Lesson 03
A Programming Primer
A crash course in the basics of C programming for the PSP.
This is part two of Lesson 03. If you haven't read part one, go back
and do that now or you will be hopelessly confused. Continuting on:
Our loop will not be of the classic form. It could easily be, but I've looked through a handful of PSP projects' code, and they always do it
this way, so I figured it would be good to introduce you to the way that you will most commonly see. Anyway, what we will be doing is having
an "infinite loop" per say. It's not really infinite though, because (as I so conveniently didn't mention earlier), there is another way to
exit a loop other than the control statement returning false. That is the "break" statement. Any time this is encountered, it will exit out
of the loop you are currently in and your program will continue executing at the end of that loop. So, here's the start of our loop:
The next line is where we set the value of "pad." It uses a function that is defined in the "pspctrl.h" file that we included earlier. This is a function call, as you saw in Lesson 02, but there's a little bit of a twist. First of all, we are passing two values to it. No big deal here, you just seperate the two values with a comma. The second thing that's a little different is that we are going to pass a variable. Not a big deal here either, just insert the variable name instead of a number or string or whatever. The third thing that's quirky is that we need to pass it with the "&" operator in front of it. This is actually the "address of" operator, and when you get deeper into programming, you will probably get familiar with it. Basically what it does is instead of passing the value of the variable, it passes the memory address. For now, just know that in order for a function to be able to change the value of a variable you pass to it, you need to use the "&" operator. So here's how we'll set "pad" to contain the current status of the PSP controls:
The next line is an "if" statement. It's how you can program logic into your applications. What it does is execute a block of code "if" a statement evaluates to true. So, what we want to do is "break" out of the loop if the [X] button has been pressed. Which, effectively will "unpause" our program and go on to the next section. If the statement evaluates to false, it will just skip the code, and if there's an "else" statement, execute that block of code, or just go on. We will not have an "else" statement, so it will go through the loop again, continually checking the status of the buttons, and seeing if the [X] button has been pressed. While we're at it, we'll close our loop by ending an ending bracket.
After they press the [X] button, we want to start a counter that will go until the user presses the [O] button. So what do we do? You guessed it, another loop! So we'll start this loop, and add in the exit code (with the substitution of "PSP_CTRL_CIRCLE" for "PSP_CTRL_CROSS").
Now, we need to use a new function. What it does is clear the screen. Since we still have the "Press [X] To Start the Timer" on the screen, we need to erase that. Plus, the next time the loop rolls around and this code is executed, there will be the text that we're going to print out on the screen. We need to clear this off. So here's how we do it:
The first line should look familiar. It's just a simple "printf" statement. The one thing that may look a little weird to you is the "\n" at the end of the string. "\n" is a special character that stands for a new line. So the new line character is the equivelant of pressing "ENTER" on your keyboard. The second "printf" statement is just a bit different. We need it to print out our "counter" variable. To do this, we will use the "%" operator. Basically this signifies that you want to display a variable, and the character that follows it tells the program what type of variable. For an integer, we will use "i" (integer; get it?). The second parameter that we will pass is which variable we want it to print. In our case, we want to pring the "counter" variable. So put these two lines in your program:
We've completed the second part of Lesson 03. Let's move on to the third part in which we complete our program. Continue.
while(1) {
As you can see, this control statement will always evaluate to "true," because "1" is the same as "true." So the following code (the lines between
the opening bracket and the ending bracket) will execute repeatedly until it encounters a "break" statement.The next line is where we set the value of "pad." It uses a function that is defined in the "pspctrl.h" file that we included earlier. This is a function call, as you saw in Lesson 02, but there's a little bit of a twist. First of all, we are passing two values to it. No big deal here, you just seperate the two values with a comma. The second thing that's a little different is that we are going to pass a variable. Not a big deal here either, just insert the variable name instead of a number or string or whatever. The third thing that's quirky is that we need to pass it with the "&" operator in front of it. This is actually the "address of" operator, and when you get deeper into programming, you will probably get familiar with it. Basically what it does is instead of passing the value of the variable, it passes the memory address. For now, just know that in order for a function to be able to change the value of a variable you pass to it, you need to use the "&" operator. So here's how we'll set "pad" to contain the current status of the PSP controls:
sceCtrlReadBufferPositive(&pad, 1);
If you're wondering what the "1" is for, don't worry about it. It sets the number of buffers to read, but I've only ever seen "1" used there.
Honestly, I don't know why you'd want anything else there (if there's a reason, someone please enlighten me, and I can add it here).The next line is an "if" statement. It's how you can program logic into your applications. What it does is execute a block of code "if" a statement evaluates to true. So, what we want to do is "break" out of the loop if the [X] button has been pressed. Which, effectively will "unpause" our program and go on to the next section. If the statement evaluates to false, it will just skip the code, and if there's an "else" statement, execute that block of code, or just go on. We will not have an "else" statement, so it will go through the loop again, continually checking the status of the buttons, and seeing if the [X] button has been pressed. While we're at it, we'll close our loop by ending an ending bracket.
if(pad.Buttons & PSP_CTRL_CROSS) {
break;
}
}
The statement in the "if()" translates to English as "if the [X] button is pressed." So if that button is pressed, it will break, if not, it
will just continue going through the loop. Now on to the counter.break;
}
}
After they press the [X] button, we want to start a counter that will go until the user presses the [O] button. So what do we do? You guessed it, another loop! So we'll start this loop, and add in the exit code (with the substitution of "PSP_CTRL_CIRCLE" for "PSP_CTRL_CROSS").
while(1) {
sceCtrlReadBufferPositive(&pad, 1);
if(pad.Buttons & PSP_CTRL_CIRCLE) {
break;
}
sceCtrlReadBufferPositive(&pad, 1);
if(pad.Buttons & PSP_CTRL_CIRCLE) {
break;
}
Now, we need to use a new function. What it does is clear the screen. Since we still have the "Press [X] To Start the Timer" on the screen, we need to erase that. Plus, the next time the loop rolls around and this code is executed, there will be the text that we're going to print out on the screen. We need to clear this off. So here's how we do it:
pspDebugScreenClear();
This will clear off the screen, so now we're ready to print out our counter (and tell the user how they can stop the counter and exit the program).The first line should look familiar. It's just a simple "printf" statement. The one thing that may look a little weird to you is the "\n" at the end of the string. "\n" is a special character that stands for a new line. So the new line character is the equivelant of pressing "ENTER" on your keyboard. The second "printf" statement is just a bit different. We need it to print out our "counter" variable. To do this, we will use the "%" operator. Basically this signifies that you want to display a variable, and the character that follows it tells the program what type of variable. For an integer, we will use "i" (integer; get it?). The second parameter that we will pass is which variable we want it to print. In our case, we want to pring the "counter" variable. So put these two lines in your program:
printf("Press [O] To Stop the Timer\n");
printf("Counter: %i", counter);
printf("Counter: %i", counter);
We've completed the second part of Lesson 03. Let's move on to the third part in which we complete our program. Continue.