if

Introduction

In its simplest form, the if conditional in Scheme evaluates a test and, based on the result, executes one of two possible code blocks. The simplest form looks like this:

(if test-is-true
  do-this)

If the test evaluates to true (#t), the code block in the consequent is executed. The block may return a value or perform other actions, such as assigning a variable or printing output.

Example

(if (< 0 1)
  (gimp-message "True!"))

In this case, the test is (< 0 1) (checking if 0 is less than 1).
Since the test evaluates to true (#t), the code block (gimp-message "True!") is executed, which prints "True!".

Adding an Else Condition: `if-else`

When using an if conditional with an alternative code block (the else case), the structure looks like this:

(if test
  do-this
  else-do-this)

If the test evaluates to true (#t), the consequent code block is executed.
If the test evaluates to false (#f), the alternative code block is executed.

(if test
  consequent
  alternative)

How It Works

Test Expression:
- The test expression is evaluated first.
Result Based on Test:
- If the test evaluates to true (#t), the consequent code block is executed.
- If the test evaluates to false (#f), the alternative code block is executed.

Both the consequent and alternative code blocks can perform any valid Scheme operation, including returning values, modifying variables, or running procedures.

Examples

Example 1: Returning a Value

(if (< 0 1)
  1
  0)

Here, the test is (< 0 1) (checking if 0 is less than 1).
Since the test evaluates to true (#t), the consequent block (1) is executed and its value is returned.

Result: 1

Example 2: Evaluating a begin Block

In cases where you need to perform multiple actions when the condition is true or false, you can use begin or a let to group them together.

(if (= 0 1)
  (begin
    (gimp-message "This won't run")
    1)
  (begin
    (gimp-message "False condition met, calculating...")
    (* 3 4)))

In this example, the test is (= 0 1) (checking if 0 equals 1).
Since the test evaluates to false (#f), the alternative block is executed:
- First, it prints "False condition met, calculating...".
- Then, it calculates (* 3 4) and returns 12.

Result: Prints “False condition met, calculating…” and returns 12.

Example 3: Evaluating a let Statement

Using a let allows us to declare local scope variables withing the code block.

(if (= 1 1)
  (let (x -1)
    (gimp-message "True condition met, calculating...")
    (* x 10))
  (let (y 4)
    (gimp-message "This won't run")
    (* 3 y)))

In this example, the test is (= 1 1) (checking if 1 equals 1).
Since the test evaluates to true (#t), the consequent block is executed:
- First, it prints "True condition met, calculating...".
- Then, it calculates (* -1 10) and returns -10.

Result: Prints “True condition met, calculating…” and returns -10.

Summary

The if conditional is a powerful tool in Scheme for evaluating tests and executing corresponding code blocks.
It can handle both simple expressions and complex code blocks that return values, modify variables, or perform side effects.
Remember: If there is no explicit else block, the if only evaluates and executes the consequent if the test is true. Otherwise it evaluates and executes the alternative.

cond

if