lc, uc, length

There are a number of simple functions such as lc and uc to return the lower case and upper case versions of the original string respectively. Then there is length to return the number of characters in the given string.

See the following example:

examples/uc_lc_length.pl

use strict;
use warnings;
use 5.010;

my $str = 'HeLlo';

say lc $str;      # hello
say uc $str;      # HELLO
say length $str;  # 5

Please note, in order to get the length of an array you don't use the length function. Instead, you put it in scalar context using the scalar function.

index

Then there is the index function. This function will get two strings and return the location of the second string within the first string.

examples/index.pl

use strict;
use warnings;
use 5.010;

my $str = "The black cat climbed the green tree";

say index $str, 'cat';             # 10
say index $str, 'dog';             # -1
say index $str, "The";             # 0
say index $str, "the";             # 22

The first call to index returned 10, because the string "cat" starts on the 10th character. The second call to index returned -1 indicating there is no "dog" in that sentence.

The 3rd call shows that index returns 0 when the second string is a prefix of the first string.

The 4th example shows that index is looking for exact matches so case also matters. Hence "the" and "The" are different.

index() looks for strings and not just for words, so even the string "e " can be looked up:

my $str = "The black cat climbed the green tree";
say index $str, "e ";              # 2

index() can also have a 3rd parameter that indicates the location where to start searching. So as we found "e " to start at the 2nd character of the first string, we could try to search starting from the 3rd place to see if there is another occurrence of "e ":

examples/index_more.pl

use strict;
use warnings;
use 5.010;

my $str = "The black cat climbed the green tree";
say index $str, "e ";              # 2
say index $str, "e ", 3;           # 24
say index $str, "e", 3;            # 19

Looking for "e" without a space will yield a different result.

Lastly, there is another function called rindex (right index) that will start searching from the right hand side of the string:

examples/rindex.pl

use strict;
use warnings;
use 5.010;

my $str = "The black cat climbed the green tree";
say rindex $str, "e";              # 35
say rindex $str, "e", 34;          # 34
say rindex $str, "e", 33;          # 29

substr

I think the most interesting function in this article is substr. It is basically the opposite of index(). While index() will tell you where is a given string, substr will give you the substring at a given locations. Normally substr gets 3 parameters. The first one is the string. The second is a 0-based location, also called the offset, and the third is the length of the substring we would like to get.

examples/substr.pl

use strict;
use warnings;
use 5.010;

my $str = "The black cat climbed the green tree";

say substr $str, 4, 5;                      # black

substr is 0 based so the character at the offset 4 is the letter b.

my $str = "The black cat climbed the green tree";
say substr $str, 4, -11;                    # black cat climbed the

The 3rd parameter (the length) can also be a negative number. In that case it tells us the number of characters from the right hand side of the original string that should NOT be included. So the above means: count 4 from the left, 11 from the right, return what is between.

my $str = "The black cat climbed the green tree";
say substr $str, 14;                        # climbed the green tree

You can also leave out the 3rd (length) parameter which will mean: return all the characters starting from the 14th place till the end of the string.

my $str = "The black cat climbed the green tree";
say substr $str, -4;                        # tree
say substr $str, -4, 2;                     # tr

We can also use a negative number in the offset, which will mean: count 4 from the right and start from there. It is the same as having length($str)-4 in the offset.

Replacing part of a string

The last example is a bit funky. So far in every case substr returned the substring and left the original string intact. In this example, the return value of substr will still behave the same way, but substr will also change the content of the original string!

The return value of substr() is always determined by the first 3 parameters, but in this case substr has a 4th parameter. That is a string that will replace the selected substring in the original string.

my $str = "The black cat climbed the green tree";
my $z = substr $str, 14, 7, "jumped from";
say $z;                                                     # climbed
say $str;                  # The black cat jumped from the green tree

So substr $str, 14, 7, "jumped from" returns the word climbed, but because of the 4th parameter, the original string was changed.

There is also an lvalue version of this example, but only look there if you are brave.

Comments

Hello:

I am new to perl and index function. For some reason, I cannot find the right location/index for a tab character.

Can you please review my code and result below and tell me what I incorrectly coded?

My Code:

# D. Test with a special string print "D. Test\n"; my $Tmp = '8614547481120 8986011880217742798 460017484706396 1111 '; # 55 characters with 4 tab characters.

print "Length of the string is " . length ($Tmp) . "\n";

if ( substr ($Tmp, 13, 1) eq chr(9)) { print " My string has a tab at 5; = chr(9) \n"; }

if ( substr ($Tmp, 13, 1) eq ' ') { print " My string has a tab at 5; = < > \n"; }

print "1st Location of a Tab is " . index ($Tmp, ' '), 0 . "\n"; # This should be 13. print "2nd Location of a Tab is " . index ($Tmp, ' '), 15 . "\n"; # This should be 33. print "3rd Location of a Tab is " . index ($Tmp, ' '), 34 . "\n"; # This should be 49.

print "1st Location of a Tab is " . index ($Tmp, chr(9)), 0 . "\n"; # This should be 13. print "2nd Location of a Tab is " . index ($Tmp, chr(9)), 15 . "\n"; # This should be 33. print "3rd Location of a Tab is " . index ($Tmp, chr(9)), 34 . "\n"; # This should be 49.

print "1st Location of a Tab is " . index ($Tmp, '/\t'), 0 . "\n"; # This should be 13. print "2nd Location of a Tab is " . index ($Tmp, '/\t'), 15 . "\n"; # This should be 33. print "3rd Location of a Tab is " . index ($Tmp, '/\t'), 34 . "\n"; # This should be 49.

print "1st Location of a Tab is " . index ($Tmp, '\t'), 0 . "\n"; # This should be 13. print "2nd Location of a Tab is " . index ($Tmp, '\t'), 15 . "\n"; # This should be 33. print "3rd Location of a Tab is " . index ($Tmp, '\t'), 34 . "\n"; # This should be 49.

print "1st Location of a Tab is " . index ($Tmp, '/t'), 0 . "\n"; # This should be 13. print "2nd Location of a Tab is " . index ($Tmp, '/t'), 15 . "\n"; # This should be 33. print "3rd Location of a Tab is " . index ($Tmp, '/t'), 34 . "\n"; # This should be 49.

Result: D. Test Length of the string is 55 My string has a tab at 5; = chr(9) My string has a tab at 5; = < > 1st Location of a Tab is 130 2nd Location of a Tab is 1315 3rd Location of a Tab is 1334 1st Location of a Tab is 130 2nd Location of a Tab is 1315 3rd Location of a Tab is 1334 1st Location of a Tab is -10 2nd Location of a Tab is -115 3rd Location of a Tab is -134 1st Location of a Tab is -10 2nd Location of a Tab is -115 3rd Location of a Tab is -134 1st Location of a Tab is -10 2nd Location of a Tab is -115 3rd Location of a Tab is -134

Thank you. Bobby

--- A tab is "\t" in double quotes. and you put the 3rd parameter outside of the parentheses. eg. you wrote:

index ($Tmp, chr(9)), 15

instead of

index($Tmp, chr(9), 15)

---

Thank you Gabor.