Naming variables, If() and Input()

When naming variables, MetaStock outlines a few rules to follow:

• Variable names cannot contain commas, parentheses, spaces, underscores and the like.
• Variable names cannot duplicate names already used by functions (e.g. mov, rsi, if).
• Variables cannot be assigned a name that matches the parameters reserved for use in formulas (e.g. open, high, low, close, simple, including their respective shortcuts).
• Variable names must contain at least one alpha letter (e.g. T1234).
• Variable names are not case sensitive: PERIODS is the same as periods.

The If() function

The if() function employs one of the most common idioms of modern programming languages, the "if... then... else..." construct. It is a conditional statement: if a predefined expression occurs, perform x; if it does not, perform y. The correct syntax is:

If({expression},{then data},{else data})

• The expression is the required condition.
• The then data is the action or value if the expression is true.
• The else data is the action or value if the expression is false.

Let's look at an example called "positive and negative volume":

if( C > O, +V, -V )

This formula plots positive volume if the security's close is greater than its open; otherwise negative volume is plotted. The formula itself doesn't have much practical use, but it shows if() in action. A more practical use appears in the On Balance Volume indicator, covered in our OBV tutorial. Its formula is a bit advanced, but it is a useful example of if() at work:

(If(C > Ref(C,-1),1,-1) * VOLUME) + PREV

We will unpack formulas like this, and their intricacies, later in the course.

The Input function

The Input function is used with custom indicators to prompt the user for input. You have probably met it without noticing: a good number of MetaStock's predefined indicators use it. When applying the moving average indicator, MetaStock prompts you to enter time periods, method, price fields and so on. Those are all input functions. The basic syntax:

input("{prompt text}",{minimum value},{maximum value},{default value});

• The prompt text is displayed next to the input box and describes what is required, e.g. moving average period.
• The minimum value is the smallest value that can be entered.
• The maximum value is the largest value that can be entered.
• The default value appears initially in the input box.

Because we define the minimum and maximum, a value outside that range triggers an error message asking for a value greater than or less than what was entered.

Here is an example. The following custom indicator asks for the time periods you wish to use for a moving average of the On Balance Volume indicator, i.e. the number of smoothing periods:

SmoothingPeriods:=Input("Enter the number of OBV smoothing periods",1,21,7);
Mov(OBV(21),SmoothingPeriods,S);

When this custom indicator is plotted, an input dialog appears prompting you to "Enter the number of OBV smoothing periods".

Next lesson

In Lesson 7 we cover inserting comments into your formulas, then give variables the full treatment: assigning values and whole expressions to shorthand names.