4 basic [control] structures of programming:

1. sequence
2. branching
3. procedure
4. repetition

...and TODAY we are introducing repetition!

*   in the 1970's, two Italian mathematicians proved that
    we can specify any problem solution statement using
    only these control structures

*   when you are at the body step of the design recipe,
    and you see multiple steps --- figure out LOGICALLY
    what these are before trying to code them;

    ... you might draw them in a flowchart (graphically),

        you might write them out in PSEUDOCODE

    *   for OUR purposes, pseudocode is WHAT we want
        to do, in the ORDER we want to do it, written in
        whatever combo of English/code makes sense to us

    *   for diff_profits1: here is pseudocode!

        print out a header

        print out all the profits for ticket prices from $4 to $5
            in dime increments

*   HANDY PICO TIP: ctrl-w then ctrl-t then line number then enter
    ... takes you STRAIGHT to that line number

*   now, to redo diff_profits1 from $1 to $7? UGH!

*   so, REPETITION.

    *   one of SEVERAL statements in Python for repetition
        is the CLASSIC while-loop

    *   while bool_expr:
            action1
            action2
            ...

    *   DRAW FLOW CHART from BOARD in your NOTES!

    *   1. when the while is reached, 
        2. evaluate bool_expr
        3. if it is true, do action1, action2, ...
        4. go to 2
 
*   SO --- diff_profits2.py will now use a loop!

    *    NOTE: almost every loop needs at least one
         LOOP CONTROL VARIABLE --- its contents affect
         when the loop ends...

*   NOTE: 4 most common loop errors:

    *  go one time too many
    *  go one time too few
    *  never enter the loop
    *  never leave the loop (infinite loop!)

*   WHAT IF... you get an infinite loop?

    1.  type ctrl-c to try to send a keyboard interrupt to
        TERMINATE the loop

    2.  open another cs-server session.

        at the cs-server prompt, type:

        cs-server> ps -ef | grep your_user_name
  
        LOOK for the python process (name is on the right)
        GRAB its process id from the 2nd column

        AT the cs-server prompt, kill it as follows:

        cs-server> kill that_id

*   some loops are unique --- others follow a few common
    styles

    *   count-controlled loop
    *   event-controlled loop
    *   sentinel-controlled loop

*   count-controlled loop

    *   you want to do something a specified number of times

    *   1st: you set up a counter variable to 0

    *   2nd: you loop while the counter is less than its limit

        *   IN the loop, you increment the counter

    *   spam1.py