Objective C － 2 - 随机数，可变字符串，字符串，SubString

int main(int argc, const char * argv[])
{
 
    @autoreleasepool {
 
        NSString *outputString = @"1234567890!@#$%^&*()";
 
        NSMutableString *resultString = [[NSMutableString alloc] init];
 
        NSLog(@"%d", (int)outputString.length);
 
        for(int j=0;j<outputString.length;j++)
        {
            NSLog(@"Index = %d  Char = %@",j, [outputString substringWithRange:NSMakeRange(j, 1)]);
        }
 
        for(int i=0;i<20;i++)
        {
            int index  = arc4random() % outputString.length;
            NSString *indexString = [outputString substringWithRange:NSMakeRange(index, 1)];
 
            [resultString appendFormat:@"%d %@ ", index, indexString];
        }
 
        if (resultString) {
            NSLog(@"%@", resultString);
        }
 
        return 0;
    }
}