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POProper and most efficient way to iterate through an NSMutableSet?
 

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    <p>Some Context: In my destructible terrain library for Cocos2D, one of the features is collapsable terrain. In order to collapse efficiently, I store the columns whose pixels have been altered in a NSMutableSet which contains NSNumbers which contain integers. I am using this data structure because I don't want to iterate over duplicate columns.</p> <p>My first instinct for how to loop through the NSMutableSet was to use a 'for in' loop.</p> <pre><code>for (NSNumber * col in [alteredColumns allObjects]) { // Works // for (NSNumber * col in alteredColumns) { // Crashes int x = col.intValue; bool shouldShift = false; bool alphaFound = false; for (int y = (size_.height -1); y &gt;= 0; y--) { if (!shouldShift) { if ([self pixelAt:ccp(x,y)].a == 0) { // Need to shift all pixels above one down alphaFound = true; } else if (alphaFound) { didCollapse = shouldShift = true; // Ensure the top pixel is alpha'ed out if a collapse will occur [self setPixelAt:ccp(x,0) rgba:ccc4(0, 0, 0, 0)]; [self setPixelAt:ccp(x,(y+1)) rgba:[self pixelAt:ccp(x,y)]]; } // end inner if } else { // Need to shift pixels down one [self setPixelAt:ccp(x,(y+1)) rgba:[self pixelAt:ccp(x,y)]]; } // end if } // end inner for // Remove column from cache if no pixels collapsed if (!shouldShift) [alteredColumns removeObject:col]; } // end outer for </code></pre> <p>However, this led to bad results. The program would crash with a Bad Access Memory error. If I alter the for loop to use [alteredColumns allObjects], then everything works just fine. So Questions... Can a 'for in' loop not be used on an unordered collection such as a NSMutableSet? If I must use a method such as allObjects, is this efficient?</p> <p>Thanks in advance!</p>
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    1. USPaul Renton
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