self.changeArray= [[NSArrayalloc]initWithObjects:@"0",@"1",@"2",@"3",@"4",@"5",@"6",@"7",@"8",@"9",@"a",@"b",@"c",@"d",@"e",@"f",@"g",@"h",@"i",@"j",@"k",@"l",@"m",@"n",@"o",@"p",@"q",@"r",@"s",@"t",@"u",@"v",@"w",@"x",@"y",@"z",nil];//存放多个数，以备随机取,也可以加上大写字母和其他符号

NSMutableString* getStr = [[NSMutableString alloc]initWithCapacity:5];

self.changeString= [[NSMutableString alloc]initWithCapacity:6];//申请内存空间

for(inti =0; i<6; i++) {

NSInteger index =arc4random()%([self.changeArray count]-1);//循环六次，得到一个随机数，作为下标值取数组里面的数放到一个可变字符串里，在存放到自身定义的可变字符串

getStr =self.changeArray[index];

self.changeString= (NSMutableString*)[self.changeString stringByAppendingString:getStr];

}

NSLog(@"%@",self.changeString);