Perl | Scalar Context Sensitivity
Introduction:
In Perl, function calls, terms, and statements have inconsistent explications which rely upon its Context. There are two crucial Contexts in Perl, namely List Context and Scalar Context. In a list context, Perl gives the list of elements. But in a scalar context, it returns the number of elements in the array. When an operator functions on Scalars then its termed as Scalar Context.
Note:
- Whenever you assign anything to a Scalar variable it will always give Scalar Context.
- In this Context, presumption is to obtain a single value.
- An array if assigned to Scalar variable will return its size.
Scalar Context can be generated with the use of Scalar variables, Numerical operator, and many more.
- Assignment to a Scalar variable:
Example:$x = @z;
$x = localtime();
$x = Scalar;
Here, localtime() displays time in human readable format whereas in List Context this function shows number depiction of time.
- Assignment to a single element of an array:
Example:$a[2] = Scalar;
Every element of an array is individually a Scalar. So, assignment to them generates Scalar Context.
- Numerical operators creating Scalar Context:
Example:3 + Scalar;
Scalar + 3;
A numerical operator can generate Scalar Context on either sides of it.
- Concatenation creating Scalar Context:
Example:"GFG" . Scalar;
Scalar . "GFG"
From the above example, it is clear that Concatenation can generate Scalar Context on both side of itself.
Example:
#!/usr/bin/perl # Perl program of creating Scalar Context # array of elements my @CS = ( 'Beginner' , 'for' , 'Beginner' , 'articles' ); # Assignment to a Scalar variable my $x = @CS ; # Assignment of a function # to a Scalar variable # Note: Time displayed here # will be the GMT my $y = localtime (); # Numerical operator creating # Scalar Context my $z = 3 + @CS ; # Displays number of elements # in an Array print "$x\n" ; # Displays time stored in array # in human readable format print "$y\n" ; # Displays sum of a number # and Scalar print "$z\n" ; # Concatenation creating # Scalar Context print "The number of elements are: " . @CS |
4 Wed Mar 27 07:01:56 2019 7 The number of elements are: 4
One must require to force Scalar Context when Perl presumes a List. So, in that case you can utilize scalar() function which generates Scalar Context as Perl is informed by this function to impart Scalar Context for its parameters.
Example:
#!/usr/bin/perl # Perl program of Forcing Scalar Context # array of elements my @x = ( 'Beginner' , 'for' , 'Beginner' ); # Forcing Scalar context to display # number of elements in an Array print scalar @x ; print "\n" ; # Displaying time in human readable # format by forcing Scalar Context print scalar localtime (); |
3 Sun Mar 17 06:12:53 2019
In order to provoke Scalar Context using an array, it is required to assign an array to a Scalar variable.
Example:
#!/usr/bin/perl # Perl program of Arrays in Scalar Context # array of elements my @x = ( 'Beginner' , 'for' , 'Beginner' ); # Assignment of an Array to # a Scalar variable my $y = @x ; # Displays number of elements in # an Array print $y ; |
3
When the condition section of the if-statement presumes a single value then that is Scalar Context. In the below program, if-statement contains array, in scalar context, array returns the number of elements in it. So, if the array is empty then it will return 0 hence, if-statement will not execute if the array passed to it as scalar context is empty.
Program 1:
#!/usr/bin/perl # Program of if-statement in Scalar Context use strict; use warnings; use 5.010; # Array with no elements my @w = (); # Statement within 'if' will be executed # only if the array is not empty if ( @w ) { print "Beginner" ; } |
No Output
Here, nothing is printed as the stated Array is empty. So, the code does not displays the content of the if-statement.
Program 2:
#!/usr/bin/perl # Program of if-statement in Scalar Context use strict; use warnings; use 5.010; # An Array of elements my @w = ( 'G' , 'f' , 'G' ); # Statement within 'if' will be executed # only if the array is not empty if ( @w ) { print "There are some elements in the Array" ; } |
There are some elements in the Array
Here, the above stated Array is not empty so, the content of the if-statement is printed.
In order to place readline operator (i.e, <STDIN>) in Scalar Context it is required to designate this operator to a scalar variable.
Example:
#!/usr/bin/perl # Program to Read input from user use strict; use 5.010; # Asking the user to provide input print "Enter your name:\n" ; # Getting input from user my $y = <STDIN>; # Printing the required output print "My name is $y\n" ; |
Output:
Above program accepts the input from the user with the use of <STDIN> and store it in the Scalar variable. Further, use that scalar variable to print the Input provided by the user.