mirror of
https://github.com/Xevion/exercism.git
synced 2025-12-07 05:15:03 -06:00
115 lines
3.9 KiB
Markdown
115 lines
3.9 KiB
Markdown
# Grep
|
|
|
|
Search a file for lines matching a regular expression pattern. Return the line
|
|
number and contents of each matching line.
|
|
|
|
The Unix [`grep`](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/grep.html) 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:
|
|
|
|
```text
|
|
hello
|
|
world
|
|
hello again
|
|
```
|
|
|
|
If we were to call `grep "hello" input.txt`, the returned string should be:
|
|
|
|
```text
|
|
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:
|
|
|
|
```text
|
|
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:
|
|
|
|
```text
|
|
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".
|
|
|
|
## Exception messages
|
|
|
|
Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to
|
|
indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not
|
|
every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include
|
|
a message.
|
|
|
|
To raise a message with an exception, just write it as an argument to the exception type. For example, instead of
|
|
`raise Exception`, you should write:
|
|
|
|
```python
|
|
raise Exception("Meaningful message indicating the source of the error")
|
|
```
|
|
|
|
## Running the tests
|
|
|
|
To run the tests, run the appropriate command below ([why they are different](https://github.com/pytest-dev/pytest/issues/1629#issue-161422224)):
|
|
|
|
- Python 2.7: `py.test grep_test.py`
|
|
- Python 3.4+: `pytest grep_test.py`
|
|
|
|
Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version):
|
|
`python -m pytest grep_test.py`
|
|
|
|
### Common `pytest` options
|
|
|
|
- `-v` : enable verbose output
|
|
- `-x` : stop running tests on first failure
|
|
- `--ff` : run failures from previous test before running other test cases
|
|
|
|
For other options, see `python -m pytest -h`
|
|
|
|
## Submitting Exercises
|
|
|
|
Note that, when trying to submit an exercise, make sure the solution is in the `$EXERCISM_WORKSPACE/python/grep` directory.
|
|
|
|
You can find your Exercism workspace by running `exercism debug` and looking for the line that starts with `Workspace`.
|
|
|
|
For more detailed information about running tests, code style and linting,
|
|
please see [Running the Tests](http://exercism.io/tracks/python/tests).
|
|
|
|
## Source
|
|
|
|
Conversation with Nate Foster. [http://www.cs.cornell.edu/Courses/cs3110/2014sp/hw/0/ps0.pdf](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.
|