Lecture 7 Discussion Questions

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Suppose that we have the following file called emails.txt, where each line contains a list of a person’s emails:
```
scot@uw.edu, scot@cs.washington.edu
may@tech-company.com
ning@nonprofit.ngo
```
What is the command to print the contents of the file such that all of the @ symbols are replaced with at (there should be spaces on either side), The output should look like:
```
scot at uw.edu, scot at cs.washington.edu
may at tech-company.com
ning at nonprofit.ngo
```
Solutions
```
sed -r 's/@/ at /g' emails.txt
```
The following file, poem.txt, is a short poem which has been poorly formatted:
```
I      see      you  driving
round town    with           the  one I love
and   I'm like   haiku
```
Write a command that outputs poem.txt such that there is only a single space between each word.
Solutions
```
sed -r 's/ +/ /g' poem.txt

# or
sed -r 's/\s+/ /g' poem.txt
```
One of the new developers at faang introduced a bug into the source code that incorrectly logged ticket id’s for product transactions. The contents of log.txt are as follows:
```
4444-1111-2222-3333
DDDD-AAAA-BBBB-CCCC
```
The problem is that the first four characters should actually appear at the end of the line. Write a command that has the corrected ticket id’s (i.e.)
```
1111-2222-3333-4444
AAAA-BBBB-CCCC-DDDD
```
Solutions
```
sed -r 's/^(.{4})-(.*)$/\2-\1/g' log.txt
```

The following file, secret.txt contains a conversation between two spies.

Spy 1: I heard that you like secret kit kats
Spy 2: Indeed. I saw Person X eating one the other day.
Spy 1: Did you drop off the secret m&ms?
Spy 2: No, I ate them.
Spy 1: ...
Spy 2: Keep that a secret.

Our goal is to replace all lines of the conversation that contain the word secret with REDACTED. Our output should look like:

REDACTED
Spy 2: Indeed. I saw Person X eating one the other day.
REDACTED
Spy 2: No, I ate them.
Spy 1: ...
REDACTED

Solutions

sed -r 's/^.*secret.*$/REDACTED' secret.txt

Write a command that converts 7 letter palindromes into two words, such that the 4^th character is replaced with a space and the last three characters are reversed. For example, if we ran
```
echo "racecar" | YOUR_COMMAND
```
The output would be
```
rac rac
```
Challenge: Can you write a similar command which has the same behavior, but prints nothing if the input string is not a 7-character palindrome?
Solutions
```
echo "racecar" | sed -r '- `s/^(...)....$/\1 \1`'

# Challenge
echo "racecar" | grep -E '^(.)(.)(.)d.\3\2\1$' | sed -r 's/^(.)(.)(.).\3\2\1/\1\2\3 \1\2\3'
```