iOS 生成自定义位数随机数

#pragma mark 生成随机数

- (NSString *)generateTradeNO

{

    static int kNumber = 15;

    NSString *sourceStr = @"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

    NSMutableString *resultStr = [[NSMutableString alloc] init];

    srand((unsigned)time(0));

    for (int i = 0; i < kNumber; i++)

    {

        unsigned index = rand() % [sourceStr length];

        NSString *oneStr = [sourceStr substringWithRange:NSMakeRange(index, 1)];

        [resultStr appendString:oneStr];

    }

    return resultStr;

}