Example 1
Variables can be given a default value before their definition.
{{#vardefine:n|{{#expr:{{#var:k|0}}+1}}}}
Now {{#var:n}}
= 1
Otherwise you may get an error or otherwise unexpected result:
{{#vardefine:n|{{#expr:{{#var:k}}*1}}}}
Now {{#var:n}}
= Expression error: Unexpected * operator.
Example 2
{{#vardefine:a|7}}{{#vardefine:b|11}} {{#vardefine:x|{{#expr:2*{{#var:a}}+{{#var:b}}}} }}
Now {{#var:x}}
= 25