cond

cond

Introduction

In Scheme, the cond conditional is used for selecting one of several possible blocks of code to execute, based on multiple tests. It is like a multi-branch if, where each branch is checked in order until a match is found.

Syntax

(cond
  (test-1 consequent-1)
  (test-2 consequent-2)
  ...
  (else fallback-consequent))

Each test is evaluated in the order they are written.
When a test evaluates to true (#t), its corresponding consequent is executed, and the cond expression stops evaluating further tests.
The else clause is optional and serves as a fallback if none of the tests evaluate to true.

How It Works

Test Each Condition:
- cond evaluates the tests in the order they are listed.
Execute the Matching Consequent:
- When the first test that evaluates to true (#t) is found, its consequent is executed.
- If no tests evaluate to true and there is an else clause, the fallback-consequent is executed.

Examples

Example 1: Single Expression Consequents

(cond
  ((< 3 2) "This won't run")
  ((= 3 3) "This will run")
  (else "Fallback"))

The first test (< 3 2) evaluates to false (#f).
The second test (= 3 3) evaluates to true (#t), so "This will run" is returned.
The else clause is not executed because a match was already found.

Result: “This will run”

Example 2: Multiple Actions Using `begin`

When a consequent involves multiple actions, use begin to group them:

(cond
  ((< 5 3) 
    (begin
      (gimp-message "This won't run")
      (* 2 3)))
  ((> 5 3) 
    (begin
      (gimp-message "Condition met")
      (* 5 5)))
  (else 
    (begin
      (gimp-message "Fallback")
      0)))

The first test (< 5 3) evaluates to false (#f).
The second test (> 5 3) evaluates to true (#t):
- It prints "Condition met".
- Then it calculates (* 5 5) and returns 25.

Result: Prints “Condition met” and returns 25.

Example 3: Using a `let` Block in a Consequent

When you need to introduce local variables, use a let block:

(cond
  ;; Case 1: If 0 is less than -1
  ((< 0 -1) 
    (let ((x 10))  
      (* x x)))    

  ;; Case 2: If 0 is greater than -1
  ((> 0 -1) 
    (let ((y 20))  
      (gimp-message "Positive condition met")  
      (+ y y)))    

  ;; Default case: If none of the above conditions are met
  (else 
    (let ((z 0))   
      z)))

The first test (< 0 -1) is false.
The second test (> 0 -1) is true, so:
- A let block is executed, binding y to 20.
- It prints "Positive condition met".
- Then it calculates (+ y y) and returns 40.

Result: Prints “Positive condition met” and returns 40.

Example 4: Fallback with `else`

(cond
  ((< 5 3) "This won't run")
  ((= 5 3) "This won't run either")
  (else "Fallback value"))

Neither of the first two tests evaluates to true.
The else clause is executed and returns "Fallback value".

Result: “Fallback value”

Summary

Use cond for handling multiple conditions in a clear and concise manner.
Consequents can be single expressions or grouped actions using begin.
Use let in consequents to declare local variables for calculations.
Always include an else clause as a fallback to handle unexpected cases.

This flexibility makes cond a powerful and readable tool for handling complex branching logic.

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