Prototyping with variables allows you to create realistic prototypes that change based on user selection, using only a few simple frames and interactions.

Expressions can help make your prototypes even more powerful. With expressions, you can generate dynamic string or number values, or even evaluate boolean expressions.

Prototyping with expressions makes it possible to do things such as:

• Create a shopping cart that calculates purchase total
• Build objects that scale in size, like volume or progress bars
• Combine and build new text strings based on user selection

Where to use expressions

Expressions provide a way to manipulate the values of variables with basic operations. They can be used in:

• The Set variable prototype action, if the selected variable is a number, string, or boolean type
• The Conditional prototype action, as a part of a conditional statement

You can write expressions directly in the following fields on the Interaction details modal:

1. [Set variable] to: Enter an expression to represent the new value of the selected variable
2. [Conditional] if: Enter a boolean expression to represent the condition required for the action

To build expressions in your prototype, you can either write them directly in the available fields using supported syntax, or use the selection panel to choose from suggested variables and operators.

Once your expression is complete, press Enter or Return. Only expressions written with supported operations and syntax will work. Invalid expressions will be outlined in red.

Write expressions

Expressions are made up of values and operators.

• Operators represent the function you are performing or evaluating (such as addition or subtraction)
• Values are the items that the operators are performing on or evaluating

For example, take a look at the following expression:

variableName + 2

In this example, the operator is addition, represented by the + plus symbol. The values are variableName and 2.

The expression type determines which values and operators are available.

Numerical expressions

Numerical expressions can be written with the following value types:

• Number variables
• Number literals (such as 0.5, 1, 10)

The following operators can be used in numerical expressions:

String expressions

String expressions can be written with the following value types:

• String literals (such as "John Doe”, “item 2”, “5”)
• Number literals
• String and number variables

String literals must be contained in quotations. Number values can be added on to a string value.

The following operator can be used in string expressions:

Boolean expressions

Boolean expressions can be written with the following value types:

• Boolean literals (true, false)
• Number literals
• String literals
• Boolean, string, and number variables

The following operators can be used in boolean expressions:

Boolean expressions must resolve to a true or false value. When setting a boolean variable with an expression, the result of the expression is evaluated to have either a true or false value—therefore setting the new value of the boolean variable.

For example, take a look at the simple interaction and expression below:

Set itemCount to 0
itemCount > 5

The value of itemCount is 0, which is not greater than 5. Therefore, the value of this expression is false. However, now take a look at the following example:

Set itemCount to 6
itemCount > 5

The value of itemCount is 6, which is greater than 5. Therefore, the value of this expression is true.

Examples of boolean expressions are listed in the tabs below. For these examples, the following statements are true:

numberVariable1 ==  1
numberVariable2 ==  2
stringVariable1 ==  red
stringVariable2 ==  blue

Set booleanVariable to:
numberVariable1 == numberVariable2

Set booleanVariable to:
numberVariable1 + 1 == numberVariable2

Set booleanVariable to:
stringVariable1 != stringVariable2

Set booleanVariable to:
numberVariable1 + 1 != numberVariable2

Set booleanVariable to:
numberVariable1 == 1 and numberVariable2 == 2

Set booleanVariable to:
numberVariable1 > 5 and numberVariable2 == 2

Set booleanVariable to:
stringVariable1 == red or stringVariable2 == purple

Set booleanVariable to:
stringVariable1 == green or stringVariable2 == orange

Complex expressions and order of operations

Complex expressions are built by using multiple operators within a single expression. Use parentheses to group expressions.

In complex expressions, basic math operations are performed in the following order:

1. Parentheses
2. Multiplication/Division
3. Addition/Subtraction

Boolean expression operators are performed in the following order:

1. Parentheses
2. Comparisons (==, !=, >, <, etc.)
3. And
4. Or

All operations are performed from left to right.

For example, in the following expression:

x + y * z

First, multiply y by z. Then, add x.

In the following boolean expression:

x == (y > z)

First, evaluate if y is greater than z. Then, evaluate if x is equal to the solution of y > z.

Concepts in expressions

Negative numbers

You can use negative numbers in expressions. To represent a negative number, use a - minus sign.

Negate boolean values

Negating a boolean value means flipping its logical state. Negating a true boolean value would make it false, and negating a false boolean value would make it true.

Negating boolean values can be valuable when buildings objects that have two opposing states—such as toggles, buttons, or other settings.

In order to negate a boolean value, enter ! or not before the boolean variable.

For example, consider the following expression: ! boolVar

• If the boolVar value is true, the statement is overall evaluated to be false.
• If the boolVar value is false, the statement, is overall evaluated to be true.