NSPredicate instead of loop to filter an array of objects - objective-c

Question

I have been told that I can use NSPredicate to duplicate the results of this method
- (void) clearArrayOut
{
bool goAgain = false;
for (int j=0; j<[array count]; j++)
{
if ([[array objectAtIndex:j] someMethod] == NO)
{
[array removeObjectAtIndex:j];
goAgain = true;
break;
}
}
if (goAgain) [self clearArrayOut];
}
How can I make an NSPredicate that will filter an array based on the results of some method of a custom class's call?

Answer 1

To make a copy with the filter applied:
NSArray *filteredArray = [someArray filteredArrayUsingPredicate:
[NSPredicate predicateWithBlock:^(id object, NSDictionary *bindings) {
return [object someMethod]; // if someMethod returns YES, the object is kept
}]];
To filter an NSMutableArray in place:
[someMutableArray filterUsingPredicate:
[NSPredicate predicateWithBlock:^(id object, NSDictionary *bindings) {
return [object someMethod]; // if someMethod returns YES, the object is kept
}]];
But I would probably just use a for loop if I were filtering a small array. However, I'd write my for loop a little differently to avoid having to either decrement the index variable or call myself recursively:
- (void)clearArrayOut:(NSMutableArray *)array {
for (int i = array.count - 1; i >= 0; --i) {
if (![[array objectAtIndex:i] someMethod]) {
[array removeObjectAtIndex:i];
}
}
}

Answer 2

You simply write it into your predicate, for example, lets assume you have an object with a method called isOdd and you want to filter your array to include only objects that return true for isOdd, you can do this:
#import <Foundation/Foundation.h>
#interface barfoo : NSObject
{
int number;
}
- (BOOL)isOdd;
- (id)initWithNumber:(int)number;
#end
#implementation barfoo
- (NSString *)description
{
return [NSString stringWithFormat:#"%i", number];
}
- (BOOL)isOdd
{
return (number % 2);
}
- (id)initWithNumber:(int)tnumber
{
if((self = [super init]))
{
number = tnumber;
}
return self;
}
#end
int main(int argc, char *argv[]) {
#autoreleasepool {
NSMutableArray *array = [NSMutableArray array];
for(int i=0; i<10; i++)
{
barfoo *foo = [[barfoo alloc] initWithNumber:i];
[array addObject:[foo autorelease]];
}
NSLog(#"%#", array); // prints 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"isOdd == true"]; // This is oure predicate. isOdd must be true for objects to pass
NSArray *result = [array filteredArrayUsingPredicate:predicate];
NSLog(#"%#", result);
}
}
Of course this also works the other way around, your predicate could also read isOdd == false or you can add even more requirements for an object to pass. Eg isOdd == true AND foo == bar. You can read more about the NSPredicate syntax in the NSPredicate documentation.

Answer 3

Your implementation is terribly inefficient to start with: rather than continuing the deletions recursively, you could change your loop to not advance if an object has been deleted, like this:
- (void) clearArrayOut {
int j = 0;
while (j < [array count]) {
if ([[array objectAtIndex:j] someMethod] == NO) {
[array removeObjectAtIndex:j];
} else {
j++;
}
}
}
You could do the same thing using filterUsingPredicate:, like this:
- (void) clearArrayOut {
NSPredicate *p = [NSPredicate predicateWithBlock:^BOOL(id obj, NSDictionary *bindings) {
return [obj someMethod] == NO
}];
[array filterUsingPredicate:p];
}