OPERATOR PRECEDENCE
Pascal, when determining how to perform calculations, works according to pre-defined rules. These rules may be overridden by the use of parenthesis ().

The priority given to the various operators, from highest to lowest, are

    NOT               Negation

    * / DIV MOD AND

    + - OR

    =  <>  <  <=  >  >=  IN
    The operators are always evaluated left to right

Class Exercise on Operator precedence
Given that

  A := 1;      B := 2;      C := 4;

What does X equal after each of the following statements,

  X := A / B / C;               ________________

  X := A + B / C;               ________________

  X := A * B * C;               ________________

  X := A * B - C;               ________________

  X := A + B + C;               ________________

  X := A / B * C;               ________________

  X := A * B / C;               ________________

  X := A + B - C;               ________________

Click here for answer

Parenthesis are used to override the order of precedence. Consider the expression

             A + B

        X = -------

             C + D

becomes in Pascal

        X := ( A + B ) / ( C + D )

and the expression

                   B

        X = A  +  ---  +  D

                   C

becomes in Pascal

        X := A + ( B / C ) + D

Self Test on Operator Precedence

Write statements in Pascal which correctly express each of the following mathematical expressions.
2 2
1. Z = X + Y 2. Z = ( X + Y )

A + B + E B
3. Z = ----------- 4. Z = A + ---
D + E C

A + B B
5. Z = ------- 6. Z = A + -------
C D - C

2 2
1. Z = X + Y 2. Z = ( X + Y )

Z := X + (Y * Y); Z := (X + Y) * (X + Y);

A + B + E B
3. Z = ----------- 4. Z = A + ---
D + E C

Z := (A+B+E) / (D+E); Z := A + (B / C);

A + B B
5. Z = ------- 6. Z = A + -------
C D - C

Z := (A + B) / C; Z := A + ( B / ( D - C ) );

STATE which of the following statements is wrong and why,

Y := 2X + A 2 * X + A {missing operator}
4 := X - Y 4 is a constant
A := 1 / ( X + ( Y - 2 ) missing bracket
-J := K + 1 rewrite as j := - ( k + 1);
S := T / * 3 one too many operators
Z + 1 := A rewrite as z:= A - 1;

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