Programming Language Concepts Perl Scalar Data Adapted by

Programming Language Concepts Perl Scalar Data Adapted by Carl Reynolds from materials by Sean P. Strout 9/17/2020 PLC - Perl Scalar Data (v 2) 1

What is Scalar Data • Scalar means “one of something” – Number – 255 or 3. 25 e 20 – String – hello • Singular vs. plural • Numbers and strings are interchangeable • A string is a sequence of characters, but Perl treats it as a single scalar value. 9/17/2020 PLC - Perl Scalar Data (v 2) 2

Numbers • Internally, Perl computes with double-precision floatingpoint values – The perl interpreter is (typically) compiled in C – Most modern systems provide for 15 digits of precision in the range of 1 e-100 to 1 e 100 • There are no integer values visible to the programmer • “A literal is the way a value is represented in the source code of the Perl program. A literal is not the result of a calculation or an I/O operation; it’s data written directly to the source code” 9/17/2020 PLC - Perl Scalar Data (v 2) 3

Floating Point Literals 1. 25 255. 000 255. 0 7. 25 e 45 -6. 5 e 24 -12 e-24 -1. 2 E-23 9/17/2020 # # 7. 25 times 10 to the 45 th power negative 6. 5 times 10 to the 24 th negative 12 times to the – 24 th same as the previous PLC - Perl Scalar Data (v 2) 4

Integer Literals 0 2001 -40 255 61298040283768 61_298_040_283_768 9/17/2020 # same as previous PLC - Perl Scalar Data (v 2) 5

Non-decimal Integer Literals • These three values are all the same 0377 0 xff 0 b 1111 # 377 octal, or 255 decimal # FF hex, or 255 decimal # 255 decimal (perl v 5. 6+) • Underscores help with readability 0 x 13770 c 77 0 x 1377_0 c 77 9/17/2020 # same as previous (perl v 5. 6+) PLC - Perl Scalar Data (v 2) 6

Numeric Operators 2 + 3 5. 1 – 2. 4 3 * 12 14 / 2 10. 2 / 0. 3 10 / 3 10 % 3 10. 5 % 3. 2 2 ** 3 9/17/2020 # # # # # 2 plus 3, or 5 5. 1 minus 2. 4, or 2. 7 3 times 12 = 36 14 divided by 2, or 7 10. 2 divided by 0. 3, or 34 3. 33333… 10 modulus 3, or 1 10 % 3, or 1 2 to the power of 3, or 8 PLC - Perl Scalar Data (v 2) 7

Strings • Strings are a sequence of zero or more characters – Shortest string is the empty string – Largest string fills all available memory • “Know thy ASCII table” – 751101111193211610412132658367737 3321169798101 • Perl can handle non printable characters in a string (values outside ASCII 32 -126) – This is perfect for manipulating raw binary data like a JPEG or a compiled program 9/17/2020 PLC - Perl Scalar Data (v 2) 8

Single-Quoted String Literals • A sequence of characters enclosed in a single quote • Any character besides a single quote, ’ , and a backslash, , stands for itself ’fred’ # the four characters: f, r, e, d ’’ # the null string ’Don’t let an apostrophe end this string!’ ’The last char is a backslash: \’ ’hellon’ # hello, backslash, n ’hello there’ # hello, newline, there (11 chars) ’’\’ # single quote, backslash 9/17/2020 PLC - Perl Scalar Data (v 2) 9

Double-Quoted String Literals • A sequence of characters enclosed in double quotes • Backslash now takes on the full power to specify a control character, or any character “barney” # same as ‘barney’ “hello worldn” # hello world, and a newline “The last character is a quote: ”” “coketsprite” # coke, a tab, and a sprite 9/17/2020 PLC - Perl Scalar Data (v 2) 10

Double-quoted String Backslash Escapes Construct n r t f b a e 07 x 7 f c. C \ ” l L u U Q E 9/17/2020 Meaning Newline Return Tab Formfeed Backspace Bell Escape (ASCII escape character) Any octal value (here 007=bell) Any hex ASCII value (here 7 f=delete) A control character (here, Ctrl-C) Backslash Double quote Lowercase next letter Lowercase all following letters until E Uppercase next letter Uppercase all following letters until E Quote non-word chars by adding a backslash until E Terminate L, U or Q PLC - Perl Scalar Data (v 2) 11

