# How to | Create Definitions for Variables and Functions

*Mathematica* has a very general notion of functions, as rules for arbitrary transformations. Values for variables are also assigned in this manner. When you set a value for a variable, the variable becomes a symbol for that value.

Here is a simple transformation rule. It says: whenever you see , replace it by 3:

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The variable has a value of 3.

Whenever you evaluate an expression, 3 is substituted for :

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You can remove the rule by defining a new one:

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The new rule says: whenever you see , replace it by . So far there are no rules associated with , so its value is itself.

Assign a value to :

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Now if you evaluate , the rule for says to replace by , and the rule for says to replace by 4, so the result is , or 16:

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If you change the value of , then the value of changes:

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Now assign a value to , like this:

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Since has already been assigned the value 3, the rule you have defined is "replace by 9", not "replace by ". So does not depend on :

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This happened because when a rule is defined using (Set), the right-hand side is evaluated before the rule is defined.

You can also define rules using (SetDelayed), like this:

When a rule is defined with the right-hand side is not evaluated before the rule is defined. So even if already has a value, this new rule says: whenever you see , replace it with . So in this case, depends on :

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Functions in *Mathematica* are defined by rules that act on patterns. Here is a simple one:

is a pattern in which stands for any expression (which is represented on the right-hand side by the name ). The rule says: if you have of any expression, replace it by that expression squared:

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Here is a function with two arguments:

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Always use to define functions, otherwise the variables on the right-hand side may not represent the associated expressions on the left-hand side, since they will be evaluated before the rule is defined:

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That happened because is 9 and is 3. This rule says that anything matching the pattern is replaced by 90:

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