forin 循環(huán)中的遍歷內(nèi)容不能被改變, 是因?yàn)槿绻淖兤浔憷膬?nèi)容會少一個, 而系統(tǒng)是不會允許這個發(fā)生的所以就會crash...但是當(dāng)改變最后一個的內(nèi)容時, 就不會crash, 是因?yàn)榇藭r遍歷已經(jīng)結(jié)束, 結(jié)束之后對內(nèi)容做修改是允許的

//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];  
    }  
}