Functions

Functions are a way to specify a computation or task that needs to be repeated.

For example: drawing an outlined box of given height and width:

print a solid row of width number of * characters

print newline

repeated height - 2 number of times {

print *

print row of height - 2 spaces

print *

print newline

}

draw a solid row of width number of * characters

print newline

What tasks are repeated in this algorithm?

print row of * characters

print row of space characters

others?

Any repeated task that is performed more than one time is an ideal candidate to become a function.

Functions are also called subprograms, and that’s exactly what they are: little programs that are used to construct a larger program.

Top-down design

Top-down design can be used for

1. Developing an algorithm

2. Implementing the algorithm as a program

Start by stating the overall problem:

Deal a hand of bridge

Break it down into sub-problems:

Shuffle

Have another player cut

while (there are cards left) {

for each player in order {

deal a card to that player

}

}

Each subproblem can then be further broken down into sub-subproblems until we have a complete description of the algorithm. The sketch of the algorithm using named subproblems is pseudo-code. It is an extremely useful technique for planning your algorithm, before you start writing the program.

Generally, each named subproblem will become a procedure when we turn the algorithm into a program. In otherwords, procedures give us an easy way to convert pseudo-code into a complete program.

Functions in C

In C, procedures are called functions.

They are not (necessarily) like mathematical functions.

Calling a Function

Using a function involves calling or invoking the function. A function call is an expression of the following general form:

identifier ( expressionList )

expressionList is a comma-separated list of zero or more expressions. These expressions are used to compute the argument values. These values are passed to the function to specify the input values to the function.

Recall that an expression computes a value.

Here is what happens when you call a function:

Each expression in the expression list is evaluated to compute its value

The computed value of each expression is “delivered” to the called function (more about this in a moment)

The code inside the function is executed using the computed values

When the code inside the function finishes executing, it can (optionally) return a value.

If you have used the function call as an expression, then the value returned by the function is the value computed by the expression.

Simple example:

sqrt(x) in <math.h>: is a function to compute the (approximate) square root of a double value.

sqrt(x) is like a mathematical function. For a given input value, there is precisely one possible output value.

printf("%lf\n", sqrt(16.0));

The function call expression sqrt(16.0) is evaluated by calling the function with the argument 16.0 and using its return value (4.0) as the value of the function call expression.

A more complex example:

#include <stdio.h>
#include <math.h>

int main(void)
{
    double area_in_square_feet, side;

    printf("Area in square feet? ");
    scanf("%lf", &area_in_square_feet);

    side = sqrt(area_in_square_feet);

    printf("%f square feet fit in a square %f feet on a side\n", 
            area_in_square_feet, side);

    return 0;
}

Some functions do not return a value. Such function calls are written as statements rather than expressions. In other words, a call to a function that does not return a value cannot be an expression.

E.g., the exit() function, which causes the program to terminate immediately:

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    int val;

    printf("Enter a non-negative value: ");
    scanf("%i", &val);

    if (val < 0) {
        printf("Invalid value: %i\n", val);
        exit(1);
    }

    ...etc...
}

Writing your own functions

Example: draw rectangle using “*” characters

What information do we need to define the possible rectangles we will draw?

height, width

These are the parameters of the function. They store the data values the function will use to carry out the computation.

Here is the general form of a C function:

returnType identifier ( parameterList )

{

statements

}

The returnType is a data type defining the type of value that is computed as the result of the function. For example, the return type of the built-in sqrt() function is double, since the value that it computes (and returns) has that type.

Some functions do not compute a value. Instead, they are used for some side-effect that they have (such as printing output). Such functions use a special data type called void. When you see a function with a void return type, it means that the function does not compute a value. The void type cannot be used to declare a variable, since there are no values of type void.

The identifier provides a name for the function. Generally, you should try to give the function a name that describes what it does. E.g., draw_rectangle.

The parameterList is a comma-separated list of variable declarations, one declaration for each parameter. These parameter variables will be initialized to the corresponding argument values each time the function is called.

The statements are simply a list of statements which define the computation that the function performs using only the parameters or other locally declared variables.

For example, the draw_rectangle() function might be:

void draw_rectangle(int width, int height)
{
    int j, i;

    for (j = 1; j <= height; j++) {
        /* Draw a line of * characters */

        for (i = 1; i <= width; i++) {
            printf("*");
        }

        printf("\n");
    }
}

Now we can use our draw_rectangle() function whenever we need to draw a rectangle of any width and height.

[Demo program]

What is happening?

When a function call is reached:

The argument values are computed. [What are the arguments for the first call to draw_rectangle()?]

The argument values are copied into the corresponding parameter variables of the called function. In the case of draw_rectangle(), the parameters are width and height. [What values are copied into the parameter variables for the first call to draw_rectangle()?]

The statements in the body of the function are executed.

If the function is one that returns a value, the last statement to be executed in the function will be a return statement. We will see those next lecture.

Once the last statement of the called function is executed, control returns to the point where the function was called from. [Where does control return after the first call to draw_rectangle() finishes?]

Functions must be defined (or declared) before being called. In order to call a function, the function must have been defined or declared earlier in the program. Otherwise, the compiler will complain that the function is unknown. One way to do this is through the use of function prototypes.

Function Prototypes

A function prototype (also called a function declaration) looks like a function, except that the body of the function is replaced by a semicolon. E.g.:

void draw_rectangle(int width, int height);

is a function prototype for a function called draw_rectangle that takes two int arguments, and does not return a value (is a void function). A function prototype is a signal to the compiler that the function described by the prototype exists somewhere else in the program.

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