BBC BASIC Programmers' Reference

by Richard Russell, July 2009

In the C programming language, an assignment operation such as:

        variable = expression

can be used either as a statement or as a function. When used as a function, it returns the new value of the variable. Whilst at first sight this might not seem very useful, it is particularly convenient in the case of a while loop, for example:

        while (variable = expression)
          {
            // Do something useful here
          }

Here variable is set equal to the value of expression and if its new value is non-zero the body of the loop is executed (note that it is not testing whether variable is equal to expression; in C you do that using the == operator).

Since in BBC BASIC an assignment is a statement, you can't straightforwardly do this, and you have to code it as follows:

        variable = expression
        WHILE variable
          REM Do something useful here
          variable = expression
        ENDWHILE

As you can see, this involves writing the assignment statement twice, once outside the loop and again inside the loop. This is inelegant and potentially error-prone, for example you might make a change to one of the assignments but forget to change the other.

To emulate the C behaviour you can utilise this simple function:

        DEF FNassign(RETURN variable, expression)
        variable = expression
        = variable

Now you can write the loop as follows:

        WHILE FNassign(variable, expression)
          REM Do something useful here
        ENDWHILE

Note that since variant numeric variables are used in the function (i.e. without a 'type' suffix character) it will work equally well with integer variables and values as with floating-point variables.