Lecture 21

Dispatch tables

MCS 260 Fall 2021
David Dumas

Reminders

• Project 2 solution will be posted Friday
• Project 3 to be announced next week; due Nov 5
• Homework 7 due tomorrow

Common situation

Chain of of/elif/elif/else checking the same variable, taking a different action for each possible value.

        if s == "exit":
            exit()
        elif s == "help":
            print(HELP_MSG)
        elif s == "next":
            x = f(x)
            print(x)
    

If we put the body of each if/elif into a function, this would look like:

        if s == "exit":
            exit()
        elif s == "help":
            do_help()
        elif s == "next":
            do_next()
    

This is ok, but the similarity of all the elif blocks is suspicious. Is there a shorter way?

We can reduce duplication by storing the mapping from values to functions in a dict.

        handlers = {
            "exit": exit,
            "help": do_help,
            "next": do_next
        }
        
        if s in handlers:
            handlers[s]() # replaces all the if/elif bodies
    

The dictionary handlers is an example of a dispatch table.

Dispatch tables

A mapping from values to actions, so looking up the value associated to a key and calling it replaces a long chain of if/elif.

Advantages:

• Possible actions are stored in an actual data structure, rather than implicitly described by code.
• Make introspection possible (program can list, examing, modify the table)
• Late extensibility: Program doesn't necessarily need to know the entire table when it starts!

Terminal

Let's refactor our mini-terminal to perform each command through a function, and to use a dispatch table to decide which one to call.