Examples of recursion: string-merge

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Lecture 3 - Page 22 : 42

Functional Programming in Scheme
Name binding, Recursion, Iteration, and Continuations

Examples of recursion: string-merge

The function string-merge zips two lists of strings to a single string. The lists are not necessarily of equal lengths

(define (string-merge str-list-1 str-list-2)
 (cond ((null? str-list-1) (apply string-append str-list-2))
       ((null? str-list-2) (apply string-append str-list-1))
       (else (string-append
              (car str-list-1) (car str-list-2)
              (string-merge (cdr str-list-1) (cdr str-list-2))))))

The recursive function string-merge. Notice that this function is a general recursive function. The recursive call, emphasized above, is not in a tail position, because of the embedding in string-append.

An application of string-merge which converts a list of strings to a string with a given separator.

This is a typical task in a web program, where a list of elements needs to be aggregated for HTML presentation purposes. Notice the merging of a list of n elements with a list of length n-1. The function make-list is another LAML function; (make-list n el) makes a list of n occurrences of el.

A tail recursive version of string-merge.