for-each

Introduction

The for-each function in Scheme is used to apply a procedure to each element of a list (or multiple lists). Unlike map, which returns a new list with the results, for-each is used for its side effects, such as printing or updating variables.

The simplest form of for-each looks like this:

(for-each procedure list)

• Procedure: A function to apply to each element of the list.
• List: The list whose elements will be processed.

Example: Print a List

(define (print-item x)
  (gimp-message (number->string x)))

(for-each print-item (list 1 2 3 4))

• Here, the function print-item is applied to each element of the list (1 2 3 4).
• This causes each number to be printed sequentially.

Output: 1 2 3 4

How It Works

1. Iterates Over Each Element:

   • The provided procedure is executed for every element in the list, in order.

2. Performs Side Effects:

   • Common side effects include printing, logging, or modifying external variables. Unlike map, for-each does not return a new list.

Example: Using with Multiple Lists

If multiple lists are provided, for-each processes corresponding elements from each list.

(define (sum-and-print x y)
  (gimp-message (number->string (+ x y))))

(for-each sum-and-print (list 1 2 3) (list 4 5 6))

• The function sum-and-print sums corresponding elements from the two lists and prints the results.

Output: 5 7 9

Summary

• The for-each function is useful for performing side effects on each element of a list.
• Unlike map, for-each does not produce a new list—it focuses solely on the procedure’s side effects.
• It can handle multiple lists simultaneously, applying the procedure to corresponding elements.

By using for-each, you can effectively process lists when the goal is to perform actions rather than transform data.