Topic 2: Working with scalars CSE2395/CSE3395 Perl Programming Learning Perl 3rd edition chapter 2, pages 19-38, Programming Perl 3rd edition chapter 2, pages 6-8, 58-67,

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

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 3 Scalar 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

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 4 Scalar 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

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 5 Scalar 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 , perlop manpage

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 6 Scalar 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

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 7 String literals  Perl provides several ways to write a literal string ► depending on your needs  Double quotes ► "blah"  Single quotes ► 'blah'  “Here document” ► <<END multiline text blah END  Perl provides several ways to write a literal string ► depending on your needs  Double quotes ► "blah"  Single quotes ► 'blah'  “Here document” ► <<END multiline text blah END

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 8 String literals  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

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 9 String literals  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

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 10 String 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"; Llama3 pages 30-31; Camel3 pages 62-63

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 11 String 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

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 12 Undefined 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 – 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 – returns undef at end of file  use defined function to test an expression ► if (defined $var) {... } Llama3 pages 37-38; Camel3 pages

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 13 Some 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 ; perlfunc manpage

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 14 Input and output  Output to screen with ► print function ► printf function ► print "testing\n";  Input from keyboard with ► operator ► reads and returns one line (including newline) from standard input ► $line = ; ► chomp function can be used to remove newline  Output to screen with ► print function ► printf function ► print "testing\n";  Input from keyboard with ► operator ► reads and returns one line (including newline) from standard input ► $line = ; ► chomp function can be used to remove newline Llama3 pages 29-30, 35-36; Camel3 pages 80-83

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 15 Timeout #!/usr/bin/perl -w # Set $pi. $pi = ; # Read a number from the user. print "Please enter radius: "; $radius = ; # 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 = ; # Read a number from the user. print "Please enter radius: "; $radius = ; # Remove the newline from $radius. chomp $radius; # Calculate the circumference. $around = $radius * 2 * $pi; # Print the result. print "The circumference is $around.\n";

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 16 Boolean 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

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 17 if / 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 ; perlsyn manpage condition evaluated in scalar context zero or more elsif clauses else clause optional all braces are compulsory, unlike C/Java

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 18 Timeout #!/usr/bin/perl -w # Read two numbers. print "Enter a number: "; chomp ($num1 = ); print "Enter another number: "; chomp ($num2 = ); # 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 = ); print "Enter another number: "; chomp ($num2 = ); # Calculate the bigger value. if ($num1 > $num2) { $max = $num1; } else { $max = $num2; } # Print result. print "Maximum is $max\n";

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 19 while statement while (condition) { # This code executed until condition is false. # Block is never entered if condition was # always false. } Llama3 pages 34-35, , 132,133; Camel3 pages , 701; perlsyn manpage condition evaluated in scalar context all braces are compulsory do {... } while (condition) (post-tested) also exists, as in C/Java

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 20 Timeout # A primitive version of Unix cat program # Read lines from standard input. # 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 = )) { # Print out the line just read. print $line; } # A primitive version of Unix cat program # Read lines from standard input. # returns the next line of input # if there is one, or undef if at end of file. # Test using defined() to see when to stop. while (defined ($line = )) { # Print out the line just read. print $line; }

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 21 unless and until  Sometimes more natural to express condition as negative ► unless (condition) same as if (!condition) ► until (condition) same as while (!condition)  Sometimes more natural to express condition as negative ► unless (condition) same as if (!condition) ► until (condition) same as while (!condition) Llama3 pages ; Camel3 pages

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 22 Loop 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 ; Camel3 pages ; perlsyn manpage

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 23 Timeout # Print the header of an 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 ($line = )) { # If line is blank (contains only a newline) # then exit the while loop. if ($line eq "\n") { last; } print $line; } # Print the header of an message. # Mail messages come in two parts: the header and # the body. The first blank line in the message # separates the header from the body. # Read each line. while (defined ($line = )) { # If line is blank (contains only a newline) # then exit the while loop. if ($line eq "\n") { last; } print $line; }

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 24 Covered in 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 with print ► reading from the keyboard with  if and while ► unless and until  Scalar values ► numbers ► strings –string interpolation  defined and undefined values  Scalar variables  Scalar operators and functions  Console input/output ► printing to the screen with print ► reading from the keyboard with  if and while ► unless and until

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 25 Going further  References ► the “other kind” of scalar value ► Topic 11  Unicode ► support for national character sets ► Camel3 pages  Operator overloading ► providing new behaviour for built-in operators ► Camel3 pages  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  References ► the “other kind” of scalar value ► Topic 11  Unicode ► support for national character sets ► Camel3 pages  Operator overloading ► providing new behaviour for built-in operators ► Camel3 pages  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

Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University 26 Next topic  Lists  Arrays  for and foreach  Context  Lists  Arrays  for and foreach  Context Llama3 chapter 3 Camel3 pages 69-76, perldata manpage