For- loop increment not passing to inner block

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

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Re: For- loop increment not passing to inner block by CountZero (Bishop) on Mar 30, 2008 at 20:16 UTC
There are two problems with your code: Assuming `SORTD` is some form of file-handle, the first time you arrive at the `while`-loop (`$i` being equal to 0), the `while`-loop will totally exhaust the filehandle. All following calls to `readline(SORTD)` (when `$i` is incremented and the `while`-loop is entered again, will just return `undef` and the `while`-loop will not even start. Re-entering the loop does not reset the filehandle to the beginning of the file. The value of `$i` remains constant throughout each iteration of the `for` loop. So the `while`-loop will see the same value of `$i` for the whole of the file you are reading. One more comment: it is more customary to write `readline(SORTD)` as `<SORTD>` CountZero A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James	[reply] [d/l] [select]
Re^2: For- loop increment not passing to inner block by Anonymous Monk on Mar 30, 2008 at 20:47 UTC
Ah! Of course! I wasn't resetting the filehandle! (Must. . . have. . . coffee. . ) The filehandle SORTD refers a textfile like this: `0912203749 0725284648 0608294149 0622424347 1219303436 1729313449 1015162331 . . .` [download] These are actually groupings of five 2-digit numbers. My goal was to traverse it by column, using i*2 as a starting index (that part, at least, works fine). I probably should have explained that at the outset but, silly me, I thought the issue was probably too simple to require further elaboration (I won't make that mistake again). Bountiful thanks, CountZero!	[reply] [d/l]
Re^3: For- loop increment not passing to inner block by graff (Chancellor) on Mar 30, 2008 at 21:51 UTC
I might not understand the overall task, but I would be inclined to invert the order of your two loops: the "while" loop over the file data should be the outer loop, and the "for" loop over columns in each line should be the inner loop. If the file in question is not very big (e.g. less than, say, 50 MB on typical desktop machines of the last few years), and you have some reason to prefer your current ordering of the loops, it would be better to read the whole file content into a memory-resident array first, then use a nested "for" loop over that array, rather than repeating your "while" loop over the file contents. The point is that file i/o is quite slow relative to memory-internal operations, so reading data from a file just once (rather than five times) just makes more sense.	[reply]
Re^3: For- loop increment not passing to inner block by GrandFather (Saint) on Mar 30, 2008 at 22:01 UTC
This is bread and butter stuff for Perl and there are many better ways to achieve what you are after. Some are likely to be outside your current comfort zone so sift through the following and see what you like the look of: `use warnings; use strict; while (<DATA>) { chomp; my @numbers = unpack ("(a2)*", $_); print "unpacked: @numbers, "; @numbers = /(..)/g; print "matched: @numbers, "; @numbers = (); push @numbers, substr $_, 0, 2, '' while length $_; print "substr'd: @numbers\n"; } __DATA__ 0912203749 0725284648 0608294149 0622424347 1219303436 1729313449 1015162331` [download] Prints: unpacked: 09 12 20 37 49, matched: 09 12 20 37 49, substr'd: 09 12 20 +37 49 unpacked: 07 25 28 46 48, matched: 07 25 28 46 48, substr'd: 07 25 28 +46 48 unpacked: 06 08 29 41 49, matched: 06 08 29 41 49, substr'd: 06 08 29 +41 49 unpacked: 06 22 42 43 47, matched: 06 22 42 43 47, substr'd: 06 22 42 +43 47 unpacked: 12 19 30 34 36, matched: 12 19 30 34 36, substr'd: 12 19 30 +34 36 unpacked: 17 29 31 34 49, matched: 17 29 31 34 49, substr'd: 17 29 31 +34 49 unpacked: 10 15 16 23 31, matched: 10 15 16 23 31, substr'd: 10 15 16 +23 31 [download] Perl is environmentally friendly - it saves trees	[reply] [d/l] [select]
Re^4: For- loop increment not passing to inner block by Anonymous Monk on Mar 31, 2008 at 05:51 UTC
Re^5: For- loop increment not passing to inner block by GrandFather (Saint) on Mar 31, 2008 at 20:06 UTC
Re^3: For- loop increment not passing to inner block by Anonymous Monk on Mar 30, 2008 at 23:35 UTC
Thanks for the tips all, particularly Grandfather. I can assure you that, to the extent I have a "comfort zone," it's simply habits and prejudices carrying over from other languages I've used. There may be more than one way to do it, but I have no interest in using a Stradavarius to pound nails. I'm always interested in a better way to do it, and if that requires learning new things, that's all the better. Thanks again!	[reply]
Re: For- loop increment not passing to inner block by FunkyMonk (Bishop) on Mar 30, 2008 at 21:42 UTC
Since nobody's mentioned it yet `for (my $i = 0; $i <= 4; $i++) {` [download] is more idiomatically written `for my $i ( 0 .. 4 ) {` [download]	[reply] [d/l] [select]
Re^2: For- loop increment not passing to inner block by GrandFather (Saint) on Mar 30, 2008 at 21:47 UTC
Not only more Perlish (idiomatically), but shorter, clearer, safer and more maintainable - what's not to like?. Perl is environmentally friendly - it saves trees	[reply]
Re: For- loop increment not passing to inner block by pc88mxer (Vicar) on Mar 30, 2008 at 19:27 UTC
By 'inner block' do you mean this: `while (my $line = readline(SORTD)) { my $number = substr($line, $i*2, 2); }` [download] If so, I don't see where you are incrementing `$i`. The way the `for` loop works is that `$i` will not be incremented until the end of the `for` block is encountered, so `$i` is not going to be incremented inside the `while` loop unless you explicitly do it yourself.	[reply] [d/l] [select]
Re: For- loop increment not passing to inner block by igelkott (Priest) on Mar 30, 2008 at 20:02 UTC
Could you possibly have meant: `for (my $i = 0; $i <= 4; $i++) { my $j=0; while (my $line = readline(SORTD)) { $j++; my $number = substr($line, $j*2, 2); } }` [download] Just a wild guess of course but when you wanted the counter of the inner loop to vary, you probably meant to have a different variable. (There are nicer ways of writing this but I wanting the changes to be obvious.)	[reply] [d/l]