Expression Evaluation Context

Perl Tutorials - Herong's Tutorial Examples

∟Expressions, Operations and Simple Statements

∟Expression Evaluation Context

This section describes expression evaluation contexts: scalar context and list context. Operations and function may behave differently in different context. A list value in a scalar context is converted by taking the last value.

Depending on where it is used, an expression will be evaluated under two different context, Scalar Context and List Context.

1. Scalar Context - When an expression is used as an argument of an operation that requires a scalar value, the expression will be evaluated under a scalar context. For example, 9.99 * EXPR requires EXPR to be evaluated in a scalar context.

2. List Context - When an expression is used as an argument of an operation that requires a list value, the expression will be evaluated under a list context. For example, sort(EXPR) requires EXPR to be evaluated in a list context.

Expressions behave differently in different contexts following these rules:

An expression in a scalar context will be evaluated to a scalar value.
An expression in a list context will be evaluated to a list value.
Some operators automatically return different values in different contexts. For example: <IN> will read and return only one line as a string in a scalar context. <IN> will read and return all lines as a list of strings in a list context.
Some functions automatically returns different values in different contexts. For example: sort(LIST) will sort all elements of the input list and return them as a list in a list context. sort(LIST) will return the 'undef' value in a scalar context.
If the expression returns scalar value in a list context, Perl will convert it into a list value with one element - the returned scalar value.
If the expression returns list value in a scalar context, Perl will convert it into a scalar value by take the last element of the returned list value.

Here is a Perl tutorial script on how expressions are evaluated in a scalar or list context:

#- ExpressionContext.pl
#- Copyright (c) HerongYang.com. All Rights Reserved.
#

   @links = ("herongyang.com", "perl.org", "cpan.org");

   $string = join(', ',sort(@links));   # sort() in a list context
   print($string, "\n");

   $string = "My bookmarks: ".sort(@links);
                                        # sort() in a scalar context
   print($string, "\n");

   $interest = 39.99;
   $capital = 1234.56;
   @rates = $interest/$capital;         # converting scalar to list
   print(@rates, "\n");

   $interest = 100*(0.02, 0.05, 0.03);  # converting list to scalar
   print($interest, "\n");

Here is the output of the tutorial script:

cpan.org, herongyang.com, perl.org
My bookmarks:
0.0323921073094868
3

Table of Contents

About This Book

Perl on Linux Systems

ActivePerl on Windows Systems

Data Types: Values and Variables

►Expressions, Operations and Simple Statements

What Is an Expression?

►Expression Evaluation Context

Simple Statements and Modifiers

User Defined Subroutines

Perl Built-in Debugger

Name Spaces and Perl Module Files

Symbolic (or Soft) References

Hard References - Addresses of Memory Objects

Objects (or References) and Classes (or Packages)

Typeglob and Importing Identifiers from Other Packages

String Built-in Functions and Performance

File Handles and Data Input/Output

Open Files in Binary Mode

Open Directories and Read File Names

File System Functions and Operations

Image and Picture Processing

Using DBM Database Files

Using MySQL Database Server

Socket Communication Over the Internet

XML::Simple Module - XML Parser and Generator

XML Communication Model

SOAP::Lite - SOAP Server-Client Communication Module

Perl Programs as IIS Server CGI Scripts

CGI (Common Gateway Interface)

XML-RPC - Remote Procedure Call with XML and HTTP

RPC::XML - Perl Implementation of XML-RPC

Integrating Perl with Apache Web Server

CGI.pm Module for Building Web Pages

LWP::UserAgent and Web Site Testing

Converting Perl Script to Executable Binary

Managing Perl Engine and Modules on macOS

Archived Tutorials

References

Full Version in PDF/EPUB