String Operators • Strings values can be concatenated with the. operator • Neither string is modified, a new result is formed “hello”. “world” # same as “helloworld” “hello”. ’ ’. “world” # same as “hello world” ’hello world!’. “n” # same as “hello world!n” • The string repetition operator, x “fred” x 3 “barney” x (1+1) 5 x 4 9/17/2020 # is “fredfred” # “barney” # What is this? ? ? PLC - Perl Scalar Data (v 2) 12

Automatic Conversion • Perl automatically converts numbers strings based on the operators. “ 12” * “ 3” “ 12 fred 34” * “ “fred” * 10 “Z”. 5 * 7 3” • Perl can notify you of suspicious code with: % perl –w my_program • or: #!/usr/bin/perl -w 9/17/2020 PLC - Perl Scalar Data (v 2) 13

Scalar Variables • A variable is a name for a container that holds one or more values • Scalar variables hold a single scalar value whose value can change throughout execution of the program $my_cool_scalar_variable_that_ends_in_1 All scalar variables begins with the dollar sign symbol The remainder is the perl identifier, which is a letter or underscore, followed by letters, digits or more underscores. 9/17/2020 PLC - Perl Scalar Data (v 2) 14

Scalar Assignment • Scalar assignment gives a value to a scalar variable $fred = $barney 17 # $fred can legally drink in 4 short years = ’hello’ # give $barney the 5 character string ’hello’ = $fred + 3 # $barney now holds the value 20 = $barney * 2 # $barney now holds the value 40 = fred + 65 # be careful! • Binary Assignment Operators $fred += 5; $barney *= 3; $str. = “ “; $fred **= 3; 9/17/2020 # # $fred = $fred + 5 $barney = $barney * 3 $str = $str. “ “ $fred = $fred ** 3 PLC - Perl Scalar Data (v 2) 15

Output With print • The print operator can take a scalar argument and print it to standard output, without embellishment print “hello worldn”; print “The answer is ”; print 6 * 7; print “. n”; print “The answer is ”, 6 * 7, “. n”; As you can imagine, this is not a scalar argument (it's a list! – later…) 9/17/2020 PLC - Perl Scalar Data (v 2) 16

Scalar/String Interpolation • Double quoted string literals are subject to variable interpolation $meal = $barney “brontosaurus = “fred ate a = ’fred ate a = “fred ate a steak”; $meal” ’. $meal; $meat”; print “$fred”; print $fred; # unneeded quote marks # the preferred style $fred = ’hello’; print “The name is $fred. n”; print ’The name is $fred’. “n”; 9/17/2020 # what happens here? # prints dollar sign # single quoted string # works same as previous PLC - Perl Scalar Data (v 2) 17

Variable Name Delimeter • Perl provides a delimiter for the variable name. Enclose the name of the variable in curly braces $what = “brontosaurus steak”; $n = 3; print “fred ate $n $whats. n”; print “fred ate $n $what”. “s. n”; print ‘fred ate ‘. $n. ‘ ‘. $what. “s, n”; print “fred ate $n ${what}s. n”; 9/17/2020 PLC - Perl Scalar Data (v 2) # # problem cumbersome ugh! easy 18

Operator Precedence and Associativity left right left left right left 9/17/2020 Operators parentheses and arguments to list operators -> ++ -- (autoincrement, autodecrement) ** ! ~ + - (unary operators) =~ !~ */%x + -. (binary operators) << >> named unary operators (-X filetests, rand) < <= > >= lt le gt ge (the “unequal” ones) == != <=> eq ne cmp (the “equal” ones) & |^ && || ? : (ternary) = += -=. = (and similar assignment operators) , => list operators (rightward) not and or xor PLC - Perl Scalar Data (v 2) 19

Operator Precedence and Associativity • Associativity resolves the order of operations when two operators of the same precedence compete for three operands. 4 ** 3 ** 2 72 / 12 / 3 36 / 6 * 3 # 4 ** (3 ** 2) # (72 / 12) / 3 # (36 / 6) * 3 • Use parentheses if you are unsure (or untrusting!) 9/17/2020 PLC - Perl Scalar Data (v 2) 20

