Difference between revisions of "Perl"
Jump to navigation
Jump to search
(14 intermediate revisions by the same user not shown) | |||
Line 1: | Line 1: | ||
+ | [[Category:Programming]] | ||
+ | =Warnings= | ||
+ | ===Put a ; at the end of each line=== | ||
+ | This is still what I forget most. | ||
+ | |||
+ | ===autovivification=== | ||
+ | :If testing for existence of a hash key beware that all intermediate keys will be created by the test. In below example <code>$hash1($key1}</code> will be created if it does not exist yet. | ||
+ | <syntaxhighlight lang=Perl> | ||
+ | if (exists $hash1{$key1}{$key2) { | ||
+ | ... | ||
+ | } else { | ||
+ | ... | ||
+ | } | ||
+ | ) | ||
+ | </syntaxhighlight> | ||
=Things that do work= | =Things that do work= | ||
− | Print out the contents of $variable in a structured format | + | Searching and matching is explained in [[Regular Expressions]]. |
+ | |||
+ | ;Print out the contents of $variable in a structured format: | ||
<syntaxhighlight lang=Perl> | <syntaxhighlight lang=Perl> | ||
use Data::Dumper; | use Data::Dumper; | ||
Line 16: | Line 33: | ||
<codeblock> | <codeblock> | ||
} | } | ||
− | |||
</syntaxhighlight> | </syntaxhighlight> | ||
Line 34: | Line 50: | ||
:Return formatted string. format is e.g. "%.3f". Check [[Python:Strings#Advanced]] for all formats. | :Return formatted string. format is e.g. "%.3f". Check [[Python:Strings#Advanced]] for all formats. | ||
− | + | ;$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. | ||
+ | |||
+ | ;$username = getpwuid($<) | ||
+ | :Get the username of the one executing the script. $< is the uid of the current user. | ||
+ | |||
+ | ==Hashes== | ||
+ | ;%newhash = (%oldhas1,%oldhash3) | ||
+ | ;$newhashref = {$oldhasref1,$oldhashref3} | ||
+ | :Merge 2 hashes, works if all keys are unique | ||
+ | |||
+ | ==Arrays== | ||
+ | ;arr1 = split(<sep>,$string) | ||
+ | :Split a string in to an array on <sep> | ||
+ | |||
+ | ;a = arr1[0] | ||
+ | :First element in arr1 | ||
+ | |||
+ | ;a = arr1[-1] | ||
+ | :Last element in arr1 | ||
+ | |||
+ | ;if ( grep(/^<value>$/,@arr1) ) | ||
+ | :Check if <value> exist in arr1 (Note the ^ and $ for an exact match | ||
+ | |||
+ | ==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 | ||
+ | |||
+ | ;while ( line = (<FH>) ) | ||
+ | :Read the file opened on FH line by 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 | ||
+ | |||
+ | ==Compare== | ||
+ | ;==, !=, >=, >, <, etc | ||
+ | :Compare numeric data | ||
+ | ;eq, ne, ge, gt, lt, etc | ||
+ | :Compare strings |
Latest revision as of 13:43, 12 November 2020
Warnings
Put a ; at the end of each line
This is still what I forget most.
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.
- $username = getpwuid($<)
- Get the username of the one executing the script. $< is the uid of the current user.
Hashes
- %newhash = (%oldhas1,%oldhash3)
- $newhashref = {$oldhasref1,$oldhashref3}
- Merge 2 hashes, works if all keys are unique
Arrays
- arr1 = split(<sep>,$string)
- Split a string in to an array on <sep>
- a = arr1[0]
- First element in arr1
- a = arr1[-1]
- Last element in arr1
- if ( grep(/^<value>$/,@arr1) )
- Check if <value> exist in arr1 (Note the ^ and $ for an exact match
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
- while ( line = (<FH>) )
- Read the file opened on FH line by 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
Compare
- ==, !=, >=, >, <, etc
- Compare numeric data
- eq, ne, ge, gt, lt, etc
- Compare strings