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Re^3: If I was forced to use only one kind of loop for the rest of my days it would be a

by Roger_B (Scribe)
on Oct 01, 2005 at 13:35 UTC ( #496638=note: print w/replies, xml ) Need Help??


in reply to Re^2: If I was forced to use only one kind of loop for the rest of my days it would be a
in thread If I was forced to use only one kind of loop for the rest of my days it would be a

While it's true you can do anything you can do with a for loop with a while loop, I picked for.

I really like the capability that for gives you to keep all the operations controlling the loop - initialisation, incrementing and testing - in one place. In C99 (and Perl) you can declare variables in the for statement, and they will have a scope of precisely the for loop. You'll need an extra block to do that with while. So taking the example above, I assume this is intended to synthesise (updated:) ambrus noted that this is equivalent to:

// UNTESTED for(int a = 1; a<10; a++) { printf("%d\n",a); }

In fact, you'll need an extra block to get the correct scope:

// UNTESTED // UPDATED: de-obfuscated { int a = 1; while(a<10) { printf("%d\n",a); a++; } }

Which is starting to look rather messy.

Context switch back to Perl here!

Synthesising while and foreach from for seems reasonably intuitive to me on the other hand.

# UNTESTED for (;EXPR;) BLOCK # leave first and third EXPRs blank to be # equivalent to while (EXPR) BLOCK # UNTESTED for (my @list = LIST, VAR = shift @list; # iterate manually through @list; # LIST in "for" to be VAR = shift @list) BLOCK # somewhat like # foreach VAR (LIST) BLOCK # without the usual aliasing

OK, the foreach is starting to look a bit funny, but a foreach from while will be worse, and spread into the block, rather than be confined to the statement itself.

One issue is that for doesn't have a continue block, but as I come from C I don't tend to use those. continue blocks also split up code that happens every time round the loop, which puts me off. for's third expression is roughly equivalent I suppose, but stuffing a large continue block there could hardly be called good style.

Recursion is another story but I don't think I could live with it as the only looping mechanism. I'd rather have the reverse issue of synthesising recursion, say with a list of hashes, on the rare occasions I really needed it.

updated in response to ambrus's comments.

Also, note:

perl -MO=Deparse -e "for (;<>;){print}" while (defined($_ = <ARGV>)) { print $_; } -e syntax OK

the for deparses as a while!

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Re^4: If I was forced to use only one kind of loop for the rest of my days it would be a
by ambrus (Abbot) on Oct 02, 2005 at 13:10 UTC

    In fact, I belive

    for(int a = 0; a<10; a++) { printf("%d\n",a); }
    is equivalent to
    { int a = 0; while(a<10) { printf("%d\n",a); a++; }
    Your other code,
    { int a = 0; while(a++,a<10) { printf("%d\n",a); } }
    doesn't run the printf statement with a = 0.

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