(octave.info)Assignment Ops

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8.6 Assignment Expressions
==========================

An “assignment” is an expression that stores a new value into a
variable.  For example, the following expression assigns the value 1 to
the variable ‘z’:

     z = 1

After this expression is executed, the variable ‘z’ has the value 1.
Whatever old value ‘z’ had before the assignment is forgotten.  The ‘=’
sign is called an “assignment operator”.

   Assignments can store string values also.  For example, the following
expression would store the value "this food is good" in the variable
‘message’:

     thing = "food"
     predicate = "good"
     message = [ "this " , thing , " is " , predicate ]

(This also illustrates concatenation of strings.)

   Most operators (addition, concatenation, and so on) have no effect
except to compute a value.  If you ignore the value, you might as well
not use the operator.  An assignment operator is different.  It does
produce a value, but even if you ignore the value, the assignment still
makes itself felt through the alteration of the variable.  We call this
a “side effect”.

   The left-hand operand of an assignment need not be a variable (Note:
Variables).  It can also be an element of a matrix (Note: Index
Expressions) or a list of return values (Note: Calling Functions).
These are all called “lvalues”, which means they can appear on the
left-hand side of an assignment operator.  The right-hand operand may be
any expression.  It produces the new value which the assignment stores
in the specified variable, matrix element, or list of return values.

   It is important to note that variables do _not_ have permanent types.
The type of a variable is simply the type of whatever value it happens
to hold at the moment.  In the following program fragment, the variable
‘foo’ has a numeric value at first, and a string value later on:

     octave:13> foo = 1
     foo = 1
     octave:13> foo = "bar"
     foo = bar

When the second assignment gives ‘foo’ a string value, the fact that it
previously had a numeric value is forgotten.

   Assignment of a scalar to an indexed matrix sets all of the elements
that are referenced by the indices to the scalar value.  For example, if
‘a’ is a matrix with at least two columns,

     a(:, 2) = 5

sets all the elements in the second column of ‘a’ to 5.

   Assigning an empty matrix ‘[]’ works in most cases to allow you to
delete rows or columns of matrices and vectors.  Note: Empty Matrices.
For example, given a 4 by 5 matrix A, the assignment

     A (3, :) = []

deletes the third row of A, and the assignment

     A (:, 1:2:5) = []

deletes the first, third, and fifth columns.

   An assignment is an expression, so it has a value.  Thus, ‘z = 1’ as
an expression has the value 1.  One consequence of this is that you can
write multiple assignments together:

     x = y = z = 0

stores the value 0 in all three variables.  It does this because the
value of ‘z = 0’, which is 0, is stored into ‘y’, and then the value of
‘y = z = 0’, which is 0, is stored into ‘x’.

   This is also true of assignments to lists of values, so the following
is a valid expression

     [a, b, c] = [u, s, v] = svd (a)

that is exactly equivalent to

     [u, s, v] = svd (a)
     a = u
     b = s
     c = v

   In expressions like this, the number of values in each part of the
expression need not match.  For example, the expression

     [a, b] = [u, s, v] = svd (a)

is equivalent to

     [u, s, v] = svd (a)
     a = u
     b = s

The number of values on the left side of the expression can, however,
not exceed the number of values on the right side.  For example, the
following will produce an error.

     [a, b, c, d] = [u, s, v] = svd (a);
     ⊣ error: element number 4 undefined in return list

   The symbol ‘~’ may be used as a placeholder in the list of lvalues,
indicating that the corresponding return value should be ignored and not
stored anywhere:

     [~, s, v] = svd (a);

   This is cleaner and more memory efficient than using a dummy
variable.  The ‘nargout’ value for the right-hand side expression is not
affected.  If the assignment is used as an expression, the return value
is a comma-separated list with the ignored values dropped.

   A very common programming pattern is to increment an existing
variable with a given value, like this

     a = a + 2;

This can be written in a clearer and more condensed form using the ‘+=’
operator

     a += 2;

Similar operators also exist for subtraction (‘-=’), multiplication
(‘*=’), and division (‘/=’).  An expression of the form

     EXPR1 OP= EXPR2

is evaluated as

     EXPR1 = (EXPR1) OP (EXPR2)

where OP can be either ‘+’, ‘-’, ‘*’, or ‘/’, as long as EXPR2 is a
simple expression with no side effects.  If EXPR2 also contains an
assignment operator, then this expression is evaluated as

     TEMP = EXPR2
     EXPR1 = (EXPR1) OP TEMP

where TEMP is a placeholder temporary value storing the computed result
of evaluating EXPR2.  So, the expression

     a *= b+1

is evaluated as

     a = a * (b+1)

and _not_

     a = a * b + 1

   You can use an assignment anywhere an expression is called for.  For
example, it is valid to write ‘x != (y = 1)’ to set ‘y’ to 1 and then
test whether ‘x’ equals 1.  But this style tends to make programs hard
to read.  Except in a one-shot program, you should rewrite it to get rid
of such nesting of assignments.  This is never very hard.