Reverse Groups challenge at CodeEval.com

Question

I am working through challenges at CodeEval.com and wanted to get your input on my code for this challenge.

Given a list of numbers and a positive integer k, reverse the elements of the list, k items at a time. If the number of elements is not a multiple of k, then the remaining items in the end should be left as is.

Input sample:

Your program should accept as its first argument a path to a filename. Each line in this file contains a list of numbers and the number k, separated by a semicolon. The list of numbers are comma delimited.

E.g. input:

1,2,3,4,5;2
1,2,3,4,5;3

Output sample:

Print out the new comma separated list of numbers obtained after reversing. E.g.

2,1,4,3,5
3,2,1,4,5

-(void)reversedGroups
{
 NSString *line = @"22,23,24,25,26,27,28,29,30;3"; //input sample
 NSArray *lineArray = [line componentsSeparatedByString:@";"];
 NSUInteger numbElements = [[lineArray lastObject] intValue];
 NSArray *elementsArray = [[lineArray objectAtIndex:0] componentsSeparatedByString:@","];
 NSMutableArray *resultLineArray = [[NSMutableArray alloc]init];
 NSUInteger pos = 0;
 while (pos<[elementsArray count]) {
 //itterating through the elements
 if (pos+numbElements <= [elementsArray count]) {
 //make sure that i have enough elements to reverse
 for (NSUInteger i = pos+numbElements; i>pos; i--) {
 //add elements in reverse to the result array
 [resultLineArray addObject:[elementsArray objectAtIndex:i-1]];
 }
 }else {
 for (NSUInteger j = pos; j<[elementsArray count]; j++) {
 //add elements that are left to the result array
 [resultLineArray addObject:[elementsArray objectAtIndex:j]];
 }
 }
 pos += numbElements; // move the postion by number of elements to reverse
 }
 //convert results array to string and add comma after each element as required for output
 NSString *resultString = [[resultLineArray valueForKey:@"description"] componentsJoinedByString:@","];
 NSLog(@"%@",resultString);
}

Answer 1

It's pretty simple task and I think code does its job well. I would point couple minor style issues:

• You don't need to use objectAtIndex method to access elements inside an array. elementsArray[i] is enough and it's more readable.

• You can access count property using dot-notation - array.count. Again, it's more readable.

• Combining result string from array looks cumbersome and complicated. And rely on description property. Why don't you create result string as you go without having to create temp array first?

• You are dividing line into lineArray. Then you're accessing first and last element using lastObject and objectAtIndex:0. I would prefer to use either [0] and [1] or lastObject and firstObject.

• Pass the code through standard Xcode formatter to clean up spacing and indentation.

• Yan: Thank you for the reply. I would use elementsArray[i] to access the elements but codeeval uses objc 2.0 and it returns an error. I thought combining the result would be easier later to print the words with comma. Next i would use a NSMutableString and just append in the code.
• nhgrif: firstObject and lastObject should always be what you use when you can.