Topic 2: Working with scalars CSE2395/CSE3395 Perl Programming Learning Perl 3rd edition chapter 2, pages 19-38, 128-138 Programming Perl 3rd edition chapter

Topic 2: Working with scalarsTopic 2: Working with scalars

CSE2395/CSE3395Perl Programming

CSE2395/CSE3395Perl Programming

Learning Perl 3rd edition chapter 2, pages 19-38, 128-138

Programming Perl 3rd edition chapter 2, pages 6-8, 58-67, 114-118

2Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University

In this topicIn this topic

Scalar values► numbers► strings

– string interpolation

defined and undefined values Scalar variables Scalar operators and functions Console input/output

► printing to the screen► reading from the keyboard

if and while




► printing to the screen► reading from the keyboard

if and while


Scalar valuesScalar values

numbers► double-precision floating-point (C double)► e.g., 87, -1.06E20, 0

strings► basic data type, not an array of char► arbitrary length, can contain ‘\0’► no pointers needed

undefined value► undef

references► covered in topic 11

numbers► double-precision floating-point (C double)► e.g., 87, -1.06E20, 0

strings► basic data type, not an array of char► arbitrary length, can contain ‘\0’► no pointers needed

undefined value► undef

references► covered in topic 11

Llama3 pages 19-20, 22-23


Scalar variablesScalar variables

Scalar values are stored in scalar variables Variables are global by default All Perl scalar variables begin with $ character

► $apples► $text2► $_

Perl doesn’t usually care if a scalar contains a number or string► numbers and strings are converted as needed

Scalar values are stored in scalar variables Variables are global by default All Perl scalar variables begin with $ character

► $apples► $text2► $_

Perl doesn’t usually care if a scalar contains a number or string► numbers and strings are converted as needed


Scalar operatorsScalar operators

Most familiar numeric operators available► +, -, ++, %, &, etc.► = (assignment), +=, *=, etc.► / (floating-point division)► <, <=, >, >=, ==, != (numeric comparison)► ** (exponentiation)

New string operators► . (join two strings)

– "cat" . "fish" (produces "catfish")► lt, le, gt, ge, eq, ne (string comparison)

Most familiar numeric operators available► +, -, ++, %, &, etc.► = (assignment), +=, *=, etc.► / (floating-point division)► <, <=, >, >=, ==, != (numeric comparison)► ** (exponentiation)

New string operators► . (join two strings)

– "cat" . "fish" (produces "catfish")► lt, le, gt, ge, eq, ne (string comparison)

Llama3 pages 22,24,25; Camel3 pages 86-110, perlop manpage


Scalar operatorsScalar operators

Both > and gt test for “greater than” in scalars► > compares numbers

– and forces both sides into numbers► gt compares strings

– and forces both sides into strings

7 > 30 is false (zero)► numerically, 7 isn’t greater than 30

7 gt 30 is true (non-zero)► alphabetically, "7" comes after "30"► absence of quotes is irrelevant because Perl converts numbers

and strings when needed

This forcing of types is related to context► Topic 3

Both > and gt test for “greater than” in scalars► > compares numbers

– and forces both sides into numbers► gt compares strings

– and forces both sides into strings

7 > 30 is false (zero)► numerically, 7 isn’t greater than 30

7 gt 30 is true (non-zero)► alphabetically, "7" comes after "30"► absence of quotes is irrelevant because Perl converts numbers

and strings when needed

This forcing of types is related to context► Topic 3


String literalsString literals

Perl provides several ways to write a literal string► depending on your needs

Double quotes► "blah"

Single quotes► 'blah'

“Here document”► <<ENDmultiline textblah blahEND

Perl provides several ways to write a literal string► depending on your needs

Double quotes► "blah"

Single quotes► 'blah'

“Here document”► <<ENDmultiline textblah blahEND



Single-quote delimited► all characters within string are literal, except

– \' (becomes ')– \\ (becomes \)

► does not interpolate variables► 'hello' (hello)► '$35.40' ($35.40)► 'it\'s' (it's)► '\n' (backslash followed by n)

Single-quote delimited► all characters within string are literal, except

– \' (becomes ')– \\ (becomes \)

► does not interpolate variables► 'hello' (hello)► '$35.40' ($35.40)► 'it\'s' (it's)► '\n' (backslash followed by n)

Llama3 page 23; Camel3 pages 60-64



Double-quote delimited► usual C backslash rules (e.g., \n) apply► "hello" (hello)► "it's \$35.40" (it's $34.50)► "\n" (newline character)► interpolates scalar and array variables

– "hello $name" (becomes hello Fred if $name contains Fred)

Double-quote delimited► usual C backslash rules (e.g., \n) apply► "hello" (hello)► "it's \$35.40" (it's $34.50)► "\n" (newline character)► interpolates scalar and array variables

