grep

Grep

Search a file for lines matching a regular expression pattern. Return the line number and contents of each matching line.

The Unix grep command can be used to search for lines in one or more files that match a user-provided search query (known as the pattern).

The grep command takes three arguments:

1. The pattern used to match lines in a file.
2. Zero or more flags to customize the matching behavior.
3. One or more files in which to search for matching lines.

Your task is to implement the grep function, which should read the contents of the specified files, find the lines that match the specified pattern and then output those lines as a single string. Note that the lines should be output in the order in which they were found, with the first matching line in the first file being output first.

As an example, suppose there is a file named "input.txt" with the following contents:

hello
world
hello again

If we were to call grep "hello" input.txt, the returned string should be:

hello
hello again

Flags

As said earlier, the grep command should also support the following flags:

• -n Print the line numbers of each matching line.
• -l Print only the names of files that contain at least one matching line.
• -i Match line using a case-insensitive comparison.
• -v Invert the program -- collect all lines that fail to match the pattern.
• -x Only match entire lines, instead of lines that contain a match.

If we run grep -n "hello" input.txt, the -n flag will require the matching lines to be prefixed with its line number:

1:hello
3:hello again

And if we run grep -i "HELLO" input.txt, we'll do a case-insensitive match, and the output will be:

hello
hello again

The grep command should support multiple flags at once.

For example, running grep -l -v "hello" file1.txt file2.txt should print the names of files that do not contain the string "hello".

Error handling

This exercise introduces the anyhow crate, which makes it easy to handle arbitrary error types. Its intent is to ensure that when you're writing an application, you don't have to worry about what particular errors your called function is returning, but to just do the right thing when propagating them.

N.B.: it is actually somewhat bad form to use anyhow when writing a library, as we are here; it's more explicit and more useful to write your own Error enum when writing a library (potentially with the aid of helper macros such as are provided by the thiserror crate). However, we are intentionally and explicitly doing so here to demonstrate the use of this crate.

To learn more about this crate refer to its documentation.

Additional reading

While this exercise asks you to implement only the most basic functions of grep, there is actually a project to fully re-implement grep in Rust - ripgrep.

If you liked the concept of rewriting the basic util programs in Rust be sure to check the following projects:

• fd - a clone of find
• exa - a clone of ls
• bat - a clone of cat
• coreutils - a rewrite of the GNU coreutils

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 which 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 ignored 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 specific 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.

Further improvements

After you have solved the exercise, please consider using the additional utilities, described in the installation guide, to further refine your final solution.

To format your solution, inside the solution directory use

cargo fmt

To see, if your solution contains some common ineffective use cases, inside the solution directory use

cargo clippy --all-targets

Submitting the solution

Generally you should submit all files in which you implemented your solution (src/lib.rs in most cases). If you are using any external crates, please consider submitting the Cargo.toml file. This will make the review process faster and clearer.

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.

Source

Conversation with Nate Foster. http://www.cs.cornell.edu/Courses/cs3110/2014sp/hw/0/ps0.pdf

Submitting Incomplete Solutions

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