• 随机字符串 - 生成指定长度的字符串

-(NSString *)randomStringWithLength:(NSInteger)len {

     NSString *letters = @"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

    NSMutableString *randomString = [NSMutableString stringWithCapacity: len];

    for (NSInteger i = 0; i < len; i++) {

        [randomString appendFormat: @"%C", [letters characterAtIndex: arc4random_uniform([letters length])]];

    }

    return randomString;

}

• 指定字符串随机生成指定长度的新字符串

-(NSString *)randomStringWithLength:(NSInteger)len String:(NSString *)letters {

    NSMutableString *randomString = [NSMutableString stringWithCapacity: len];

    for (NSInteger i = 0; i < len; i++) {

        [randomString appendFormat: @"%C", [letters characterAtIndex: arc4random_uniform([letters length])]];

    }

    return randomString;

}

//调用方法

-(NSString *)generateCard{

    //HXMF   123456789TJQK

    NSString *color =[self randomStringWithLength:1 String:@"HXMF"];

    NSString *number = [self randomStringWithLength:1 String:@"123456789TJQK"];

    return [NSString stringWithFormat:@"%@%@",color,number];

}