– "hello $name" (becomes hello Fred if $name contains Fred)

Llama3 pages 23-24; Camel3 pages 60-63


String interpolationString interpolation

In C, use printf or sprintf to insert variable values into string► printf("The sum is %d\n", total);► this works in Perl too

In Perl, can place variable name inside double-quoted string► print "The sum is $total\n";► Perl substitutes current value of $total into the

string

Can also use string concatenation operator► print "The sum is " . $total . "\n";

In C, use printf or sprintf to insert variable values into string► printf("The sum is %d\n", total);► this works in Perl too

In Perl, can place variable name inside double-quoted string► print "The sum is $total\n";► Perl substitutes current value of $total into the

string

Can also use string concatenation operator► print "The sum is " . $total . "\n";



String interpolationString interpolation

Single-quoted strings do not interpolate► print 'the sum is $total\n' prints “the sum

is $total\n”

If variable name is ambiguous, use braces► want to print “Today is the 6th” when $day is 6► print "Today is the $dayth\n";

– wrong, what is $dayth?► print "Today is the ${day}th\n";

– right, uses $day

Can always use braces like this in any situation► see Topic 11 for further use of this in nested data

structures

Single-quoted strings do not interpolate► print 'the sum is $total\n' prints “the sum

is $total\n”

If variable name is ambiguous, use braces► want to print “Today is the 6th” when $day is 6► print "Today is the $dayth\n";

– wrong, what is $dayth?► print "Today is the ${day}th\n";

– right, uses $day

Can always use braces like this in any situation► see Topic 11 for further use of this in nested data

structures


Undefined valueUndefined value

undef► $x = undef;► this is the only undefined value

Neither number nor string► converted to empty string ("") in string context► converted to zero in numeric context

Used to indicate when a scalar doesn’t have a value► unassigned scalar variables return undef► similar uses to NULL in C► returned by some functions and operators on out-of-range input

– <STDIN> returns undef at end of file

use defined function to test an expression► if (defined $var) { ... }

undef► $x = undef;► this is the only undefined value

Neither number nor string► converted to empty string ("") in string context► converted to zero in numeric context

Used to indicate when a scalar doesn’t have a value► unassigned scalar variables return undef► similar uses to NULL in C► returned by some functions and operators on out-of-range input

– <STDIN> returns undef at end of file

use defined function to test an expression► if (defined $var) { ... }



Some scalar functionsSome scalar functions

chomp $string► removes a newline from the end of $string, if any

length $string► returns length of $string in characters

uc $string► returns a version of $string entirely in uppercase► also lc (lower case)

rand $number► returns pseudorandom number from 0 to $number

substr $string, $start, $length► returns substring of $string, starting at $start (zero origin)

for $length characters And many more

chomp $string► removes a newline from the end of $string, if any

length $string► returns length of $string in characters

uc $string► returns a version of $string entirely in uppercase► also lc (lower case)

rand $number► returns pseudorandom number from 0 to $number

substr $string, $start, $length► returns substring of $string, starting at $start (zero origin)

for $length characters And many more

Camel3 chapter 29, pages 677-830; perlfunc manpage


Input and outputInput and output

Output to screen with► print function► printf function► print "testing\n";

Input from keyboard with► <STDIN> operator► reads and returns one line (including newline) from

standard input► $line = <STDIN>;► chomp function can be used to remove newline

Output to screen with► print function► printf function► print "testing\n";

Input from keyboard with► <STDIN> operator► reads and returns one line (including newline) from

standard input► $line = <STDIN>;► chomp function can be used to remove newline

Llama3 pages 29-30, 35-36; Camel3 pages 80-83


TimeoutTimeout

#!/usr/bin/perl -w

# Set $pi.$pi = 3.1415926535898;

# Read a number from the user.print "Please enter radius: ";$radius = <STDIN>;# Remove the newline from $radius.chomp $radius;

# Calculate the circumference.$around = $radius * 2 * $pi;

# Print the result.print "The circumference is $around.\n";

#!/usr/bin/perl -w

# Set $pi.$pi = 3.1415926535898;

# Read a number from the user.print "Please enter radius: ";$radius = <STDIN>;# Remove the newline from $radius.chomp $radius;

# Calculate the circumference.$around = $radius * 2 * $pi;

# Print the result.print "The circumference is $around.\n";


Boolean truthBoolean truth

All Boolean conditions evaluated in scalar context

False values are► undef (the undefined value)► "" (the empty string)► 0 (number) and "0" (string)

True values are► everything else

All Boolean conditions evaluated in scalar context

False values are► undef (the undefined value)► "" (the empty string)► 0 (number) and "0" (string)

True values are► everything else

Llama3 pages 34-35; Camel3 page 29-30; perldata manpage


if/else statementif/else statement

if (condition){ # These lines execute if condition is true.}elsif (condition2){ # These lines execute if condition2 is true.}else{ # These lines execute # if all conditions are false.}

