ios - What is the difference between newArray = array and newArray = [array copy]? -

i have lines of code:

- (void)redrawgraphwitharray:(nsarray *)array {     _graphview.graphpoints = [array copy];     [_graphview setneedsdisplay]; } 

graphpoints nsarray too.

if use

_graphview.graphpoints = array; 

the graph looks same , there's no problem.

i wanna know difference between

_graphview.graphpoints = [array copy] ,

_graphview.graphpoints = array;

thanks.

_graphview.graphpoints = array; assigns graphpoints same array original object. you're creating pointer exact same memory. instance, happen:

nsmutablearray *array = [[nsmutablearray alloc] init]; array[0] = @{@"x": @1, @"y": @2}; array[1] = @{@"x": @1, @"y": @3}; _graphview.graphpoints = array;  // let's make changes original array // note used nsmutablearray explain differences between // 2 assignment types. array[0] = @{@"x": @1, @"y": @1}; // nsarray wouldn't able array[1][@"y"] = @5;              // can still happen  // happens graphpoints? _graphview.graphpoints[0]; // <== {x: 1, y: 1} _graphview.graphpoints[1]; // <== {x: 1, y: 5} 

in case, graphpoints points exact same object array 2 remain same. it's not illustrated in code example, it's important remember "sameness" points in both directions, making changes graphpoints change array.

on other hand, [array copy] creates new array object copy of original, above code have different result:

nsmutablearray *array = [[nsmutablearray alloc] init]; array[0] = @{@"x": @1, @"y": @2}; array[1] = @{@"x": @1, @"y": @3}; _graphview.graphpoints = [array copy];  // let's make same changes original array array[0] = @{@"x": @1, @"y": @1}; array[1][@"y"] = @5;  // happens graphpoints? _graphview.graphpoints[0]; // <== {x: 1, y: 2} _graphview.graphpoints[1]; // <== {x: 1, y: 5} 

the first object didn't change in graphpoints, because wrote whole new object array. since graphpoints copy, didn't affected.

however, second object did change, because didn't write new object array, instead modified existing object, contained both arrays. illustrates important subtlety object copying. what's known "shallow" copy, means container gets copied, contents not. end 2 arrays, 1 set of contained objects.

there relatively straightforward ways copy of contained objects making sure contained objects implement nscopying protocol (if you're using foundation classes nsdictionary, nsarray, nsnumber, nsstring, don't need worry, done you) , using - initwitharray:copyitems: initializer on nsarray. create shallow copies of contained objects, there lots of info available how implement full "deep" copy, should need so.