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map

map

Introduction

The map function in Scheme is used to apply a procedure to each element of a list (or multiple lists) and return a new list containing the results. This makes it ideal for transforming data.

The simplest form of map looks like this:

(map procedure list)
  • Procedure: A function to apply to each element of the list.
  • List: The list whose elements will be transformed.

Example: Double Each Element 

(define (double x)
  (* x 2))

(map double (list 1 2 3 4))
  • Here, the function double is applied to each element of the list (1 2 3 4).
  • The result is a new list with each element doubled.

Output: (2 4 6 8)

How It Works

  1. Creates a New List:

    • map applies the provided procedure to each element of the list and collects the results into a new list.

  2. Transforms Data:

    • It is primarily used for data transformations rather than performing side effects.

Example: Using with Multiple Lists

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

(define (sum x y)
  (+ x y))

(map sum (list 1 2 3) (list 4 5 6))
  • The function sum adds corresponding elements from the two lists and returns the results as a new list.

Output: (5 7 9)

Summary

  • The map function is a powerful tool for transforming lists by applying a procedure to each element.
  • Unlike for-each, map produces a new list containing the results of applying the procedure.
  • It supports multiple lists, allowing element-wise operations across them.

By using map, you can efficiently create transformed versions of your data while keeping the original lists unchanged.

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