Declare equations or execute statements conditionally

#### Examples

#### If clause

**parameter** Boolean linear=true;
**parameter** Boolean quadratic=false;
Real x, y;
**equation**
**if** linear **then**
y = x + 1;
**elseif** quadratic **then**
y = x^2 + x + 1;
**else**
y = Modelica.Math.sin(x) + 1;
**end if**;

#### If expression

Integer i;
Integer sign_of_i=**if** i<0 **then** -1 **else if** i==0 **then** 0 **else** 1;

#### Syntax

In equation sections:

**if** expression **then**
{ equation ";" }
{ **elseif **expression **then**
{ equation ";" }
}
[ **else**
{ equation ";" }
]
**end if**

In algorithm sections:

**if** expression **then**
{ algorithm ";" }
{ **elseif **expression **then**
{ algorithm ";" }
}
[ **else**
{ algorithm ";" }
]
**end if**

#### Description

#### If clause

The expression of an **if** and
**elseif**-clause must be scalar Boolean expression.
One **if**-clause, and zero or more
**elseif**-clauses, and an optional
**else**-clause together form a list of branches. One
or zero of the bodies of these **if**-,
**elseif**- and **else**-clauses is
selected, by evaluating the conditions of the **if**-
and **elseif**-clauses sequentially until a condition
that evaluates to true is found. If none of the conditions evaluate
to true the body of the **else**-clause is selected
(if an **else**-clause exists, otherwise no body is
selected). In an algorithm section, the selected body is then
executed. In an equation section, the equations in the body are
seen as equations that must be satisfied. The bodies that are not
selected have no effect on that model evaluation.

If clauses in **equation** sections which do not
have exclusively parameter expressions as switching conditions
shall have an **else** clause and each branch shall
have the *same number* of *equations*. *[If this
condition is violated, the single assignment rule would not hold,
because the number of equations may change during simulation
although the number of unknowns remains the same].*

#### If expression

An expression

**if** expression1 **then** expression2 **else** expression3

is one example of if-expression. First expression1, which must
be Boolean expression, is evaluated. If expression1 is true
expression2 is evaluated and is the value of the if-expression,
else expression3 is evaluated and is the value of the
if-expression. The two expressions, expression2 and expression3,
must be type compatible and give the type of the if-expression.
If-expressions with **elseif** are defined by
replacing **elseif** by **else
if**.*[Note: elseif is added for symmetry with
if-clauses.]*

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