Precedence Example

The precedence is worked out as follows

1. in an expression find the operator(s) with the tightest binding.
2. if there are more than one occurrence of this operator then the separate instances are evaluated left to right.
3. place the first executed subexpression in brackets to signify this.
4. continue with the second and subsequent subexpressions.
5. move to next most tightest binding operator and follow the same procedure.

It is easy to make mistakes by forgetting the implications of precedence. The following expression,

    x = a+b/5.0-c**d+1*e

is equivalent to

    x = ((a+(b/5.0))-(c**d))+1*e

The following procedure has been followed to parenthesise the expression,

• tightest binding operator is **. This means c**d is the first executed subexpression so should be surrounded by brackets.
• / and * are the second most tightly binding operators and expressions involving them will be evaluated next, put b/5.0 and 1*e in brackets.
• + and - have the next highest precedence. Since they are of equal precedence, occurrences are evaluated from left
• at last the assignment is made.

Likewise, the following expression,

    .NOT.A.OR.B.EQV.C.AND.D.OR..NOT.E

is equivalent to

    ((.NOT.A).OR.B).EQV.((C.AND.D).OR.(.NOT.E))

here,

• the tightest binding operator is: .NOT. followed by .AND. followed by .OR..
• the two subexpressions containing the monadic .NOT. are evaluated first, as there are two of these the leftmost, .NOT.A is done first followed by .NOT.E.
• the subexpression C.AND.D is evaluated next followed by .OR. (left to right)
• finally the .EQV. is executed

Parentheses can be added to any expression to modify the order of evaluation.

Now try this question

Return to corresponding overview page