Recursive atoi

Learning Goals

  • Deepen an understanding of strings
  • Practice creating recursive functions


Image that you travel back in time to the 1970’s, when the C programming language was first created. You are hired as a programmer to come up with a way to convert strings to ints. (You may recall that you used a function just like this in Week 2, called atoi). You want to be thorough in your development process and plan to try several approaches before deciding on the most efficient.

In this problem, you will start with a simple implementation of atoi that handles positive ints using loops. You want to rework this into an implementation that uses recursion. Recusive functions can be memory intensive and are not always the best solution, but there are some problems in which using recursion can provide a simpler and more elegant solution.

(Scroll to the bottom of this page to see what an implementation of atoi might actually look like.)

  • Hints
    • Start by getting the index of the last char in the string (the char before the \0).
    • Convert this char into its numeric value. Can you subtract some char to do this?
    • Remove the last char from the string by moving the null terminator one position to the left.
    • Return this value plus 10 times the integer value of the new shortened string.
    • Remember you need a base case when creating a recursive function.


Getting Started

  1. Log into using your GitHub account.
  2. Click inside the terminal window and execute cd.
  3. Execute wget followed by Enter in order to download a zip called in your codespace. Take care not to overlook the space between wget and the following URL, or any other character for that matter!
  4. Now execute unzip to create a folder called atoi.
  5. You no longer need the ZIP file, so you can execute rm and respond with “y” followed by Enter at the prompt.
  6. Finally, right-click or control-click on the atoi folder and click “Open in CS50 Lab”. You should see the specification for this problem on the left-hand side and its distribution code on the right-hand side.

Implementation Details

In the recursive version of convert, start with the last char and convert it into an integer value. Then shorten the string, removing the last char, and then recursively call convert using the shortened string as input, where the next char will be processed.

Thought Question

Why do you need a base case whenever you create a recursive function?

How to Test Your Code

Your program should behave per the examples below.

atoi/ $ ./atoi
Enter a positive integer: 3432
atoi/ $ ./atoi
Enter a positive integer: 98765

No check50 for this one!

To evaluate that the style of your code, type in the following at the $ prompt.

style50 atoi

How to Submit

No need to submit! This is an optional practice problem.

A More Thorough Implementation

The actual version of atoi must handle negative numbers, as well as leading spaces and non-numeric characters. It might look something like this:

#include <stdio.h>
// Iterative function to implement `atoi()` function in C
long atoi(const char S[])
    long num = 0;
    int i = 0, sign = 1;
    // skip white space characters
    while (S[i] == ' ' || S[i] == '\n' || S[i] == '\t') {
    // note sign of the number
    if (S[i] == '+' || S[i] == '-')
        if (S[i] == '-') {
            sign = -1;
    // run till the end of the string is reached, or the
    // current character is non-numeric
    while (S[i] && (S[i] >= '0' && S[i] <= '9'))
        num = num * 10 + (S[i] - '0');
    return sign * num;
// Implement `atoi()` function in C
int main(void)
    char S[] = " -1234567890";
    printf("%ld ", atoi(S));
    return 0;