Using Block with objectAtIndex method of NSArray - iphone

Question

Here is the code:
TrailLayer * layer = (TrailLayer*)[_layers objectAtIndex:(int)^{
if (_segmentNumber < [_segmentArray count]) {
return _segmentNumber;
} else {
return _segmentNumber - 1;
}
}];
what is the problem here? Here, all the _Variables are IVARs. I am first time using Blocks, can someone help me identifying this problem. Its giving EXC_BAD_INSTRUCTION here.
Thanks.

Answer 1

Try this code, First write block to get the index number, and use that number to retrieve value from array
int (^segmentIndex)(int) = ^(int segmentNumber){
if (segmentNumber < [_layers count]) {
return segmentNumber;
} else {
return segmentNumber - 1;
}
};
NSLog(#"Trail Layer %#", [_layers objectAtIndex:segmentIndex(_segmentNumber)]);
TrailLayer * layer = (TrailLayer*)[_layers objectAtIndex:segmentIndex(_segmentNumber)];

Answer 2

A block is a runnable thing like a function, basically a piece of code that you can store, pass around, and run later (or not). You want to pass an integer to objectAtIndex:, so passing a block makes absolutely no sense whatsoever.
What you could have meant to do was run the block, and then pass its result to objectAtIndex:. In that case, you would have to run the block:
TrailLayer * layer = [_layers objectAtIndex:^{
if (_segmentNumber < [_segmentArray count]) {
return _segmentNumber;
} else {
return _segmentNumber - 1;
}
}()]; // <-- run the block
However, defining a block (which is a full-fledged object, and does a lot of fancy stuff) just to run it immediately is kinda silly. If you want to just be able to use a piece of code as an expression, you could use the "statement expressions" language extension supported by both GCC and LLVM:
TrailLayer * layer = [_layers objectAtIndex:({
int result;
if (_segmentNumber < [_segmentArray count]) {
result = _segmentNumber;
} else {
result = _segmentNumber - 1;
}
result;
})];
But really you should just do a regular C conditional expression in this case:
TrailLayer * layer = [_layers objectAtIndex:
_segmentNumber < [_segmentArray count] ?
_segmentNumber : _segmentNumber - 1];