ios 有没有办法遍历字典？
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Is there a way to iterate over a dictionary?
提问by Adam Rosenfield
I know NSDictionariesas something where you need a keyin order to get a value. But how can I iterate over all keysand valuesin a NSDictionary, so that I know what keys there are, and what values there are? I know there is something called a for-in-loopin JavaScript. Is there something similar in Objective-C?
我知道NSDictionaries你需要 akey才能获得value. 但是我如何遍历 allkeys和valuesa NSDictionary，以便我知道有哪些键以及有哪些值？我知道有个东西叫一个用于在回路中JavaScript。有没有类似的东西Objective-C？
回答by Adam Rosenfield
Yes, NSDictionarysupports fast enumeration. With Objective-C 2.0, you can do this:
是的，NSDictionary支持快速枚举。使用 Objective-C 2.0，你可以这样做：
// To print out all key-value pairs in the NSDictionary myDict
for(id key in myDict)
NSLog(@"key=%@ value=%@", key, [myDict objectForKey:key]);
The alternate method (which you have to use if you're targeting Mac OS X pre-10.5, but you can still use on 10.5 and iPhone) is to use an NSEnumerator:
另一种方法（如果您的目标是 Mac OS X 10.5 之前的版本，则必须使用该方法，但您仍然可以在 10.5 和 iPhone 上使用）是使用NSEnumerator：
NSEnumerator *enumerator = [myDict keyEnumerator];
id key;
// extra parens to suppress warning about using = instead of ==
while((key = [enumerator nextObject]))
NSLog(@"key=%@ value=%@", key, [myDict objectForKey:key]);
回答by Rok Strni?a
The block approachavoids running the lookup algorithm for every key:
块方法避免为每个键运行查找算法：
[dict enumerateKeysAndObjectsUsingBlock:^(id key, id value, BOOL* stop) {
NSLog(@"%@ => %@", key, value);
}];
Even though NSDictionaryis implemented as a hashtable(which means that the cost of looking up an element is O(1)), lookups still slow down your iteration by a constant factor.
即使NSDictionary是作为一个哈希表（这意味着查找元素的费用O(1)），查找仍然通过你的迭代放慢一个常数因子。
My measurements show that for a dictionary dof numbers ...
我的测量表明，对于d数字字典......
NSMutableDictionary* dict = [NSMutableDictionary dictionary];
for (int i = 0; i < 5000000; ++i) {
NSNumber* value = @(i);
dict[value.stringValue] = value;
}
... summing up the numbers with the block approach ...
...用块方法总结数字...
__block int sum = 0;
[dict enumerateKeysAndObjectsUsingBlock:^(NSString* key, NSNumber* value, BOOL* stop) {
sum += value.intValue;
}];
... rather than the loop approach ...
... 而不是循环方法 ...
int sum = 0;
for (NSString* key in dict)
sum += [dict[key] intValue];
... is about 40% faster.
...快了大约 40%。
EDIT: The new SDK (6.1+) appears to optimise loop iteration, so the loop approach is now about 20% faster than the block approach, at least for the simple case above.
编辑：新的 SDK (6.1+) 似乎优化了循环迭代，所以循环方法现在比块方法快 20%，至少对于上面的简单情况。
回答by Javier Calatrava Llavería
This is iteration using block approach:
这是使用块方法的迭代：
NSDictionary *dict = @{@"key1":@1, @"key2":@2, @"key3":@3};
[dict enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
NSLog(@"%@->%@",key,obj);
// Set stop to YES when you wanted to break the iteration.
}];
With autocompletion is very fast to set, and you do not have to worry about writing iteration envelope.
自动完成设置非常快，您不必担心编写迭代信封。
