forin循环中的遍历之特殊之处

forin 循环中的遍历内容不能被改变, 是因为如果改变其便利的内容会少一个, 而系统是不会允许这个发生的所以就会crash...但是当改变最后一个的内容时, 就不会crash, 是因为此时遍历已经结束, 结束之后对内容做修改是允许的

//init a NSArray  
NSMutableArray *arr2 = [NSMutableArray array];  
for (int i=0; i<10; ++i) {  
    NSMutableDictionary *d = [NSMutableDictionary dictionaryWithObjectsAndKeys:[NSString stringWithFormat:@"%d", i], @"a", nil nil];  
    [arr2 addObject:d];  
}  
  
//Wrong  
for (NSMutableDictionary *s in arr2) {  
    if ([s[@"a"] isEqualToString:@"3"]) {  
        [arr2 removeObject:s]; //while here is safe for s[@"a"] = @"eeee";  
    }  
}  
  
  
//Safe 1.  
[arr2 enumerateObjectsUsingBlock:^(NSMutableDictionary *obj, NSUInteger idx, BOOLBOOL *stop) {  
    if ([obj[@"a"] isEqualToString:@"3"]) {  
        *stop = YES;  
        [arr2 removeObject:obj]; // or obj[@"a"] = @"4ewewewe";  
    }  
}];  
  
//Safe 2.  
NSArray *tmp = [NSArray arrayWithArray:arr2];  
for (NSMutableDictionary *s in tmp) {  
    if ([s[@"a"] isEqualToString:@"3"]) {  
        s[@"a"] = @"eeee";  
        [arr2 removeObject:s];  
    }  
}