workshop12

Previous: Pointers

If - Else branching

If my bills are bigger than 1500 USD I need to find a new job, else I will keep the existing job.

Yes, my brain is still working ok, thank you for asking. This is just an example of if - else branching; the associated code is listed below:

if (my_bills > 1500)
{
find_new_job( );
}
else
{
keep_existing_job( );
}

You use "if" instructions when you want your program to behave differently depending on some conditions: user input, a random number, the result of a mathematical operation, etc. Here's the basic form of the "if" instruction:

if (some condition is true)
{
// execute the instructions that are placed inside the curly brackets
}

A more complex form of the "if" instruction is listed below:

if (some condition is true)
{
// execute the instructions that are placed inside the curly brackets
}
else
{
// execute the instructions that are placed inside these curly brackets
}

The instructions placed inside the "else" curly brackets are executed only if "some condition" isn't true. Here's a practical example:

if (my_age > 65)
{
income = 2000;
}
else // age less than or equal to 65?
{
income = 3000;
}

It is pretty clear that income can be either 2000 or 3000 because only one of the branches will be executed (income = 2000 or income = 3000, not both).

Let's draw some conclusions:
- "if" branching instructions start with the if keyword followed by a pair of parentheses;
- the parentheses contain an expression ("some condition") that can be true or false;
- if the expression is true, the set of instructions placed inside the first pair of curly brackets is executed;
- if the expression is false and we don't use "else", the set of instructions placed between the curly brackets is skipped (it isn't executed);
- if the expression is false and we are using the "else" branch too, the set of instructions placed inside the first pair of curly brackets is skipped, and the set of instructions placed inside the second pair of curly brackets is executed.

Now that we've learned a few things let's open the script12 file in SED:

////////////////////////////////////////////////////////////////////
var video_mode = 7;
var screen_color[3] = 150,0,0;

////////////////////////////////////////////////////////////////////
var weight = 300;

STRING fluffy_str = "Fluffy!";
STRING skinny_str = "Skinny!";

////////////////////////////////////////////////////////////////////
TEXT weight_txt
{
	pos_x = 380;
	pos_y = 250;
	flags = VISIBLE;
}

/////////////////////////////////////////////////////////////////////
function main()
{
	while (1)
	{
		if (weight > 200)
		{
			weight_txt.string = fluffy_str;
		}
		else
		{
			weight_txt.string = skinny_str;
		}
		wait (1);
	}
}

The code doesn't look that complicated; we have the usual vars that set the video resolution and color depth, a new var named weight which receives an initial value of 300, a string that is used for the name of the level and two strings: "Fluffy!" and "Skinny!". We will display a text that uses the built-in font. The interesting part of the code is the while (1) loop placed inside function main:

while (1)
{
      if (weight > 200)
      {
           weight_txt.string = fluffy_str;
      }
      else
      {
           weight_txt.string = skinny_str;
      }
      wait (1);
}

I hope that you have noticed the "wait (1);" instruction inside the while loop; this instruction interrupts the loop from time to time, allowing the other actions and functions to run at the same time. The rest of the code is simple: if weight is bigger than 200, we set fluffy_str as the string used by the text named weight_txt (using the dot method); if weight is smaller than or equal to 200, we set skinny_str as the string used by weight_txt.

Run the script and you will see the following text in the center of the screen:

Now press Tab on your keyboard and type the following line of code at the console:

weight = 100;

The text changes from "Fluffy!" to "Skinny!" because the "else" branch is executed if weight is less than or equal to 200.

We needed a while loop this time because we need to keep checking the value of "weight" all the time in order to display the proper text when the user types in a new value at the console.

Let's assume that you want to shut down the program when the user inputs this line:

weight = 0;

at the console. How would you do that? You must add a new "if" that compares weight with zero; copy and paste the code below inside the while loop in the function main:

if (weight == 0)
{
sys_exit(NULL);
}

The new code doesn't look too complicated; if weight is zero, the engine will shut down (you should remember "sys_exit" from one of the previous workshops). Run the script and type

weight = 0;

at the console. The engine will shut down because the new "if" has done its job.

Wait a minute! What's with that "==" in the first new line of code? Is this a typing error? No, it isn't a typing error. Whenever you compare two expressions (weight and zero in the example above) you have to use "==" instead of "=", because a line of code that looks like this:

if (weight = 0)
{
// do some stuff
}

will set weight to zero instead of comparing weight with zero. The engine executes what's inside the parenthesis first (weight = 0) and then what's outside the parenthesis. The guys at Conitec have made our lives easier: the engine will issue an error message when we use a single "=" sign to compare two expressions:

Important tip
Using one instead of two equality signs for comparing two expressions inside a while loop, if branching, etc. is a very frequent mistake. Please keep in mind:

if (a == 3) // correct
{
do_some_stuff
}

if (a = 3) // wrong!
{
do_some_stuff
}

We've talked a lot about expressions, but what is an expression? An (arithmetic) expression is a sequence of variables, numbers, operators, etc that are linked together. While it is nice to have simple expressions that look like the one below:

if (energy < 50) // simple expression inside the parenthesis
{
health = health - 10;
}

it is entirely possible to work with more complicated expressions that could look like the ones below:

if ((armor + energy * 0.25) > (health * (1 - class / 3))) // more complicated expression
{
health = health + 5;
}

Important tip
You can combine as many expressions as you want, using parenthesis, just like in the example above. The same rules from your math classes apply here, too. Sadly, you have to know some math if you want to be a good programmer.

Take a look at the table to see how you can compare two different expressions (you can use the same table for "if", "while", etc):

Sometimes you need to check if several conditions are met at once. Let's imagine that you want to fire a bullet; the code that takes care of that should check if:
a) Your health is above zero (dead players shouldn't be able to fire bullets);
b) Your ammo is bigger than zero (you need to have at least one bullet if you want to be able to shoot something or somebody).

There are several methods to solve this problem but one of them uses nested if branches, which translated to English means "if branches placed inside the curly brackets of other if branches". Let's see a clear, simple example:

if (player_health > 0)
{
    if (ammo > 0)
    {
        fire_bullets();
    }
}

What's happening here? First of all, player's health is compared with zero; if player_health <= 0, the rest of the code will be ignored. If the player is alive (player_health > 0), we will get to check if ammo is greater than zero. If the player hasn't got enough ammo (ammo <= 0) the rest of the code will be ignored; otherwise, if ammo > 0, function fire_bullets will run.

Next: Using the keyboard