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Equality and looks like tests - Perl

Whenever you do an equality check in a Perl program, you must think whether you're checking if two numbers are equal, if two test strings are equal, or if a string looks like a pattern. And you write different code in each case:

Checking numbers: If ($stuff == 6) { ...
Tests whether $stuff contains the number 6 (or a string that evaluates to the number 6, or the number 6.0)

Checking text string equality: If ($stuff eq "Well House") { ...
Tests whether $stuff contains exactly the string "Well House"

Checking a text string against a pattern: If ($stuff =~ /hotel/i) { ...
Tests whether the string in $stuff contains, somewhere within it, the word "hotel" in upper case, lower case, or a mixture.

You need to be especially careful not to use the == (numeric) operator to check text strings, as text strings that do not start with digits return zero - so (for example) it would always tell you that two names are the same!

Here's an example to illustrate that:

print "What is your name? ";
chop($hes = <STDIN>);
 
# Numeric equality
 
if ($hes == "Graham") {
   print "Hello and welcome\n";
} else {
   print "Not known\n";
}
 
# String equality - EXACT match!
 
if ($hes eq "Graham") {
   print "Hello and welcome\n";
} else {
   print "Not known\n";
}
 
# String match to regular expression
 
if ($hes =~ /Graham/) {
   print "Hello and welcome\n";
} else {
   print "Not known\n";
}


And some results from testing it:

Dorothy:cs2 grahamellis$ perl u
What is your name? Graham
Hello and welcome
Hello and welcome
Hello and welcome
Dorothy:cs2 grahamellis$ perl u
What is your name? I am Graham Ellis
Hello and welcome
Not known
Hello and welcome
Dorothy:cs2 grahamellis$ perl u
What is your name? Simon Smith
Hello and welcome Test zero against zero
Not known
Not known
Dorothy:cs2 grahamellis$

(written 2008-07-29)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P212 - Perl - More on Character Strings
  [453] Commenting Perl regular expressions - (2005-09-30)
  [583] Remember to process blank lines - (2006-01-31)
  [586] Perl Regular Expressions - finding the position and length of the match - (2006-02-02)
  [597] Storing a regular expression in a perl variable - (2006-02-09)
  [608] Don't expose your regular expressions - (2006-02-15)
  [737] Coloured text in a terminal from Perl - (2006-05-29)
  [928] C++ and Perl - why did they do it THAT way? - (2006-11-16)
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [1222] Perl, the substitute operator s - (2007-06-08)
  [1230] Commenting a Perl Regular Expression - (2007-06-12)
  [1251] Substitute operator / modifiers in Perl - (2007-06-28)
  [1305] Regular expressions made easy - building from components - (2007-08-16)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1510] Handling Binary data (.gif file example) in Perl - (2008-01-17)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1947] Perl substitute - the e modifier - (2008-12-16)
  [2230] Running a piece of code is like drinking a pint of beer - (2009-06-11)
  [2379] Making variables persistant, pretending a database is a variable and other Perl tricks - (2009-08-27)
  [2657] Want to do a big batch edit? Nothing beats Perl! - (2010-03-01)
  [2801] Binary data handling with unpack in Perl - (2010-06-10)
  [2834] Teaching examples in Perl - third and final part - (2010-06-27)
  [2874] Unpacking a Perl string into a list - (2010-07-16)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2993] Arrays v Lists - what is the difference, why use one or the other - (2010-10-10)
  [3059] Object Orientation in an hour and other Perl Lectures - (2010-11-18)
  [3100] Looking ahead and behind in Regular Expressions - double matching - (2010-12-23)
  [3322] How much has Perl (and other languages) changed? - (2011-06-10)
  [3332] DNA to Amino Acid - a sample Perl script - (2011-06-24)
  [3411] Single and double quotes strings in Perl - what is the difference? - (2011-08-30)
  [3546] The difference between dot (a.k.a. full stop, period) and comma in Perl - (2011-12-09)
  [3630] Serialsing and unserialising data for storage and transfer in Perl - (2012-02-28)
  [3650] Possessive Regular Expression Matching - Perl, Objective C and some other languages - (2012-03-12)
  [3707] Converting codons via Amino Acids to Proteins in Perl - (2012-04-25)
  [3927] First match or all matches? Perl Regular Expressions - (2012-11-19)
  [4452] Binary data handling - Python and Perl - (2015-03-09)

P204 - Perl - Conditionals and Loops
  [353] Wimbledon Neck - (2005-06-20)
  [930] -> , >= and => in Perl - (2006-11-18)
  [1191] Smart English Output - via PHP and Perl ? : operator - (2007-05-18)
  [1468] Lexical v Arithemetic testing, Bash and Perl - (2007-12-11)
  [1477] Decisions - small ones, or big ones? - (2007-12-18)
  [1607] Learning to program in Perl - (2008-04-11)
  [1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
  [2351] Ternary operators alternatives - Perl and Lua lazy operators - (2009-08-12)
  [2550] Do not copy and paste code - there are much better ways - (2009-12-26)
  [2711] For loop - checked once, or evety time? Ruby v Perl comparison and contrast - (2010-04-07)
  [2832] Are you learning Perl? Some more examples for you! - (2010-06-27)
  [3004] Increment operators for counting - Perl, PHP, C and others - (2010-10-18)
  [3397] Does a for loop evaluate its end condition once, or on every iteration? - (2011-08-18)
  [3895] Flowchart to program - learning to program with Well House - (2012-10-14)
  [4031] Showing what programming errors look like - web site pitfall - (2013-03-06)
  [4322] Learning to Program - the conditional statement (if) - (2014-11-21)
  [4323] Learning to program - Loop statements such as while - (2014-11-22)


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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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