# Ada Programming/Expressions

## Expressions[edit]

The LRM defines an expression as a formula that defines the computation or retrieval of a value.

.^{[1]} There are numerous forms of expression, ranging from primaries such as literals or names, to quantified expression.

An expression is typically used in an assignment, or as part of a bigger expression. The value of an expression usually involves computation. However, some expressions' values are determined at compile time; these are called *static* expressions. A so-called simple expression (which happens to be a *term*) is seen in

Area := Length * Height;

The text `Length * Height`

has the form of an expression, and is used on the right hand side of an assignment statement. Computing the value of the expression means multiplying the value named `Length`

by the one named `Height`

. Using the same expression as part of a bigger expression is demonstrated in the following example:

`if`

Cost (Paint, Area => Length * Height) > 900 * Dollar`then`

...

The bigger expression starts with `Cost`

and ends with `Dollar`

. It features another form of expression, a *relation*, and places a function call and another multiplicative expression to the left and right, respectively, of the relation's relational operator `>`

. The two are called *operands* and the result is a Boolean expression.

### Kinds of Expressions[edit]

Among basic expressions are *literals*, for example, decimal (real), enumeration, string, and access value literals:

```
2.5e+3
False
"и"
````null`

Involving many of those one can write *aggregates* (a *primary*),

(X => 0.0, Y => 1.0, Z => 0.0)

but arbitrarily complex sub-components are possible, too, creating an aggregate from component expressions,

```
(Height => 1.89 * Meter,
Age => Guess (Picture => Images.Load (Suspects, "P2012-Aug.PNG"),
Tiles => Grid'(1 .. 3 => Scan, 4 => Skip)),
Name =>
````new`

Nickname'("Herbert"))

`Age`

is associated with the value of a nested *function call*. The actual parameter for `Tiles`

has type name `Grid`

qualify the array aggregate following it; the component `Name`

is associated with an *allocator*.

The well known ‘mathematical’ expressions have closely corresponding *simple expressions* in Ada syntax, for example `2.0*π*r`

, or the *relation*

Area = π*r**2

Other expressions test for *membership* in a range, or in a type:

X`in`

1 .. 10 | 12 Shape`in`

Polygon'Class

### Conditional Expressions, Quantified Expressions[edit]

New in Ada 2012, there are forms for making the value of an expression depend on the value of another expression, conditionally. They have a pair of parentheses around them and they start with a keyword, which clearly distinguishes them from other kinds of expression. For example:

Area := (`if`

Is_Circular`then`

Pi * Radius**2`else`

Length * Height);

In this example, the value of `Is_Circular`

determines the part of the expression that is used for computing the value of the entire expression. A similar construct exists for `case`

and also one for `for`

. These kinds of expressions are frequently used in assertions, like in the conditions of contract based programming.