Methods and Loops

We will soon ad the web stuff, and actually download the web page. But the method main is getting a bit long now, so perhaps it is time to divide or program into several methods. Instead of putting the web stuff in main, we write a method called handle_url, which will do all the handling of a URL.

void handle_url(string this_url)
{
  write("URL: " + this_url + "\n");
} // handle_url

So far, it only writes the URL on the screen, just like we did in main.

Some interesting things to note:

• main returns an integer, to indicate if the program succeeded or not. We don’t care if handle_url fails, so we set the return type to void, which means it returns “nothing”.

• main had two parameters. handle_url only has one, this_url. this_url can contain a string.

For this method to be executed by Pike, we must call it from main. We replace the statement that printed the URL with:

handle_url(url);

This sends the value in the variable url, which is a local variable in main, to handle_url, where it will be put in the variable this_url. The thing that is sent, url, is usually called an argument, and the variable that receives it, this_url, is usually called a parameter.

A problem with our program is what happens if the user doesn’t give an argument, and then just hits the return key when the program asks for a URL. Stdio.stdin->gets() will return an empty sting, just as it should, but that is not a valid web address. If we want the program to keep asking until it gets a non-empty answer, we can put the two ask-and-read statements inside a loop:

do
{
  write("Type the address of the web page:\n");
  url = Stdio.stdin->gets();
} while (sizeof(url) == 0);

This is a do-while loop, and such a loop follows the pattern

do
  *something*
while ( *condition* );

The do-while loop will do the something at least once, and then keep doing it as long as the condition is true. First it does the something, then it checks the condition, and then it either leaves the loop and continues after it, or goes back to the start of the loop and does the something again. In our case, the condition is that sizeof(url) == 0, i e that the string url is empty.

Another problem with our program is that it only checks if argc is 2. If you give several arguments, the program will ignore them. We change the if test a bit, so that it prints an error message and then terminates the program if we give too many arguments. The complete program now looks like this:

#! /usr/local/bin/pike


// The Very Simple World Wide Web Browser

void handle_url(string this_url)
{
  write("URL: " + this_url + "\n");
} // handle_url

int main(int argc, array(string) argv)
{
  write("Welcome to the Very Simple WWW Browser!\n");
  string url;
  if (argc == 1)
  {
    do
    {
      write("Type the address of the web page:\n");
      url = Stdio.stdin->gets();
    } while (sizeof(url) == 0);
  }
  else if (argc == 2)
  {
    url = argv[1];
  }
  else
  {
    write("Too many arguments. Goodbye. Sorry.\n");
    return 1;
  }
  handle_url(url);
  return 0;
} // main

Note how we have chained two if statements together with an else if.

Neither the curly brackets around url = argv[1]; nor those around the do loop are necessary, since they only encircle a single statement each, but some programmers feel that it is to be on the safe side having them there either way, in case they would add yet another statement later on and forget about adding the braces. Others feel that if you have curly brackets around one of the possible cases in an if statement, you should have curly braces around all of them. Uniformity makes it easier to grasp the structure of the whole thing at a glance.

Also note how the use of indentation, i e the varying amount of white space at the beginning of each line, makes it easy to follow the “block structure” of the program. For example, you can easily see that the do-while loop is inside the first case of the if statement.