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I'm looking for the most efficient and memory friendly way.
Let's say I have an array of Person objects. Each person has a hair color represented by an NSString. Let's then say I want to remove all Person objects from the array where their hair color is brown.
How do I do this?
Keep in mind that you cannot remove an object from an array that is being enumerated upon.
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We can use the following JavaScript methods to remove an array element by its value. indexOf() – function is used to find array index number of given value. Return negavie number if the matching element not found. splice() function is used to delete a particular index value and return updated array.
To remove an element from an array by ID in JavaScript, use the findIndex() method to find the index of the object with the ID in the array. Then call the splice() method on the array, passing this index and 1 as arguments to remove the object from the array.
Java arrays do not provide a direct remove method to remove an element. In fact, we have already discussed that arrays in Java are static so the size of the arrays cannot change once they are instantiated. Thus we cannot delete an element and reduce the array size.
Your answer You can use the pop() method to remove specific elements of a list. pop() method takes the index value as a parameter and removes the element at the specified index. Therefore, a[2] contains 3 and pop() removes and returns the same as output.
There are two general approaches. We can test each element and then immediately remove the element if it meets the test criteria, or we can test each element and store the indexes of elements meeting the test criteria, and then remove all such elements at once. With memory usage a real concern, the storage requirements for the latter approach may render it undesirable.
With the "store all indexes to remove, then remove them" approach off the table, we need to consider the details involved in the former approach, and how they will effect the correctness and speed of the approach. There are two fatal errors waiting in this approach. The first is to remove the evaluated object not based on its index in the array, but rather with the removeObject: method. removeObject: does a linear search of the array to find the object to remove. With a large, unsorted data set, this will destroy our performance as the time increases with the square of the input size. By the way, using indexOfObject: and then removeObjectAtIndex: is just as bad, so we should avoid it also. The second fatal error would be starting our iteration at index 0. NSMutableArray rearranges the indexes after adding or removing an object, so if we start with index 0, we'll be guaranteed an index out of bounds exception if even one object is removed during the iteration. So, we have to start from the back of the array, and only remove objects that have lower indexes than every index we've checked so far.
Having laid that out, there's really two obvious choices: a for loop that starts at the end rather than the beginning of the array, or the NSArray method enumerateObjectsWithOptions:usingBlock: method. Examples of each follow:
[persons enumerateObjectsWithOptions:NSEnumerationReverse usingBlock:^(Person *p, NSUInteger index, BOOL *stop) { if ([p.hairColor isEqualToString:@"brown"]) { [persons removeObjectAtIndex:index]; } }]; NSInteger count = [persons count]; for (NSInteger index = (count - 1); index >= 0; index--) { Person *p = persons[index]; if ([p.hairColor isEqualToString:@"brown"]) { [persons removeObjectAtIndex:index]; } } My tests seem to show the for loop marginally faster - maybe about a quarter second faster for 500,000 elements, which is a difference between 8.5 and 8.25 seconds, basically. So I would suggest using the block approach, as it's safer and feels more idiomatic.
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