Comparison Operators • The logical comparison operators returns true or false Comparison Equal Not equal Less than Greater than Less than or equal Greater than or equal to 35 != 30 + 5 35 == 35. 0 ’ 35’ eq ’ 35. 0’ ’fred’ lt ’barney’ ’fred’ lt ’free’ ’fred’ eq “fred” ’fred’ eq “Fred” ‘ ‘ gt ‘‘ 9/17/2020 Numeric == != < > <= >= String eq ne lt gt le ge # # # # false true false (why? ) false true PLC - Perl Scalar Data (v 2) 21

The if Control Structure if ($name gt ’fred’) { print “’$name’ comes after ’fred’ in sorted order. n”; } else { print “’$name’ does not come after ’fred’. n”; } • Curly braces are required around all conditional statements (unlike C) • The conditional can come after the statement, and the parentheses can be omitted { print “’$name’ > ’fred’. n” } if $name gt ’fred’; • You can reverse the test sense by substituting “unless” for “if” unless($name le ‘fred’){print “’$name’ > ‘fred’. n”; } 9/17/2020 PLC - Perl Scalar Data (v 2) 22

Boolean Values • Boolean values may be stored in a scalar $is_bigger = $name gt ’fred’; if ($is_bigger) {…} • Perl has no separate boolean type. The rules: 1. The special variable undef is false 2. Zero is false, all other numbers are true 3. The empty string (’’) is false; all other strings are normally true 4. Since numbers and strings are equivalent, the string form of zero, ’ 0’, has the same value as its numeric form: false 5. Everything else is true • Use the "unary not” operator, !, to get the opposite boolean value if (! $is_bigger ) {…} 9/17/2020 PLC - Perl Scalar Data (v 2) 23

Getting User Input • Use the line operator <STDIN> in place of a scalar variable to read the next complete line from standard input (a blocking call) • The string value will maintain the newline character! $line = <STDIN>; if ($line eq “n”) { print “That was just a blank line!n”; } else { print “That line of input was: $line”; } • It’s often useful to be able to discard the newline character 9/17/2020 PLC - Perl Scalar Data (v 2) 24

chomp • The chomp operator removes the newline character from a line $text = “a line of textn”; chomp($text); # or from <STDIN> # bye newline • “Anytime you need a variable in Perl, you can use assignment instead” # $text stores chomped string # $removed is the number of characters removed $removed = chomp($text = <STDIN>); • chomp only removes one newline. – If there is no newline, it does nothing and returns 0 9/17/2020 PLC - Perl Scalar Data (v 2) 25

while Control Structure • The classic looping structure: $count = 0; while ($count < 10) { $count += 1; print “count is now $countn”; } • Truth value works the same as with if control structure • Curly braces are required • The conditional expression is evaluated before the first iteration 9/17/2020 PLC - Perl Scalar Data (v 2) 26

undef • Variables have a special undef value before they are given a value • As a numeric, undef is treated as 0, and • as a string it is treated as the empty string • But, undef is neither a number nor a string, it’s an entirely separate scalar value $n = 1; while ($n < 10) { $sum += $n; $n += 2; } print “The total was $sum. n”; $string. = “more textn”; 9/17/2020 # using undef initially as 0 # using undef as empty string PLC - Perl Scalar Data (v 2) 27

defined • To tell whether a value is undef and not the empty string, use the defined function, which – returns false for undef and – true for everything else $madonna = <STDIN>; if ( defined($madonna) ) { print “The input was $madonnan”; } else { print “No input available!n”; } • To force a variable to be undefined: $madonna = undef; 9/17/2020 # as if it had never been touched PLC - Perl Scalar Data (v 2) 28

Revision History • Revision History – – -- 9/17/2020 v 1. 00, 10/6/2003 2: 48 PM, sps Initial revision. v 1. 01, 1/14/2004 10: 54 AM, sps 20032 updates. v 1. 02, 4/6/2004 2: 54 PM, sps 20033 updates. v 1. 03, 4/7/2004 10: 47 AM, sps Touched up some examples. v 2. 0, 1/9/2005, chr PLC - Perl Scalar Data (v 2) 29