[tutorial] loops and faking goto line

[Tony]

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.

[atrain]

only problem is you end up with a ton of loops........
If you constanty need to go to something, i use procedures or it statments


if you have a main loop and a sub loop and a loop inside that it gets anoying...

[theguru]

atrain wrote:

only problem is you end up with a ton of loops........
If you constanty need to go to something, i use procedures or it statments


if you have a main loop and a sub loop and a loop inside that it gets anoying...

I know what you mean. I've been trying to use that for my 'Rock, Paper, Scissors' program to let the user to go back and fourth between the different screens and I have 6 loops nested with each other. it's probably really inefficient but not like i care

You should just use that 'fake goto' trick if you want to verify a users input.

code:

put "enter choice: " .. get choice loop   put "you entered ", choice, " are you sure (n=no)?" ..   getch (sure)   exit when ord(sure) ~= <insert ordinal value of 'n' here> end loop

[Raknarg]

I've been doing this for a while and my teacher says its bad programming practice. Would you agree with this?

[unoho]

since im feeling like trolling,
in line 2, should be "concepts" rather than "consepts"
and "sertain" should be "certain" under loop tricks

hehe

[Tony]

Since I'm feeling like reminding you that the original post was published in 2003 -- except to find high-school level writing (meaning sub-par )

Also, @Raknarg -- what exactly is "this" in "I've been doing this"? goto is generally considered to be bad due to issues with legibility (spaghetti code from jumping all over the place), and safety (jumping into arbitrary spots). The above code is closer to Java's "goto" (labelled breakpoints), which address both points explicitly, _but_ Turing's code doesn't have labels and relies on tricky exit conditions, so the clarify is possibly not ideal.

[unoho]

ouchhh... burnn..u got me..i didn't see that coming lol

[Raknarg]

Dear god Tony, I was refering to the fake goto line in general. I posted here because there was a topic here already and I didn't have time to make an actual topic. If your going t reprimand me, don't be an ass about it.

I know full well its necro posting, but I beleive there's an exception that if you had something worthwhile to add, its allowed. I felt it was a worthwhile question.

[Tony]

Sorry, the "2003" part of the comment was directed towards unoho. Your question was rather vague in the context it was posted, but I hope that I've addressed the technical parts.

[Raknarg]

Ok thanks.

[mirhagk]

Not if it helps much, but bad programming practice is generally whatever you define it to be. Often there's a constant struggle between speed of writing vs speed of execution. Generally you need to find something that works for you, or rather just listen to what your boss/teacher tells you to do.

[Raknarg]

Oh yeah. I personally don't see the error really, buyt me teacher told me today that he really dislikes it when I do it, so I'll have to find some other means for the time being :/

[mirhagk]

The error is mostly in the fact that it looks messy, and there are ways to do it that look a lot nicer. GOTO's are never really needed, they are just useful, and they were removed from modern languages for a reason. (mostly cuz what happens to local variables? what happens with try/catch blocks? There were too many unanswered questions)

[user23]

Ahh there must be some better / more efficient way to get it done?

[Raknarg]

Well, there's different ways to do it. The way my teacher is telling me to do it is just by using an if statement.

Instead of:

Turing:

loop      stuff      exit when (condition)      more stuff      exit end loop

You could use:

Turing:

if (condition) then      stuff else      stuff      more stuff end if

Get it?