Difference between revisions of "Perl"

From wiki
Jump to: navigation, search
m
Line 1: Line 1:
 +
[[Category:Programming]]
 
=Warnings=
 
=Warnings=
 
===Put a ; at the end of each line===
 
===Put a ; at the end of each line===

Revision as of 16:03, 9 June 2019

Warnings

Put a ; at the end of each line

autovivification

If testing for existence of a hash key beware that all intermediate keys will be created by the test. In below example $hash1($key1} will be created if it does not exist yet.
if (exists $hash1{$key1}{$key2) {
    ...
} else {
    ...
}
)

Things that do work

Searching and matching is explained in Regular Expressions.

Print out the contents of $variable in a structured format
use Data::Dumper;
print Dumper($variable);

if-then-else:

if < expr > {
 <codeblock>
} elsif < expr ) {
 <codeblock>
} else {
 <codeblock>
}
$var += <value>
Add <value> to $var (works for -, *, / too)
$string .= $addstring
Concatenate $string and $addstring
$string =~ s/<regexp>/<newvalue>/[g]
Substitute <regexp> with <newvalue> in $string. The g modifier makes the all occurrences of <regexp> are substituted.
$newstring = substr($string,start,length)
Return substring of $string
sprintf(format,$string)
Return formatted string. format is e.g. "%.3f". Check Python:Strings#Advanced for all formats.
$exitcode = system("a command")
Executed the command in a subprocess and return the exitcode
$output = `a command`
Execute the command and return all command output
sleep(x)
Sleep for x seconds
$|++
Set standard output to unbuffered, every is put on screen immediately.

Hashes

%newhash = (%oldhas1,%oldhash3)
$newhashref = {$oldhasref1,$oldhashref3}
Merge 2 hashes, works if all keys are unique

Files

open (FH, "< filename") || die("Open failed for filename")
Open filename for reading on filehandle FH. Exit the program when the open failed
@array = <FH>
Read the file opened on FH into @array (1 element per line

Flow control

The usual works (if-then-else, for , foreach)

||die("message")
|| causes the statement only to be executed if the previous failed. Stop the program and write messaged to standard error.
|| do {codeblock};
Like die but not exiting the program, use to do additional actions on the failure of the previous command