Introduction

An expression is a representation of a value or of the computation of a value. Expressions are classified as either element (scalar) expressions or array expressions.

Element expressions represent element values, including elementary names within a structure or a subscripted name that specifies a single element of an array. Array expressions represent either an entire array or a cross-section of an array. Element variables and array variables may be used in the same expression.

Expressions are used for the following reasons:

To indicate constant values or scalar variables. For example:
```
D = 40;
```
To perform algebraic or logical calculations on constants or variables. For example:
```
A = B + 10;
```
To compare values of two or more expressions and obtain a Boolean result. For example:
```
IF C < D THEN E = 12;
```
To concatenate bit-string or character-string values. For example:
```
ADDRESS = CITY | STATE;
```

All expressions, except for single constants and references, are made up of an operator and one or more operands. An operator requires specific types of operands (arithmetic, character-string, or bit-string) and produces a result of a specific type. Operands can be constants, variable references, user-defined function references, built-in function references, or other expressions that match the type required by the operator.

Operators can be either infix or prefix. An infix operator is written between its two operands, and a prefix operator is written in front of its operands, as shown in the following table of operators.

Operator	Category	Example
+ – * / **	arithmetic infix	B+C
+ –	arithmetic prefix	-C
= ^= > < >= <= ^< ^>	relational	B=C
\| or &	bit-string infix	BIC
^	bit-string prefix	^C
\|\| or !!	concatenate	B\|\|C

One expression can contain both an infix and a prefix operator. For example:

A*-B

Note:

Open PL/I supports the use of a tilde (~) as an operator symbol equivalent to a caret (^).

Open PL/I supports the use of an exclamation point (!) as an operator symbol equivalent to a single bar (|), and a double exclamation point (!!) as equivalent to a double bar (||).