRange = [composedQuery rangeOfString:placeholder options:0 range:searchRange];
    }

    return composedQuery;
}

Drop that in somewhere, then you can get what you want with:

NSString *completeSQL = [sql stringBySubstitutingParameters:params forInstancesOfPlaceholder:@"?"];

edited Jul 23, 2012 at 17:49

answered Jul 23, 2012 at 17:18

warrenm

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2 Comments

Julian Over a year ago

As a beginning Objective-C developer I haven't had the chance to really use Categories for much. This is a great example! I had to modify it a little bit to add a check for when currentParam is an NSNumber. Since this always expects a string, it will crash. Other than that, it's a perfect one-liner! Thanks!

warrenm Over a year ago

Glad this worked for you. I thought about the case where you might have non-string parameters and almost suggested that you use [currentParam description] instead of currentParam where appropriate. Glad you figured it out on your own :)

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