4.3: Variables and Assignment

The assignment statement is the most basic operation. We proceed by example (using the command line input mode):

>> pi_approx = 3.14159
pi_approx =
    3.1416
>>

where we enter the material after the prompt >> and Matlab responds as indicated in the lines below the prompt line. In general, Matlab will display the result of an operation in the command window unless we put a semi-colon at the end of the instruction.

It is important to understand what this statement actually does. The variable pi_approx is a name to which Matlab will associate a unique address — a location in memory; hence the variable pi_approx is in fact an address (a name), not a value. The assignment statement puts the value \(3.14159\) at the address pi_approx. Again, pi_approx is the address, or reference, or pointer, or box, which contains the value. If we change the value

>>pi_approx = 3.14
pi_approx =
    3.14
>>

we are simply replacing the earlier value \(3.14159\) at address pi_approx with the new value \(3.14\) at the same address pi_approx.

In the above assignment statement the \(=\) is an operator (and an operator which is part of the core Matlab functionality). An operator is a function which takes one or two arguments and produces an output; operators, given the few operands, can be conveniently represented as a symbol. In the next few sections we will introduce a number of useful operators. In terms of an operator, = looks to the right for the value and then places this value in the address indicated to the left.

In our assignment for pi_approx the way in which we have entered the number ensures that pi_approx is of type floating point double. But we can be more explicit as in

>> pi_approx = double(3.14159)
pi_approx =
    3.1416
>> floatcheck = isfloat(pi_approx)
floatcheck =
    1
>>

which confirms that the double function indeed produces a variable of data type double.

We emphasize that both double and isfloat are functions. The general syntax for a function is simple: the input or inputs are included in parentheses following the function name, and the output (later, many outputs) - the evaluation of the function - is assigned to the variable to the left of the \(=\). For example, isfloat takes as input a variable - here the input is pi_approx - and returns as output a logical variable which indicates whether the input is a floating point number here the output is a 1 (true), since pi_approx is indeed a floating point number (or more precisely, an instance of the floating point double data type).

It is worth dissecting the above example a bit further. In fact, the statement

>> floatcheck = isfloat(pi_approx)
floatcheck =
    1
>>

is a very simple example of a very powerful capability — composition. In particular, in this case when = looks to the right it finds not a value but a function, isfloat; the function isfloat is then evaluated and the output of isfloat ( a logical 1) is then taken as the (right) input to the = (assignment) operator; the = then looks to the left, floatcheck, to know where in memory to write this value (a logical 1) (or to create a memory location with the “name” floatcheck, as needed).

We invite the reader to input a logical with logical and check that the result is logical with islogical, and an integer with int32 and check that the result is integer with isinteger. We will discuss characters and strings later in a separate section. Most often we will use the double data type, which is the Matlab default and requires no special attention — we just enter the number as \(3.14159\). We will also make extensive use of the logical data type.