Cervantes wrote a more basic tutorial on loops and forloops that explain things better and is available here
Mine covers a bit more advanced consepts and should be read after understanding of the basics
Loops allow you to loop through a part of your program until a condition, so that you dont have to type up the same code over and over again.
the most basic loop:
| code: |
loop put "compsci.ca" end loop |
this will keep on outputing compsci.ca on the screen until program terminates... Alt F4 in most cases
Now knowing of how loops behave (as soon as end loop line is reached, control jumps back to loop line) we can add a little counter in there to tell us how many times we runned through the loop. There are two kinds
a) pre-incroment
thats when you have your counter at the BEGINNING of the loop
| code: |
loop counter:= counter + 1 put counter end loop |
and b) post-incroment
thats when you have your counter at the END of the loop
| code: |
loop put counter counter:= counter +1 end loop |
the difference? Not much... mostly has to do with where you put your exit statment (read down v ). But some teachers have an issue with what counters you use. Just to compare two, here're their outputs
pre-incroment: 1 2 3 4 5..
post-incroment: 0 1 2 3 4..
Basically you put your counter on the OPPOSITE side of your exit statment (more on that).
EXIT WHEN STATMENT (finally!)
you'd have to exit your loop sooner or later... thats where exit statment comes in. Its a kind of if statment that exits your loop (jumps to end loop line).
| code: |
loop counter:=counter+1 put counter exit when counter = 5 end loop put "the end" |
the output will be 1 2 3 4 5 the end
Now if you would put your exit when statment in the beginning of the loop and used a post-incroment counter, its possible to skip through the whole loop. (you can use an exit to get out of the loop of put a true condition into exit when like exit when 1=1 if you want to get out on purpose).
LOOP TRICKS
There aren't much tricks, but you can use your loops to do something they weren't ment to do... You can fake goto line that turing is missing.
If at sertain point you want to skip a part of the code, you put that code into a loop with exit on the line right before end loop. This way the loop will NEVER repeat, but now you have the power to skip right to the end with another exit statment in the middle of your code!!
here's how:
| code: |
var text:string put "this will fake a goto line" loop put "would you like to skip next few lines? y/n" get text exit when text = "y" put "you chose not to skip this lines" put "now you will suffer" put "bwahahaha" put "some more unwanted lines" exit end loop put "the end" |
I suppose you could do the same thing with an if statment, BUT you cant if you want to have 2 or more exit points
More tricks - abusing fake goto line
Good programmer doesnt just memorize programming syntax but understands it. Great programmer explains to others how you can use that sytax in the new ways 8)
More of faking goto line. Before you could only jump farward to the end of the loop. But if you understand basic syntax you can figure out a way to jump BACK few lines
Here's how:
| code: |
var text:string loop put "beginning of the program" loop put "would you like to go back to the beginning? y/n" get text exit when text = "y" put "you didnt chose to go to the beginning and fix your errors" put "now you have to live in regret" put "compsci.ca is cool" exit end loop exit when text not= "y" end loop put "the end" |
In the inside loop, we use the same goto end trick, but now we also have an OUTSIDE loop that will bring us back to the beginning of the program. Notice how outside loop exit when is structured. It will exit the loop ONLY if you chose not to go back to the beginning.
The advantages of being able to skip through a portion of the code (or perhaps go back
) will prove useful when aim for efficiency of your program. Such as if you already know the result, there's no point in making computer do unnececary calculations.
Also you gain more control over your code, such as in a game - an ability to go back and change your stats if you made an mistake while chosing them, or being able to exit the intro story at vertually any point.
)
