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What is an lvalue? (Perl, C)

An lvalue is an expression that you can write on the left hand side of an assignment statement - in other words an expression that defines a specific memory address of a variable.

The most common lvalues are simple variables or array / list / hash / dictionary members ... for example

$hello = "Hello World";
$greet[5] = "Hello World";
$greet{"UK"} = "Hello World"; # all these in Perl


val = 15;
jolly[posn] = 27; # These two in C


There are some surprises until you think about it.

In Perl you can write $lhs[$n+4] = 17; but not $lhs+4 = 17;. Yet in C, if vvv is a pointer to an array you can write vvv+4 = 15; quite happily - altering the fifth element of the array.

You'll find the term lvalue come up most usually for the programmer in a compiler error message - "invalid lvalue" you will be told. What the compiler really means is that the twit who wrote the code is trying to save a value into an expression that doesn't actually define the memory address of a variable.
(written 2008-03-18)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
C212 - C and C based languages - Memory Management
  [4635] Encapsulating logic in functions and structs - the C approach to Object Oriented techniques - (2016-01-30)
  [4634] Regression testing - via a very short C testing framework - (2016-01-29)
  [4340] Simple C structs - building up to full, dynamic example - (2014-12-03)
  [4128] Allocating memory dynamically in a static language like C - (2013-06-30)
  [3416] Storing Tcl source code encoded, and running via your own C program - (2011-09-02)
  [3386] Adding the pieces together to make a complete language - C - (2011-08-11)
  [3144] Setting up arrays in C - fixed size at compile time, or dynamic - (2011-01-24)
  [3118] Arrays of arrays - or 2D arrays. How to program tables. - (2011-01-02)
  [2848] C course - final course example puts it all together - (2010-07-02)
  [2669] Efficient use of dynamic memory - C and realloc - (2010-03-10)
  [1845] Passing a table from Lua into C - (2008-10-18)
  [1670] Dynamic Memory Allocation in C - (2008-06-09)
  [1589] Dynamic Memory Allocation in C - calloc, realloc - (2008-03-22)
  [1497] Training Season Starts again! - (2008-01-07)

P301 - Variables in Perl
  [4608] Introspecion in Perl 6 - (2016-01-02)
  [4398] Accessing variables across subroutine boundaries - Perl, Python, Java and Tcl - (2015-01-18)
  [3430] Sigils - the characters on the start of variable names in Perl, Ruby and Fortran - (2011-09-10)
  [3059] Object Orientation in an hour and other Perl Lectures - (2010-11-18)
  [2972] Some more advanced Perl examples from a recent course - (2010-09-27)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2374] Lead characters on Perl variable names - (2009-08-24)
  [2241] Perl references - $$var and \$var notations - (2009-06-15)
  [1946] Variable Types in Perl - (2008-12-15)
  [975] Answering ALL the delegate's Perl questions - (2006-12-09)


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