Executors and Command Systems

An executor is a person, animal or device that carries out an algorithm. A computer, a robot vacuum cleaner, a sprite in a programming environment and even a cook following a recipe are all executors. An executor does not think about the meaning of an algorithm; it simply carries out the written commands exactly.

The set of all commands an executor can carry out is called its command system. For example, a simple robot knows the commands “forward”, “turn left”, “turn right” and “paint the square”. An executor cannot perform a command that is not in its command system, so an algorithm must be built only from those commands.

The place where the executor works is called the executor’s environment. For a robot it is a grid field, and for Scratch it is the stage. The rules of the environment matter too: a robot cannot pass through a wall, and a sprite cannot leave the stage.

An example: a robot stands on a grid in square (1; 1) and must be moved to square (1; 3). The algorithm is “forward, forward”. If the executor does not know the command “forward”, the algorithm cannot be carried out. That is why, before writing an algorithm, you should find out what the executor can do.

Key takeaways

• An executor is a person or device that carries out an algorithm.
• A command system is the set of all commands the executor can perform.
• The environment is the place where the executor works, with its rules.
• An algorithm must use only the commands the executor knows.