For issue

[Csaba]

Hi,

In this simple for demo program:

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class ForIssue
   def main is shared
      for i in 1 : 5
         print i

The output is:
>C:\Cobra\Cobra.bat "ForIssue.cobra"
1
2
3
4
>Exit code: 0

Note, the upper limit, 5 in this case, is not printed..
In other words, the test seems to be <upper limit, not <=upper limit.
The behavior is the same when counting down (for I in 5 :-5 :-1)
Is it intended to be that way in Cobra?

Regards
Csaba

[Charles]

Yes, it's the open half interval approach where the upper bound is exclusive. It works well in languages where valid indexes are 0 through n-1 as in:

t = [1, 2, 3]
for i in 0 : t.count
    t[i] = -t[i]

# btw you can leave out the start when it's zero:
for i in t.count
    t[i] = -t[i]

Cobra ranges, which are modeled after Python ranges, are the same way. t[i:j] --> [t[i], t[i+1], ..., t[j-1]]

[hopscc]

Theres an existing discussion/explanation of this back in
Cobra and the Newbie

[Csaba]

Hi,
The c language has something like “for(0,<a.cont,1)” or you if you want you can write “for(0, a.cont-1,1)” so this is clearer (I never thought I ever in my life would use an C example to show that something is clearer! ).

In Cobra, the implicit <a.count is OK when counting up:

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      fileNames = ["a.xls", "b.ppt", "c.ppt", "a.doc"]
      for i in 0 : fileNames.count # OK
         print i, fileNames[i]

But see what it looks like when counting down:

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      for i in fileNames.count-1: 0-1 :-1         
        print i, fileNames[i]

In my opinion, this looks terrible and is error prone. To avoid this, Cobra could have an iterator that is guaranteed in order (have no better wording for this right now). Iterating up is not really needed, see above (except for symmetry reason), but for iterating down, it would be nice to avoid the example above:

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      forup i, fileName in fileNames
         print i, fileName

      fordown i, fileName in fileNames
         print i, fileName

Or does it already exist? (I think I have somewhere read a discussion about in-order iterators). Or are there better ways already? Anyhow, the count down example in an array has to be documented for newbies to Cobra.

Regards
Csaba

[Charles]

Here is what I came up with:

class X

    def main is shared
        print
        for i in 0 : 5 : 1
            print i stop
        print

        # same as above:
        print           
        for i in 5
            print i stop
        print
        
        # reversed:
        print
        for i in 5 : 0 : -1
            print i stop
        print
        
        print
        t = ['a', 'b', 'c', 'd', 'e']
        for i in t.count : 0 : -1
            print t[i-1] stop
        print

        print
        t = ['a', 'b', 'c', 'd', 'e']
        i = t.count
        while i > 0
            print t[i-1] stop
            i -= 1
        print

And the output is:

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01234

01234

54321

edcba

edcba


Note that the while loop starts at t.count and stops at > 0. Consequently, it has to use i-1. The for loop is no different.

Also, the for loops use of "start : stop : step" is modeled directly after Python's slicing and it's range functionality and matches their "end excluded" semantics.

Whether you want to use "i-1" in the loop, as I did, or adjust the boundaries as you did:

for i in fileNames.count-1 : -1 : -1
    ...

is a matter of style. Actually the "-1 : -1 : -1" forms a neat visual pattern. Pull the "0-" out and make the spacing around ":" uniform and I don't think it looks so bad.

Another approach:

for i, name in fileNames.reversed.numbered
    print i, name

I supposed we could make a more efficient method by having a version of .numbered that goes in reverse without having to produce a new list. Is .reverseNumbered an okay name for that?

I agree this needs documentation.

[hopscc]

.numberedDown (pref)
or
.numberedBack/.numberedBackwards

[Csaba]

Hi Chuck,
Thank you for the answers! I have not programmed so much in Python so I did not know that the For-statements worked like this. I think it is good to mention in the documentation that the behavior is inherited from Python.

For counting down, I would also prefer the suggestion from hopscc: numberedDown.

Regards
Csaba

[Charles]

.numberedDown it is and checked in just now.