Variable Definitions

Next: Finger Exercises on Composing Up: Programs are Function Plus Previous: Composing Functions

Variable Definitions

When a number occurs many times in our program(s), we should give it a name using a VARIABLE DEFINITION, which associates a name with a value. One example is 3.14, which we have used in place of . Here is how we could give this number a name:

(define PI 3.14)

Now, every time we refer to PI, DrScheme replaces it with 3.14.

Using a name for a constant makes it easier to replace it with a different value. Suppose our program contains the definition for PI, and we decide that we need a better approximation of for the entire program. By changing the definition to

(define PI 3.14159)

the improvement is used everywhere where we use PI. If we didn't have a name like PI for

, we would have to find and all instances of 3.14 in the program and replace them with 3.14159.

Let us formulate this observation as our second guideline:

Guideline on Variable Definitions
Give names to frequently used constants and use the names instead of the constants in programs.

Initially, we won't use many variable definitions for constants, because our programs are small. But, as we learn to write larger programs, we will make more use of variable definitions. As we will see, the ability to have a single point of control for changes is important for variable and function definitions.

Exercises

Exercise 3.2.1

Provide variable definitions for all constants that appear in the profit program of figure and replace the constants with their names. Solution

PLT