Overview

• Preliminaries.
  • Admin.
• Fun with GitHub.
• Go over homework.
• Exercise: The spaces problem.
• Raymond, chapter 1.
• Thinking about basic tools.

Preliminaries

Admin

• I encourage you to attend CS table on Friday and talk about the ACM code of ethics.
• Status check 1: How many of you know the C bitwise operations? (&, |, etc.)
• Status check 2: Could anyone not make class if I moved it to 3:15pm? (Hands closed, blackball-style question.)
  • We are sticking with 1:15 p.m.
• Note: I was overly optimistic at the start of the semester. Given the amount of time daily homework is taking in my three classes, I probably won't get a lot of chance to write the "book" for this class. I will try to set up some lab problems, though.

Questions

Fun with GitHub

• What happens when someone sends a pull request?
  • GitHub sends you a nice message with instructions.
  • You can browse the request to see how someone wants to screw up the repository.
• Isn't our site lovely?
  • Careful instructions and agreed-upon policies help

Go over homework

That fun hypothetical C problem. (Did anyone write code to cause

the error?)

int *x;     // Our array of data.
int 
main (int argc, char *argv[])
{
  x = malloc (...);      
  foo ();
  bar ();
  free (x);   // Crash
} // main

• Multi-threaded program, foo or bar accesses x after it should.
  • foo and bar have terminated.
• malloc keeps track of how much space it has allocated

• Note: "You went beyond the null at the end of the array."

  • Arrays are not null terminated in C.
  • Strings are null terminated.

  char *str = malloc (sizeof (char) * 16); strcpy (str, "hello"); printf ("%d\n", strlen (str));

Spell checking

• Finding information, mechanism one: Google
• Check the manual pages man -k spell
  • The -k is "keyword"
• So, our first solution is to use spell, but it gives us duplicates. What next?
  • Read the manual page
  • Use uniq - not quite
• spell file | sort | uniq

URL extraction

grep -i -o '<img[^>]*src=[^>]*>' input | grep -o '"[^"]*"'

The first part gets you lines that look like

The second part extracts the stuff in quotation marks

Flaws in this solution?

• What if there are other things in quotation marks? Those get printed too. Boo.
  • grep -o 'src="[^"]"' | sed -e 's/^.src="//' | sed -e 's/".*$'//'
• What if there are multiple images on the same line? Works okay.
• What if the thing in quotation marks has a greater-than sign? (Questionable HTML.) Punt.
• What if the image is in single quotes rather than double quotes?

The CSV Problem

"I want the names of the people with the five highest grades on HW2." File has

fname:lname:assignment:grade

• grep to find the lines that contain HW2
• sort to put them in numeric order (sort -k4 -n -r)
• cut to remove columns
• head to take the top 5

Exercise: The spaces problem

    sed -e 's/ *$//'

Write it in C. You have ten minutes.

While input characters remain read the next input character switch (ch) case space: shove it in the buffer break; case newline: clear the buffer break; default: print the buffer clear the buffer print ch; break

Raymond, chapter 1

Thinking about basic tools

Homework

1. Learn Markdown so that we can use it for these assignments.

2. Learn about how you set and get environment variables. Put your explanations in the submissions directory, with a file named after you.

   • Command line
   • Bash
   • C (get only)

3. Update our list of "things every *nix user should know" (either in the repo

4. Come up with two or three tasks, similar to those we did for assignment 1, and put notes about them in the tasks directory. (You can include some sample solutions in your file.)

5. Read the first chapter of the Make book.