Class 20: Control Flow: Miscellaneous

Held: Wednesday, 29 September 2010

Summary: We continue to consider loops in C, trying not to break our understanding.

Related Pages:

• EBoard.
• Lab: Loops in C.
• Reading: K&R 3.7-3.8.
• Due: Exam 1.

Notes:

• Reading for Friday: K&R 4.1-4.3.
• Don't forget questions on the reading!
• EC for this week's CS Extra on Drupal.
• EC for this week's Convocation on Chemistry.
• Office hours shifted until 2:00-4:00ish today.
• Questions on Exam 1?

Overview:

• Modifying loop control.
• More extreme modifications with goto.
• Questions.
• Lab.

Modifying Control with break and continue

• Two new ways to affect the behavior of loops
  • break - exit the loop now!
  • continue - skip the rest of the body.
• Normally used with a conditional
• Neither is necessary.
  • continue can be handled by an if.
  • break can be handled with a sentinel and a continue

Extreme Modifications to Control: goto

• You can label parts of your code with a name and a colon.
• You can then jump to that part of your code with goto.
• Goto was one of the first control structures invented, and it handles most forms of looping (although some recursive procedures are hard to write with goto).
• Unconstrained use of goto leads to unreadable code.
  • Goto considered harmful was a famous paper in CS.

Questions

Received: SD, RK, CP

Missing: BB, CB, DB, MD, MH, SH, RK, MM, TM, WM, AW

Leftover questions

• How do you decide when to make a function void and when to make a function return int?
• What is /=? Did we learn this and I just forgot?
• j-=gap is in the place that would be occupied by increment things, but I don't know what j- is. Or perhaps it is -=.
• I don't quite understand comma operators. They seem to have something to do with two things being connected, or perhaps both true. I don't understand the advantage of using

    c=s[i], s[i]=s[j], s[j]=c;
  

  as opposed to

    c = s[i];
    s[i] = s[j];
    s[j] = c;"
  

• I don't understand ShellSort. Or maybe I just don't understand nested loops.

New questions

• Why do we use labels?

Lab

• Do the lab.
• Be prepared to reflect.