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derekdreery's solution

to Binary Search in the Rust Track

Published at Jul 13 2018 · 0 comments
Test suite


This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Implement a binary search algorithm.

Searching a sorted collection is a common task. A dictionary is a sorted list of word definitions. Given a word, one can find its definition. A telephone book is a sorted list of people's names, addresses, and telephone numbers. Knowing someone's name allows one to quickly find their telephone number and address.

If the list to be searched contains more than a few items (a dozen, say) a binary search will require far fewer comparisons than a linear search, but it imposes the requirement that the list be sorted.

In computer science, a binary search or half-interval search algorithm finds the position of a specified input value (the search "key") within an array sorted by key value.

In each step, the algorithm compares the search key value with the key value of the middle element of the array.

If the keys match, then a matching element has been found and its index, or position, is returned.

Otherwise, if the search key is less than the middle element's key, then the algorithm repeats its action on the sub-array to the left of the middle element or, if the search key is greater, on the sub-array to the right.

If the remaining array to be searched is empty, then the key cannot be found in the array and a special "not found" indication is returned.

A binary search halves the number of items to check with each iteration, so locating an item (or determining its absence) takes logarithmic time. A binary search is a dichotomic divide and conquer search algorithm.


Rust provides in its standard library already a binary search function. For this exercise you should not use this function but just other basic tools instead.


Slices have additionally to the normal element access via indexing (slice[index]) many useful functions like split_at or getting subslices (slice[start..end]).

You can solve this exercise by just using boring old element access via indexing, but maybe the other provided functions can make your code cleaner and safer.

For bonus points

Did you get the tests passing and the code clean? If you want to, there are some additional things you could try.

  • Currently your find function will probably only work for slices of numbers, but the Rust type system is flexible enough to create a find function which works on all slices which contains elements which can be ordered.
  • Additionally this find function can work not only on slices, but at the same time also on a Vec or an Array.

You can find tests (commented out) for these bonus tasks in the test file.

Then please share your thoughts in a comment on the submission. Did this experiment make the code better? Worse? Did you learn anything from it?

Hints for Bonus Points

  • To get your function working with all kind of elements which can be ordered, have a look at the Ord Trait.
  • To get your function working directly on Vec and Array, you can use the AsRef Trait

Rust Installation

Refer to the exercism help page for Rust installation and learning resources.

Writing the Code

Execute the tests with:

$ cargo test

All but the first test have been ignored. After you get the first test to pass, open the tests source file wich is located in the tests directory and remove the #[ignore] flag from the next test and get the tests to pass again. Each separate test is a function with #[test] flag above it. Continue, until you pass every test.

If you wish to run all tests without editing the tests source file, use:

$ cargo test -- --ignored

To run a specific test, for example some_test, you can use:

$ cargo test some_test

If the specfic test is ignored use:

$ cargo test some_test -- --ignored

To learn more about Rust tests refer to the online test documentation

Make sure to read the Modules chapter if you haven't already, it will help you with organizing your files.

Feedback, Issues, Pull Requests

The exercism/rust repository on GitHub is the home for all of the Rust exercises. If you have feedback about an exercise, or want to help implement new exercises, head over there and create an issue. Members of the rust track team are happy to help!

If you want to know more about Exercism, take a look at the contribution guide.


Wikipedia http://en.wikipedia.org/wiki/Binary_search_algorithm

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


extern crate binary_search;

use binary_search::find;

fn finds_a_value_in_an_array_with_one_element() {
    assert_eq!(find(&[6], 6), Some(0));

fn finds_first_value_in_an_array_with_two_element() {
    assert_eq!(find(&[1, 2], 1), Some(0));

fn finds_second_value_in_an_array_with_two_element() {
    assert_eq!(find(&[1, 2], 2), Some(1));

fn finds_a_value_in_the_middle_of_an_array() {
    assert_eq!(find(&[1, 3, 4, 6, 8, 9, 11], 6), Some(3));

fn finds_a_value_at_the_beginning_of_an_array() {
    assert_eq!(find(&[1, 3, 4, 6, 8, 9, 11], 1), Some(0));

fn finds_a_value_at_the_end_of_an_array() {
    assert_eq!(find(&[1, 3, 4, 6, 8, 9, 11], 11), Some(6));

fn finds_a_value_in_an_array_of_odd_length() {
        find(&[1, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 634], 144),

fn finds_a_value_in_an_array_of_even_length() {
        find(&[1, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377], 21),

fn identifies_that_a_value_is_not_included_in_the_array() {
    assert_eq!(find(&[1, 3, 4, 6, 8, 9, 11], 7), None);

fn a_value_smaller_than_the_arrays_smallest_value_is_not_included() {
    assert_eq!(find(&[1, 3, 4, 6, 8, 9, 11], 0), None);

fn a_value_larger_than_the_arrays_largest_value_is_not_included() {
    assert_eq!(find(&[1, 3, 4, 6, 8, 9, 11], 13), None);

fn nothing_is_included_in_an_empty_array() {
    assert_eq!(find(&[], 1), None);

/* --------------------------------------- Optional Bonus Tests -------------------------------*/

//fn works_for_arrays() {
//    assert_eq!(find([6], 6), Some(0));
//fn works_for_vec() {
//    let vector = vec![6];
//    assert_eq!(find(&vector, 6), Some(0));
//    assert_eq!(find(vector, 6), Some(0));
//fn works_for_str_elements() {
//    assert_eq!(find(["a"], "a"), Some(0));
//    assert_eq!(find(["a", "b"], "b"), Some(1));
use std::cmp::Ordering;

pub fn find<R, T>(haystack: R, needle: T) -> Option<usize>
where T: Ord,
      R: AsRef<[T]>
    let mut haystack = haystack.as_ref();
    let mut idx_acc = 0;
    loop {
        let mid_idx = haystack.len() / 2;
        let mid_key = match haystack.get(mid_idx) {
            Some(k) => k,
            None => { return None; }
        match needle.cmp(mid_key) {
            Ordering::Less => { haystack = &haystack[..mid_idx]; }
            Ordering::Equal => { return Some(mid_idx + idx_acc); }
            Ordering::Greater => {
                haystack = &haystack[mid_idx+1..];
                idx_acc += mid_idx + 1;

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