Objectives
In this section you'll learn how to get input from the keyboard to make programs interactive.
You'll also learn how to create and call a procedure. Procedures are key to the idea of structured programming and are one of the most important concepts in programming.
You'll also learn how to create and call a procedure. Procedures are key to the idea of structured programming and are one of the most important concepts in programming.
Making a paddle move
We're going to continue making a basic game by adding some user interaction. We'll have a paddle at the bottom of the screen that the player can move to the left and right by pressing the 'a' and 'd' keys.
To do this we need to read the keyboard and respond appropriately.
In this program we will introuce the idea of a procedure.
A procedure is like a little program inside a program. It's a group of code lines that do something specific and can be called by name
We'll start by setting up the program. We need to add the console unit to our uses section and create two variables for the x and y position of the paddle. We then create an infinite loop using "while true do" and, for the moment, just draw the paddle wherever it happens to be.
To do this we need to read the keyboard and respond appropriately.
In this program we will introuce the idea of a procedure.
A procedure is like a little program inside a program. It's a group of code lines that do something specific and can be called by name
We'll start by setting up the program. We need to add the console unit to our uses section and create two variables for the x and y position of the paddle. We then create an infinite loop using "while true do" and, for the moment, just draw the paddle wherever it happens to be.
uses
System.SysUtils,console;
var
xpos, ypos : integer; // define variables for x and y
key: char; // and a char variable for our keypress
begin
xpos := 40; // set those variables to the initial position
ypos := 12;
while true do // loop forever
begin
gotoxy(xpos,ypos); // display the paddle
write('XXX');
end;
end.
That's OK but it doesn't actually do anything, just sits in the middle of the screen. We need to read the keyboard and actually make something happen. First we check to see if a key has been pressed. If not there's nothing to do. If a key has been pressed we call readkey to find out if it's a key we are interested in
while true do
begin
if keypressed then // check if a key has been pressed
begin
key := readkey;
if key = 'a' then
begin
// move left
end;
if key = 'd' then
begin
// move right
end;
end;
gotoxy(xpos,ypos);
write('XXX');
Defining a procedure
Instead of putting all the code for moving left and right in the main part of the program, we're going to create a procedure. We do that by adding some code before the main program starts, between the variable declaration section and the first 'begin' that indicates the main program.
To make a procedure we just use the keyword 'procedure' followed by the name we wish to give it. The name is up to you, but as with variables, it can't start with a number or be an existing keyword. It's best, as you'll see, to make the procedure name as descriptive as possible.
To make a procedure we just use the keyword 'procedure' followed by the name we wish to give it. The name is up to you, but as with variables, it can't start with a number or be an existing keyword. It's best, as you'll see, to make the procedure name as descriptive as possible.
var
xpos, ypos : integer;
key: char;
procedure moveleft;
begin
xpos := xpos -1;
end;
procedure moveright;
begin
xpos := xpos +1;
end;
begin
xpos := 40;
ypos := 12;
We've now created two procedures called moveleft and moveright. It's as if we had added new keywords to Delphi. Now whenever we want to run the code between the begin and end of the procedure, we just call it by name, like this:
if keypressed then // check if a key has been pressed
begin
key := readkey;
if key = 'a' then
begin
moveleft; // move left
end;
if key = 'd' then
begin
moveright; // move right
end;
end;
Making procedures is incredibly important in programming. It allows complex bits of code to be sectioned off into their own area and given a name that describes what the code does. It makes programs much more readable - as you can see, and allows us to work on different bits at a time. In addition, if we make a good general procedure we can copy it into other programs.
This is the beginning of a big idea:
This is the beginning of a big idea:
Structured programming
Structured programming is essential when our programs become large. It means that we split the program into self-contained blocks that talk to each other. We can draw out the structure of our program using a flowchart or structure chart - diagram it as a series of blocks.
Can you complete this program so that it displays properly (hint: use clrscr) and doesn't flicker quite so much?
