iOS开发 银行卡号每4位插一个空格

#pragma mark - 拼接成中间有空格的字符串
+ (NSString *)jointWithString:(NSString *)str
{
    NSString *getString = @"";
    
    int a = (int)str.length/4;
    int b = (int)str.length%4;
    int c = a;
    if (b>0)
    {
        c = a+1;
    }
    else
    {
        c = a;
    }
    for (int i = 0 ; i<c; i++)
    {
        NSString *string = @"";
        
        if (i == (c-1))
        {
            if (b>0)
            {
                string = [str substringWithRange:NSMakeRange(4*(c-1), b)];
            }
            else
            {
                string = [str substringWithRange:NSMakeRange(4*i, 4)];
            }
            
        }
        else
        {
            string = [str substringWithRange:NSMakeRange(4*i, 4)];
        }
        getString = [NSString stringWithFormat:@"%@ %@",getString,string];
    }
    return getString;
}