Llama3 pages 34-35, 132,133; Camel3 pages 114-115; perlsyn manpage

condition evaluated in scalar context

zero or more elsif clauses

else clause optional

all braces are compulsory, unlike C/Java


TimeoutTimeout

#!/usr/bin/perl -w

# Read two numbers.print "Enter a number: ";chomp ($num1 = <STDIN>);print "Enter another number: ";chomp ($num2 = <STDIN>);

# Calculate the bigger value.if ($num1 > $num2){ $max = $num1;} else { $max = $num2;}

# Print result.print "Maximum is $max\n";

#!/usr/bin/perl -w

# Read two numbers.print "Enter a number: ";chomp ($num1 = <STDIN>);print "Enter another number: ";chomp ($num2 = <STDIN>);

# Calculate the bigger value.if ($num1 > $num2){ $max = $num1;} else { $max = $num2;}

# Print result.print "Maximum is $max\n";


while statementwhile statement

while (condition){ # This code executed until condition is false. # Block is never entered if condition was # always false.}

Llama3 pages 34-35, 128-129, 132,133; Camel3 pages 112-115, 701; perlsyn manpage

condition evaluated in scalar context

all braces are compulsory

do { ... } while (condition)(post-tested) also exists, as in C/Java


TimeoutTimeout

# A primitive version of Unix cat program

# Read lines from standard input.# <STDIN> returns the next line of input# if there is one, or undef if at end of file.# Test using defined() to see when to stop.while (defined ($line = <STDIN>)){ # Print out the line just read. print $line;}

# A primitive version of Unix cat program

# Read lines from standard input.# <STDIN> returns the next line of input# if there is one, or undef if at end of file.# Test using defined() to see when to stop.while (defined ($line = <STDIN>)){ # Print out the line just read. print $line;}


unless and untilunless and until

Sometimes more natural to express condition as negative► unless (condition) same asif (!condition)► until (condition) same aswhile (!condition)

Sometimes more natural to express condition as negative► unless (condition) same asif (!condition)► until (condition) same aswhile (!condition)



Loop control commandsLoop control commands

Perl has commands which can alter flow within loop last

► exits the innermost containing loop► like C/Java break statement

next► jumps to end of innermost containing loop

– with for loop, executes the increment code► re-tests loop condition, then continues from beginning of loop► like C/Java continue statement

redo► jumps to start of innermost containing loop► does not re-test loop condition► no equivalent in C without goto

Perl has commands which can alter flow within loop last

► exits the innermost containing loop► like C/Java break statement

next► jumps to end of innermost containing loop

– with for loop, executes the increment code► re-tests loop condition, then continues from beginning of loop► like C/Java continue statement

redo► jumps to start of innermost containing loop► does not re-test loop condition► no equivalent in C without goto

Llama3 pages 138-142; Camel3 pages 120-123; perlsyn manpage


TimeoutTimeout

# Print the header of an email message.

# Mail messages come in two parts: the header and# the body. The first blank line in the message# separates the header from the body.

# Read each line.while (defined ($line = <STDIN>)){ # If line is blank (contains only a newline) # then exit the while loop. if ($line eq "\n") { last; }

print $line;}

# Print the header of an email message.

# Mail messages come in two parts: the header and# the body. The first blank line in the message# separates the header from the body.

# Read each line.while (defined ($line = <STDIN>)){ # If line is blank (contains only a newline) # then exit the while loop. if ($line eq "\n") { last; }

print $line;}


Covered in this topicCovered in this topic




► printing to the screen with print► reading from the keyboard with <STDIN>

if and while► unless and until




► printing to the screen with print► reading from the keyboard with <STDIN>

if and while► unless and until


Going furtherGoing further

References► the “other kind” of scalar value► Topic 11

Unicode► support for national character sets► Camel3 pages 401-410

Operator overloading► providing new behaviour for built-in operators► Camel3 pages 347-362

Loops, blocks and goto► putting labels on blocks to change last, next and redo► more scary things to do with control structures, including

implementing C’s switch► Camel3 pages 123-127

References► the “other kind” of scalar value► Topic 11

Unicode► support for national character sets► Camel3 pages 401-410

Operator overloading► providing new behaviour for built-in operators► Camel3 pages 347-362

Loops, blocks and goto► putting labels on blocks to change last, next and redo► more scary things to do with control structures, including

implementing C’s switch► Camel3 pages 123-127


Next topicNext topic

Lists Arrays for and foreach Context

Lists Arrays for and foreach Context

Llama3 chapter 3Camel3 pages 69-76, 112-120perldata manpage

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Topic 2: Working with scalars CSE2395/CSE3395 Perl Programming Learning Perl 3rd edition chapter 2, pages 19-38, 128-138 Programming Perl 3rd edition chapter