Single Mode Arithmetic Expressions

An arithmetic expression is an expression using additions +, subtractions -, multiplications *, divisions /, and exponentials **. A single mode arithmetic expression is an expression all of whose operands are of the same type (i.e. INTEGER, REAL or COMPLEX). However, only INTEGER and REAL will be covered in this note. Therefore, those values or variables in a single mode arithmetic expression are all integers or real numbers.

In single mode arithmetic expressions, the result of an operation is identical to that of the operands. The following is a table showing this fact. The empty entries will be discussed in mixed mode arithmetic expressions.

Operator	INTEGER	REAL
INTEGER	INTEGER	mixed mode
REAL	mixed mode	REAL

Simple Examples:

1 + 3 is 4
1.23 - 0.45 is 0.78
3 * 8 is 24
6.5/1.25 is 5.2
8.4/4.2 is 2.0 rather than 2, since the result must be of REAL type.
-5**2 is -25
12/4 is 3
13/4 is 3 rather than 3.25. Since 13/4 is a single mode arithmetic expression and since all of its operands are of INTEGER type, the result must also be of INTEGER type. The computer will truncate the mathematical result (3.25) making it an integer. Therefore, the result is 3.
3/5 is 0 rather than 0.6.

Rules for Evaluating Expressions

The following are rules of evaluating a more complicated single mode arithmetic expression:

Expressions are always evaluated from left to right
If an operator is encountered in the process of evaluation, its priority is compared with that of the next one:

if the next one is lower, evaluate the current operator with its operands
```
3 * 5 - 4
```
In the above expression, in the left to right scan, operator * is encountered first. Since the the operator - is lower, 3 * 5 is evaluated first transforming the given expression to 15 - 4. Hence, the result is 11.
if the next one is equal to the current, the associativity rules are used to determine which one should be evaluated. For example, if both the current and the next operators are *, then 3 * 8 * 6 will be evaluated as (3 * 8) * 6. On the other hand, if the operator is **, A ** B ** C will be evaluated as A ** (B ** C).
if the next one is higher than the current, the scan should continue with the next operator. For example, consider the following expression:
```
4 + 5 * 7 ** 3
```
if the current operator is +, since the next operator * has higher priority, the scan continues to *. Once the scan arrives at *, since the next operator ** is higher, 7 ** 3 is evaluated first, transforming the given expression to
```
4 + 5 * 343
```
Then, the new expression is scan again. The next operator to be evaluated is *, followed by +. Thus, the original expression is evaluated as 4 + (5 * (7 ** 3)).

More Complicated Examples:

In the following examples, brackets are used to indicated the order of evaluation.

The result is 4 rather than 4.444444 since the operands are all integers.

2 * 4 * 5 / 3 ** 2
     --> [2 * 4] * 5 / 3 ** 2
     --> 8 * 5 / 3 ** 2
     --> [8 * 5] / 3 ** 2
     --> 40 / 3 ** 2
     --> 40 / [3 ** 2]
     --> 40 / 9
     --> 4

As in mathematics, subexpressions in parenthesis must be evaluated first.

100 + (1 + 250 / 100) ** 3
     --> 100 + (1 + [250 / 100]) ** 3
     --> 100 + (1 + 2) ** 3
     --> 100 + ([1 + 2]) ** 3
     --> 100 + 3 ** 3
     --> 100 + [3 ** 3]
     --> 100 + 27
     --> 127

In the following example, x**0.25 is equivalent to computing the fourth root of x. In general, taking the k-th root of x is equivalent to x**(1.0/k) in Fortran, where k is a real number.

1.0 + 2.0 * 3.0 / ( 6.0*6.0 + 5.0*44.0) ** 0.25
     --> 1.0 + [2.0 * 3.0] / (6.0*6.0 + 5.0*44.0) ** 0.25
     --> 1.0 + 6.0 / (6.0*6.0 + 5.0*55.0) ** 0.25
     --> 1.0 + 6.0 / ([6.0*6.0] + 5.0*44.0) ** 0.25
     --> 1.0 + 6.0 / (36.0 + 5.0*44.0) ** 0.25
     --> 1.0 + 6.0 / (36.0 + [5.0*44.0]) ** 0.25
     --> 1.0 + 6.0 / (36.0 + 220.0) ** 0.25
     --> 1.0 + 6.0 / ([36.0 + 220.0]) ** 0.25
     --> 1.0 + 6.0 / 256.0 ** 0.25
     --> 1.0 + 6.0 / [256.0 ** 0.25]
     --> 1.0 + 6.0 / 4.0
     --> 1.0 + [6.0 / 4.0]
     --> 1.0 + 1.5
     --> 2.5

Click here to continue with mixed mode arithmetic expressions.