iOS 生成随机字符串 从指定字符串随机产生n个长度的新字符串

-(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;
}

-(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];
}