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:
The variable has a value of 3.
Whenever you evaluate an expression, 3 is substituted for :
You can remove the rule by defining a new one:
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 :
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:
If you change the value of , then the value of changes:
Now assign a value to , like this:
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 :
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 :
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:
Here is a function with two arguments:
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:
That happened because is 9 and is 3. This rule says that anything matching the pattern is replaced by 90: