Re: For loop trouble

Replies are listed 'Best First'.
Re^2: For loop trouble by cheech (Beadle) on Jun 13, 2009 at 16:55 UTC
I was taught that for structure was like this: (starting value, ending value, increment). But I infer from your response that it must be: (starting value, continue while this is true, increment)?	[reply]
Re^3: For loop trouble by JavaFan (Canon) on Jun 13, 2009 at 17:03 UTC
It's `for (EXPR1; EXPR2; EXPR3) {...}`. Note that there are semi-colons between the expressions, not commas. The first expression is run before entering the loop. Before each iteration the second expression is run; if false, the loop terminates. After each iteration, the third expression is run. For further details, see `man perlsyn`.	[reply] [d/l] [select]
Re^4: For loop trouble by afoken (Chancellor) on Jun 13, 2009 at 17:14 UTC
Note that there are semi-colons between the expressions, not commas. In this case, the OP has constructed a foreach loop iterating `$_` over a list of three elements: `$i=500, $i=900 , $i += 100`, which evaluates to `500, 900, 1000` and `$i` set to 1000 for ALL iterations of the loop. Alexander -- Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)	[reply] [d/l] [select]
Re^4: For loop trouble by cheech (Beadle) on Jun 13, 2009 at 18:03 UTC
Ugh the carelessness.. Thank you, I just needed to realize semi colons were to be used.	[reply]
Re^3: For loop trouble by afoken (Chancellor) on Jun 13, 2009 at 17:07 UTC
Perls three-argument for loop is exactly like in C: initializer, test, and counting. See also http://en.wikipedia.org/wiki/For_loop#Three-expression_for_loops. Other languages may implement for loops differently, notably BASIC and its derivates often have constructs like `for i=1 to 100 step 3` (counting up from 1 with increments to 3 until 100 is reached or exceeded) or even `for i=100 downto 1 step 3` (counting down in steps of 3, starting at 100, stopping when 1 is reached or exceeded). Other languages have even stranger for loops, see the wikipedia article. Alexander -- Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)	[reply] [d/l] [select]
Re^3: For loop trouble by ikegami (Patriarch) on Jun 13, 2009 at 20:02 UTC
I was taught that for structure was like this: (starting value, ending value, increment). But I infer from your response that it must be: (starting value, continue while this is true, increment)? Won't hurt you to read the docs... What's wrong: The first expression has nothing to do with a "starting value". (Starting value of what??? It's not like C-style loops have counters.) The last expression has nothing to do with an "increment". (Incrementing what??? It's not like C-style loops have counters.) What it really means The first expression (if specified) is evaluated unconditionally before the loop is started. The second expression is evaluated at the start of every loop pass (incl the first). If an expression is specified and it evaluates to something false, the look exits. The third expression (if specified) is evaluated at the end of every loop pass (even if `next` is called). A common usage: `for (my $i = 0; $i < $n; ++$i) { ... }` [download] The above is equivalent to the following, except `$i` is scoped to the loop. `my $i = 0; while ($i < $n) { ... } continue { ++$i }` [download]	[reply] [d/l] [select]