Division
Keywords: pame

Subsections

- Exercises

#
Division

In the preceding sections, all the operators mentioned yield results which have the same mode as the operand or operands. In this and following sections, we shall see that this is not always the case.

Division poses a problem because division by integers can have two different meanings. For example, 3 ÷ 2 can be taken to mean `1` or `1.5`. In this case, we use two different operator symbols.

Integer division is represented by the symbol `%`. It takes operands of mode `INT` and yields a value of mode `INT`. It has the alternative representation `OVER`. The formula `7 % 3` yields the value `2`, and the formula `-7 % 3` yields the value `-2`. The priority of `%` is 7, the same as multiplication. Here are some identity declarations using the operator:

       INT r = 23 OVER 4, s = -33 % 3;
       INT q = r * s % 2

Using the given values of `r` and `s`, the value of `q` is `-27`. When a formula containing consecutive dyadic operators of the same priority is elaborated, elaboration is always left-to-right, so in this case the multiplication is elaborated first, followed by the integer division. Of course, `%` can be combined with subtraction as well as all the other operators already discussed.

The modulo operator `MOD` gives the remainder after integer division. It requires two operands of mode `INT` and yields a value also of mode `INT`. Thus `5 MOD 3` yields `2`, and `12 MOD 3` yields `0`. It does work with negative integers, but the results are unexpected. You can explore `MOD` with negative integers in an exercise. `MOD` can also be written as `%*`. The priority of `MOD` is 7.

Division of real numbers is performed by the operator `/`. It takes two operands of mode `REAL` and yields a `REAL` result. Thus the formula `3.0/2.0` yields `1.5`. Again, `/` can be combined with `*` and the other operators already discussed. It has a priority of 7. The operator is also defined for integer operands. Thus `3/2` yields the value `1.5`. No widening takes place here since the operator is defined to yield a value of mode `REAL` when its operands have mode `INT`.

Here are some identity declarations using the operators described so far:

       REAL pi by 2 = pi / 2,
            pm3 = pi - 3.0 * -4.1;
       INT c = 22 % 3 - 22 MOD 3;
       INT d = c MOD 6 + SIGN -36

## Exercises

2.5
  What is the value yielded by each of the following formulæ, and what is its mode? Ans
  (a)
    `5 * 4`

  (b)
    `5 % 4`

  (c)
    `5 / 4`

  (d)
    `5 MOD 4`

  (e)
    `5.0 * 3.5 - 2.0 / 4.0`

2.6
  Write a short program to print the results of using `MOD` with negative integer operands. Try either operand negative, then both operands negative. Ans
2.7
  Give an identity declaration for the identifier `two pi`. Ans