input-processing-output-ipo-model-example.md

Input Processing Output (IPO) Model example

Declare a 4-byte (32bit) integer variable named speed. Speed would normally always be a represented as positive number, never negative so we should use the unsigned type unit32_t

uint32_t int speed;

Assign value to variable, initialise the variable.

uint32_t speed = 20; // km/h

Assign value to variable time, initialise the variable.

uint32_t time = 7; // hours

Can declare and initialise on one line multiple variables of same type.

uint32_t speed = 20, time = 7, distance = 5;

Use expression to calculate distance

uint32_t distance = speed * time; // result = 140

Output results of distance variable.

In c you cannot do this:

printf(distance); // invalid in c

You must use place holder, i.e %d. %d means integer.

printf("dist: %d\n", distance)

Compile file d.c to create new file d

Open terminal and type:

$ gcc -o d d.c

Execute file d.

$ ./d

Expected output

$ ./d
distance: 140
$

This approach means if we change the variable values the program needs to be recompiled.

A better approach is to ask the user for the variable values speed and time, then the program can output the result for different values without needing to be recompiled.

To do this we can use scanf function. This is also in the stdio.h library.

scanf has several limitations, other functions exist that do overcome these limitations.

scanf works similar to printf but backward.

We first have to initialise the variables speed and time. It is best practice to assign a value to variables, even if the value will be updated by the user. This initialising a variable with a value, creates a space in memory and overwrites any previous data in that memory location. In our example we are defining integer variables so we should assign the value 0 to the variables.

    uint32_t speed = 0;
    uint32_t time = 0;

Ask user to enter value for speed.

    // Ask user for value of speed
    printf("Enter value for speed: ");
    // Expects integer %d from input, will assign to speed variable using & operator.
    scanf("%d", &speed);

Ask user to enter value for time.

Expects integer %d from input.

scanf will then assign the user input to the time variables memory location.

& operator in front of a variable name, returns the memory location.

    printf("Enter value for time: ");
    
    scanf("%d", &time);

Recompile and run program.

Expected output

$ ./d
Enter value for speed: 4
Enter value for time: 5
distance: 20
$

The program can be run multiple times with user entering different values without need for recompilation.