CSC302 2007S, Class 04: Goto Considered Harmful

Admin:
* Thursday at noon: Summer research opportunities.
* Questions on HW2?
* Friday's reading is the same as Wednesday's, 
  so no readings distributed today.

Overview:
* Context.
* Dijkstra's Famous Letter.
* Permutation with Repetitions.

/Context/

* The Story of Mel, A Real Programmer
  * Mel was programming in hexadecimal, rather than a "real"
    programming language
  * Why?
    * Macho
	  * Awesome hack: Integer rollover exits loop
	* Wanted to control details that a higher-level language 
	  would make it impossible to control
	  * Where instructions were on the memory drum
	    * Permits instructions to be used as instructions AND
		  as data
		* Could produce more efficient code - optimized so that
		  seek is not costly
		  * Also pessimized
  * The problem?
    * Really hard to tell what the code does
	* Really hard to change

* The context of GTSCH
* Current growth in "high level" languages
* But goto is "low level"
* So why was it in high-level languages?
  * The focus was less on control and more on other issues
    * Data representation
	* Location of data and program in memory
    * Temporaries and order-of-operations in arithmetic expressions
* Control structures were "discovered"/"invented"
  * But goto remained
* Go to identified as a problem by various programmers
  * Go-to ridden code is hard to read/understand and modify/fix
* Dijsktra served as "spokesperson" for a wide variety of
  computer scientists

/Go To Statement Considered Harmful/

* Published in 1968 in "Communications of the ACM"
* A "letter to the editor"
* Thesis
  * Programs with goto have difficult-to-understand dynamics
    * "The unbridled use of the go to statement has an immediate consequence that it becoms terribly hard to find a meanigful set of corrdinates in which to describe the process progress."
	* "The go to statement should be abolished from all "higher level" programming languages"j
* Argumentation strategy: Figuring out the sequence of textual/dynamic indices is harder in goto-laden programs
* Also: You can do everthing that goto can without goto.
*  Why is the sequence of textual/dynamic indices is harder in goto-laden programs so important?
  * To understand the value of a variable, you need to know where you are in the program (example of "number of people in the room")
  * Can help the reader, as he/she tries to "hand execute" the program

Counter-arguments?
* There are ways to use "goto" correctly
  * E.g., to make it more like the while loop
  * Don't always want to enter a while loop at the top
  * Sometimes want early exit from loops
* Do you trust the programmer?

Permutation with Repetitions on Wednesday