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Variables

Neil Fraser edited this page Oct 15, 2014 · 4 revisions

We use the term variable the same as it is used in mathematics and in other programming languages: a named value that can be changed (varies). Variables can be created in several different ways.

  • Every count with and for each block uses a variable and defines its values. These values can only be used within the block. A traditional computer science term for these are loop variables.
  • User-defined functions (also known as "procedures") can define inputs, which creates variables that can be used only within the function. These are traditionally called "parameters" or "arguments".
  • Users may create variables at any time through the "set" block. These are traditionally called "global variables". Blockly does not support local variables.

Default names

While users can choose any name for a variable, core Blockly provides a default name, "item", as shown in the below picture. Some applications provide other default values, such as "value", also shown below.

Dropdown menu

Clicking on a variable's dropdown symbol (triangle) gives the following menu:

The menu provides the following options.

  • the names of all variables defined in the program.
  • "Rename variable...", which changes the name of this variable wherever it appears in the program. Selecting this opens a small window that prompts the user for the new name with the text: "Rename all %1 variables to:", where %1 is replaced by the old name (here "item").
  • "New variable...", which enables the user to enter a new name for the variable, without replacing or changing variables with the old name (here "item"). Selecting this opens a small window that prompts the user for the new name with the text "New variable name:".

Blocks

Set

The set block assigns a value to a variable, creating the variable if it doesn't already exist. For example, this sets the value of the variable named "age" to 12.

Get

The get block provides the value stored in a variable, without changing it.

It is possible, but a bad idea, to write a program in which a get appears without a corresponding set.

Example

Consider the following example code:

The first row of blocks creates a variable named "age" and sets its initial value to the number 12. The second row of blocks gets the value 12, adds 1 to it, and stores the sum (13) into the variable. The final row displays the message: "Happy birthday! You are now 13"

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