Lecture Overview

Before talking more about objects and classes, I want a little time for you to do some practice problems with classes and objects so you have a little experience with objects and classes. Therefore, I am going to talk about a different topic and we will come back to classes in a few days.

Digital Circuits

To implement a CPU we create electronic circuits using transistors and resistors. Physics tells us that to understand any circuit we must compute the voltage as a function of time at all points on the circuit, and physics gives formulas for this. When designing the circuits such as those appear inside a CPU, we will design the circuit to make sure each wire is only in one of two possible states: at zero voltage or high voltage (which is usually around 3 volts). We make sure that all points in the circuit are either at zero voltage or high voltage and no other values. (The actual voltage might not be exactly zero or exactly 3 volts, but the design is such that during the operation the voltage will be near one of these two values and nowhere else.) Therefore, when reasoning about these circuits we can just assume that each wire is either zero or high (which we interpret as zero or one). These types of circuits are called digital circuits.

• Transistors. Transistors and resistors form the majority of digital circuits.
  • This page gives much more detail than I went into about transistors. I talked only about the NPN transistor which has a drawing on that page.
  • The main thing to take away (for this class) is the following:
    • If the base is zero, the transistor is OFF and no current can flow between the emitter and collector.
    • If the base is high, the transistor is ON and current can flow between the emitter and collector.
• Logic Gates
  • A logic gate is a circuit element implemented using transistors and resistors with "input" wires and an "output" wire. Thinking of each input wire as zero or one, the output wire will be zero or high depending on the state of the input wires.
  • This page has pictures of the three gates if you scroll down to the "Digitial Logic" section.
  • This page has an overview of how we draw and think about gates. Usually we just ignore the internals of the gate and just think of zeros or ones "flowing" down each wire.

• Any boolean function can be implemented using logic gates

• Combining gates to form an ALU
  • All arithmetic and logic operations are just boolean functions, and therefore can be implemented using gates. For example, the adder.
  • Use a multiplexer to select the output of the various operations.

Exercises

Problem 1

In the simulation from last time, the Bear starves at 20 turns and this is hard-coded into the live_a_little function. Update the Bear class to instead of having a fixed number of turns before it starves, instead each Bear starves after a random number of turns between 16 and 24. You will need to create a property to store the number of turns before starving, set that property from the __init__ method with a call to the random module, and update the live_a_little function to use your new property.

Problem 2

Design a class to represent a bank account. It should store a customer name, account ID, and balance. The __init__ method should take parameters for the name and ID and initialize the balance to zero. You should have the following methods. You should come up with sensible names for the properties and methods yourself (keep PEP8 section on Naming Conventions in mind).

• A method to print out the name, ID, and balance to the screen.

• Two methods to deposit or withdraw an amount of money which modifies the balance. If someone tries to withdraw more money than there is balance, print an error message to the screen.

• A transfer method which takes two parameters: a balance and another account object. The money should be transfered from the self account to the account passed as a parameter. Again, if there is not enough money then don't transfer anything and instead print an error message.

Write a few lines of test code to show your class is working properly. Create a couple accounts, add and remove money by calling the methods, and then transfer a few times, printing out the balances between transfers.