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Iterators - expressions tha change each time you call them

If you're programming and you write the same expression into your code twice without changing any of the variables, you're simply writing the same expression twice, right? For example, here's a piece of Perl code that exits if $userval is 0, but adds it in to a total and keep going if it's not zero.

if ($userval == 0) { 
print "job done\n";
exit;
}
$total += $userval;


If you're reading from a file, though, things are different. Refer to the same file handle twice, and you'll get the next line read in each time - thus:


if (<FH> == 0) { 
print "job done\n";
exit;
}
$total += <FH>;


will read from a file handle TWICE - if the first line read is zero, the program exits. If the first line was NOT zero, a second line is read and that value in that line is added to $total.

Why the difference? Because the read from operator - <> in Perl - is what we call an iterator, and each time we refer to it, it moves on to the next value.

This one often catches new programmers ... the solution is to read the value just once and save it into a variable - thus:

$userval = <FH>;
if ($userval == 0) {
print "job done\n";
exit;
}
$total += $userval;

(written 2006-04-27)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P207 - Perl - File Handling
  [3839] Spraying data from one incoming to series of outgoing files in Perl - (2012-08-15)
  [3830] Traversing a directory in Perl - (2012-08-08)
  [3548] Dark mornings, dog update, and Python and Lua courses before Christmas - (2011-12-10)
  [3326] Finding your big files in Perl - design considerations beyond the course environment - (2011-06-14)
  [2833] Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27)
  [2821] Chancellor George Osborne inspires Perl Program - (2010-06-22)
  [2818] File open and read in Perl - modernisation - (2010-06-19)
  [2405] But I am reading from a file - no need to prompt (Perl) - (2009-09-14)
  [2233] Transforming data in Perl using lists of lists and hashes of hashes - (2009-06-12)
  [1861] Reactive (dynamic) formatting in Perl - (2008-10-31)
  [1860] Seven new intermediate Perl examples - (2008-10-30)
  [1841] Formatting with a leading + / Lua and Perl - (2008-10-15)
  [1709] There is more that one way - Perl - (2008-07-14)
  [1467] stdout v stderr (Tcl, Perl, Shell) - (2007-12-10)
  [1442] Reading a file multiple times - file pointers - (2007-11-23)
  [1416] Good, steady, simple example - Perl file handling - (2007-10-30)
  [1312] Some one line Perl tips and techniques - (2007-08-21)
  [867] Being sure to be positive in Perl - (2006-09-15)
  [618] Perl - its up to YOU to check your file opened - (2006-02-23)
  [616] printf - a flawed but useful function - (2006-02-22)
  [255] STDIN, STDOUT, STDERR and DATA - Perl file handles - (2005-03-23)
  [114] Relative or absolute milkman - (2004-11-10)
  [12] How many people in a room? - (2004-08-12)


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