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Expressions

Published 17 Aug, 2017
561 words
~2-4 mins

In events, expressions are used to calculate sums or retrieve information from objects, such as a Sprite's X co-ordinate. Expressions are entered in to the Parameters dialog when adding or editing a condition or action which has parameters. The Expressions dictionary is also shown at the same time and provides a dictionary of all the system and object expressions available in a project.

Some examples of expressions, which can range from a simple number to a complex calculation, are given below:

  • 0
  • random(360)
  • Sprite.X
  • (Player1.X + Player2.X) / 2
  • Sprite.8Direction.Speed
  • Sprite.X + cos(Sprite.Angle) * Sprite.Speed * dt

Numbers

Numbers are simply entered as digits with an optional fractional part separated by a dot, e.g. 5 or -1.2.

Text (strings)

Text is also known as strings in software development, and Construct also sometimes uses this naming convention. Text in expressions should be surrounded by double-quotes, e.g. "Hello"

The double-quotes are not included as part of the text, so setting a text object to show the expression "Hello" will make it show Hello, without any double-quotes. To include a double-quote in a string, use two double-quotes next to each other (""), e.g. "He said ""hi"" to me" will return He said "hi" to me.

You can use & to build strings out of mixed text and numbers, e.g. "Your score is: " & score

To add a line break to a string use the system expression newline, e.g. "Hello" & newline & "world"

Operators

You can use the following operators in expressions:

Operator Description
+ Addition
- Subtraction
* Multiplication
/ Division
% Modulo (remainder after division)
^ Raise to power, e.g. 5 ^ 2 = 25
& Build strings, e.g. "Your score is: " & score
=, <>, <, <=, >, >= Comparison operators, e.g. score < 10. Return 1 if comparison is true or 0 if false.
?: Conditional operator, in the form condition ? result_if_true : result_if_false. Allows testing conditions in expressions. The condition counts as true if it is non-zero, and false if it is zero. E.g. score < 0 ? "Game over!" : "Keep going!"

Object expressions

Objects have their own expressions to retrieve information about the object. These are written in the form Sprite.X (the object name, a dot, then the expression name). The Expressions dictionary lists all the available expressions in the project, and they are further documented in the reference section of the manual.

The expression Self can be used as a short-cut to refer to the current object. For example, in an action for the Player object, Self.X refers to Player.X.

You can add a zero-based object index to get expressions from different object instances. For example Sprite(0).X gets the first Sprite instance's X position, and Sprite(1).X gets the second instance's X position. For more information see index IDs (IIDs) in instances. You can also pass another expression for the index. Negative numbers start from the opposite end, so Sprite(-1).X gets the last Sprite's X position.

Behavior expressions

If an object has a behavior with its own expressions, they are written in the form Object.Behavior.Expression, e.g. Sprite.8Direction.Speed.

System expressions

The built-in system expressions are listed in the reference. These include some basic mathematical functions like sqrt (square root).