Question

I have an issue that (I think) might have to do with scope, but I’m not sure. I’m trying to do something that I think should be simple, but I am getting a strange result, and I could truly use some advice. I would say I’m an early-objective-c programmer, but not a complete newb.

I have written a function in objective-c that I would like to use to change the key-names in a mutable array of mutable dictionary objects. So, I want to pass in a mutable array of mutable dictionary objects, and return the same mutable array with the same dictionary objects, but with some of the key-names changed. Make sense?

I have tried several log statements in this code, which seem to indicate that everything I’m doing is working, except when the for loop is finished executing (when I try to test the values in the temp array), the array appears to contain only the LAST element in the source array, repeated [source count] times. Normally, this would lead me to believe I’m not writing the new values correctly, or not reading them correctly, or even that my NSLog statements aren’t showing me what I think they are. But might this be because of scope? Does the array not retain its changes outside of the for loop?

I have put a fair amount of time into this function, and I have exhausted my bag of tricks. Can anyone help out?

-(NSMutableArray *)renameKeysIn:(NSMutableArray*)source {
/*
// Pre:
// The source array is an array of dictionary items.
// This method renames some of the keys in the dictionary elements, to make sorting easier later. 
// - "source" is input, method returns a mutable array
 */

// copy of the source array
NSMutableArray *temp = [source mutableCopy];

// a temporary dictionary object:
NSMutableDictionary * dict = [[NSMutableDictionary alloc] init];

// These arrays are the old field names and the new names
NSMutableArray *originalField = [NSMutableArray arrayWithObjects:@"text", @"created_at",nil];
NSMutableArray *replacedField = [NSMutableArray arrayWithObjects:@"title", @"pubDate", nil];

// loop through the whole array
for (int x =0; x<[temp count]; x++) {
    // set the temp dictionary to current element
    [dict setDictionary:[temp objectAtIndex:x]]; 

    // loop through the number of keys (fields) we want to replace (created_at, text)... defined in the "originalField" array 
    for (int i=0; i<[originalField count]; i++)
    {   
            // look through the NSDictionary item (fields in the key list)
            // if a key name in the dictionary matches one of the ones to be replaced, then replace it with the new one
            if ([dict objectForKey:[originalField objectAtIndex:i]] != nil) {
                // add a new key/val pair: the new key *name*, and the old key *value* 
                [dict setObject:[dict objectForKey:[originalField objectAtIndex:i]] 
                         forKey:[replacedField objectAtIndex:i]];
                // remove the old key/value pair
                [dict removeObjectForKey:[originalField objectAtIndex:i]];
            }// end if dictionary item not null

    }// end loop through keys (created_at, text)

    [temp replaceObjectAtIndex:x withObject:dict];

}// end loop through array

// check array contents
for (int a=0; a<[temp count]; a++){
    NSLog(@"Temp contents: ############ %@",[[temp objectAtIndex:a] objectForKey:@"pubDate"]);
}

return temp;    
} // end METHOD

Answer 1

I think the issue is on the line with:

[dict setDictionary:[temp objectAtIndex:x]];

Since these things are almost all working in pointers (instead of copying contents), every element of your temp array will point to the dict dictionary, which is set to be whatever the latest key’s dictionary is. I think setting the actual pointer will fix the issue.

dict = [temp objectAtIndex:x];