Lecture 7 Discussion Questions

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1. 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
   

2. 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
   

3. 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
   

4. 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
   

5. 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 '^(.)(.)(.).\3\2\1$' | sed -r 's/^(.)(.)(.).\3\2\1/\1\2\3 \1\2\3/'