Reverse String in Scheme

Published on 07 May 2018 (Updated: 15 May 2023)

Welcome to the Reverse String in Scheme page! Here, you'll find the source code for this program as well as a description of how the program works.

Current Solution

(define (reverse-string x)
  (list->string (reverse (string->list x))))

(if (> (length (command-line)) 1)
  (display (reverse-string (list-ref (command-line) 1)))

Reverse String in Scheme was written by:

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How to Implement the Solution

As you can see, we can write a script to reverse a string in five lines of code (not counting the blank line). In the following sections, we'll take a look at a breakdown of each of these lines.

The Function Definition

(define (reverse-string x)

The keyword define binds a function's definition to the specified name. This keyword is followed by the name of the function and the function's argument. In this particular case, the argument x is the string we want to reverse.

The Function Body

(list->string (reverse (string->list x))))

The whole magic happens in the second line. We will go through it step by step from the inside out, since this makes understanding it easier.

The most inner pair of parentheses calls the conversion of our string x to a list. Then, this list is reversed by calling the function reverse which is part of the standard library. Finally, the reversed list is converted back to a string. This is also the return value of the function reverse-string.

Check if Enough Command-Line Arguments

(if (> (length (command-line)) 1)

Let's look at this starting from the inner pair of parentheses. command-line is a list containing the command-line arguments. The first value is the name of the script. The rest are the user-specified arguments. The length method returns the number of command-line arguments. The > takes two arguments and indicates a "true" value if the first argument is greater than the second. The if statement will only execute the code in the outermost parentheses for a "true" value. Therefore, we only do something if the number of command-line arguments is greater than 1. Otherwise, the program exits.

The Display

(display (reverse-string (list-ref (command-line) 1)))

The first thing you see there, is the display function. You'll probably remember it from our Hello World experience, so have a look there, if you need a quick refresher of what it does.

The argument to this function is the reverse-string function defined above. It receives the second command-line argument (the string to reverse), which is accessed with the function list-ref. list-ref takes two arguments: the list and an index. In this case, the list is the command-line and the index is 1, which is the string to reverse. Finally, display outputs to reversed string.

If you replace (list-ref (command-line) 1) with a string, the program prints that string reversed. To show you an example, the output of (display (reverse-string "Hello")) is olleH.

How to Run the Solution

The quickest way is probably to try use an online Scheme interpreter. Just copy the code above, drop it into the editor, fill in some input and hit run.

As an alternative, you can download CHICKEN Scheme and a copy of the solution file from Github. Assuming CHICKEN Scheme is on your path, you can run the script from a command line with the following command:

csi -s reverse-string.scm

This will run the Scheme file which will print out whatever you enter on the command line.