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 (see Variables). It can also be an element of a matrix (see Index Expressions) or a list of return values (see 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. See 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.