TIGCC Programming Lessons

Lesson 2 - Basic Keyboard Input

Step 3 - Alternative to ngetchx() 1: The kbhit() function

As we have seen in the previous example, the ngetchx() function waits for the user to press a key before the program can continue. This is often a big drawback, and something we need to avoid; so we need some alternative form of handling keyboard input. Enter kbhit(). The kbhit() function will tell us if a key has been pressed. So, if we switch the condition in the previous example for the kbhit() function, we can work the body of the loop while no keys have been pressed.

Start a new project in TIGCC with a C Source file named kbhit. Modify the program so that is looks like this.

kbhit.c

#include <tigcclib.h>

void _main(void) {
    int key, number = 0;

    // clear the screen
    clrscr();

    // loop until user presses a key
    while (!kbhit()) {
        printf("%d\n", number);     // print the number
        ++number;                   // increment the number
    }

    // grab the keycode and display it
    key = ngetchx();
    printf("You pressed keycode %d\n", key);

    // wait for input before exiting
    ngetchx();
}

Step 3a - Compile and Run the Program

Build the program and send it to TiEmu. It should look something like this, depending upon what key you press and how long you wait to press it.

Step 3b - Analyze the Program

This is another fairly straightforward program, but it never hurts to undergo program analysis.

int key, number = 0;

// clear the screen
clrscr();

This is pretty simple. As we talked about in part 1, we can declare multiple variables and assign values to them all on a single line.

// loop until user presses a key
while (!kbhit()) {
    printf("%d\n", number);     // print the number
    ++number;                   // increment the number
}

This is where the new segment comes in. We have another while loop, but with a new condition this time. Instead of assigning key values from the ngetchx() function, we check if a key has been hit. Just like in !=, the ! means "not". The kbhit() function returns true if a key has been pressed; false otherwise. So we will stay in the while loop if no key has been hit.

The loop body is simple, but has some nuances you may not have seen before if you are new to C. First, we have a new kind of value in the printf() function. The '%d' is called a format specifier. Format specifiers are used in formatted strings (the f in printf means print formatted) to stand in place of another value which will be substituted later. The printf() function replaces these place holders with the variables we specify as arguments to the function. The specific '%d' specifier is used for int variables. So, %d means 'replace me with the value of an integer'. So, this printf function prints the integer value stored by the 'number' variable followed by a new line character.

The last statement in the loop body is the pre-increment statement. It increases the value of the variable by 1.

// grab the keycode and display it
key = ngetchx();
printf("You pressed keycode %d\n", key);

// wait for input before exiting
ngetchx();

From the previous lessons and examples, these statements should be no problem for you, but the surprise may be that ngetchx() does not have to wait this time, because we already know the user pressed a key. This was taken care of in the while condition. Since a key has already been pressed, we can use ngetchx() to retrieve this value for us. Then we just print out the value of the keycode to the screen.

As always, we wait for one more key press before exiting out program.

Step 3c - Programmatic Conclusions

This program demonstrates a different method of key reading, and can be quite useful in programs that need to do other things while checking for input. This will work fine for simple games that don't need a ton of speed, but you will need faster methods for real speed.

In the final part of the lesson, we will learn about the fastest built-in method of keyboard reading.

Continue with Part III