NSArray category that extends it with abilities inspired by Microsoft LINQ. See Readme.md below. Update 13/1/13: Heavily modified as per @jamiepinkham's recommendations. Thanks!

Readme.md

NSArray category that extends it with abilities inspired by Microsoft LINQ

These methods allow you to:

• Take a subset of an array
• Take all the elements in an array that are of a certain class
• Get a singular item/property off each object in an array
• Get a subset (stored in a dictionary) of items off each object in an array
• Get the first object in an array that matches a predicate, or nil if none found.
• Get the last object in an array that matches a predicate, or nil if none found.

A couple examples:

NSArray *array = @[ @"Casey", @"Erin", @"Chris" ];

NSString *firstC = [array firstOrNil:[NSPredicate predicateWithFormat:@"SELF BEGINSWITH \"C\""]]; // firstC = @"Casey"

NSString *lastC = [array lastOrNil:[NSPredicate predicateWithBlock:^BOOL(id evaluatedObject, NSDictionary *bindings) {
  return [[evaluatedObject substringToIndex:1] isEqualToString:@"C"];
}]];  // lastC = @"Chris".  This could have been done using the same predicate as firstC; just showing another approach.

NSArray *selections = [array select:^id(id source) {
  return [source uppercaseString];
}];   // selections = @[ @"CASEY", @"ERIN", @"CHRIS" ]

arr = @[ @"Casey", @30, @"Erin", @29, @"Adam", @26 ];
NSArray *strings = [arr ofClass:[NSString class]];  // Strings = @[ @"Casey", @"Erin", @"Adam" ]
NSArray *numbers = [arr ofClass:[NSNumber class]];  // Numbers = @[ @30, @29, @26 ]

arr = // Some array of NSAttributedStrings
NSArray *values = [arr valueForKeys:@[ @"string", @"length" ]]
// values = @[  @{ @"length" : 5, @"string" : @"Casey" },
//              @{ @"length" : 4, @"string" : @"Erin", },
//              @{ @"length" : 5, @"string" : @"Chris" } ]  

NSArray+CLinq.h

//
//  NSArray+CLinq.h
//
//  Created by Casey Liss on 12/1/13.
//  Copyright (c) 2013 Casey Liss. All rights reserved.
//

#import <Foundation/Foundation.h>

typedef id(^CLSelector)(id source);

@interface NSArray (CLinq)

// Gets all the objects in the array that satisfy the predicate.
// Note you can use [NSPredicate predicateWithBlock:] to provide a block.
- (NSArray *)where:(NSPredicate *)predicate;

// Gets all objects in the array that are of the desired class
- (NSArray *)ofClass:(Class)desiredClass;

// Gets an item off each object in the array.
// For example, getting the length of every NSString in the array.
- (NSArray *)select:(CLSelector)selector;

// Gets multiple items off each object in the arrya.
// Provide it an array of NSStrings of selector names which take no parameters.
// The result is an array of NSDictionary's where the key is the name and the value is the value for that object.
- (NSArray *)valueForKeys:(NSArray *)namesOfItems;

// Gets the first object that matches the predicate, or nil if none found.
- (id)firstOrNil:(NSPredicate *)predicate;

// Gets the last object that matches the predicate, or nil if none found.
- (id)lastOrNil:(NSPredicate *)predicate;

@end

NSArray+CLinq.m

//
//  NSArray+CLinq.m
//
//  Created by Casey Liss on 12/1/13.
//  Copyright (c) 2013 Casey Liss. All rights reserved.
//

#import <objc/message.h>
#import "NSArray+CLinq.h"

@implementation NSArray (CLinq)

- (NSArray *)where:(NSPredicate *)predicate
{
    return [self filteredArrayUsingPredicate:predicate];
}

- (NSArray *)ofClass:(Class)desiredClass
{
    return [self filteredArrayUsingPredicate:[NSPredicate predicateWithBlock:^BOOL(id evaluatedObject, NSDictionary *bindings) {
        return [evaluatedObject isKindOfClass:desiredClass];
    }]];
}

- (NSArray *)select:(CLSelector)selector
{
    NSMutableArray *retVal = [[NSMutableArray alloc] initWithCapacity:self.count];
    NSEnumerator *en = [self objectEnumerator];
    
    id current = [en nextObject];
    while (current)
    {
        id selected = selector(current);
        if (selected)
        {
            [retVal addObject:selected];
        }
        current = [en nextObject];
    }
    
    return retVal;
}

- (NSArray *)valueForKeys:(NSArray *)keys
{
    NSMutableArray *retVal = [[NSMutableArray alloc] initWithCapacity:self.count];
    NSMutableDictionary *keysToArrays = [NSMutableDictionary dictionaryWithCapacity:keys.count];
    
    for (NSString *key in keys)
    {
        [keysToArrays setValue:[self valueForKey:key] forKey:key];
    }
    
    for (int i=0; i < self.count; ++i)
    {
        NSMutableDictionary *dict = [NSMutableDictionary dictionaryWithCapacity:keys.count];
        for (NSString *key in keys)
        {
            [dict setValue:keysToArrays[key][i] forKey:key];
        }
        
        [retVal addObject:dict];
    }
    
    return retVal;
}

- (id)firstOrNil:(NSPredicate *)predicate
{
    return [self processPredicate:predicate usingEnumerator:[self objectEnumerator]];
}

- (id)lastOrNil:(NSPredicate *)predicate
{
    return [self processPredicate:predicate usingEnumerator:[self reverseObjectEnumerator]];
}

#pragma mark - Private Methods

- (id)processPredicate:(NSPredicate *)predicate usingEnumerator:(NSEnumerator *)en
{
    id current = [en nextObject];
    while (current)
    {
        if ([predicate evaluateWithObject:current])
        {
            return current;
        }
        current = [en nextObject];
    }
    
    return nil;
}

@end