In-class notes for 02/17/2014

CS 121B (CS1), Spring 2014

• Homework assignment

• Piazza, Sign up for an appointment, daily questions form

Submitted questions on assignments and technology

Upcoming

• Homework 4

• Read online text: Selection
  submit at least one reading question by 9am before the next class meeting

• (If you haven't already, also read Functions and submit at least one reading question by 9am before the next class meeting)

• Quiz on Wednesday. See below for topics.

Submitted questions on readings

Functions

• Function definitions

  Example: Write a function with two numerical arguments that returns the average (mean) of those argument values.

      def mean(num1, num2):
  	return (num1 + num2)/2
  

• Syntax for a function definition

• Exercise: write functions that accomplish the following:

  1. A function invite() with one argument, a list of names, that prints an invitation to a party for each name

     Hint:

         for name in ["Joe", "Amy", "Brad", "Angelina", "Zuki"]:
     	print("Hi", name, "Please come to my party on Saturday!")
     

  2. A function square() with two arguments, a turtle and a length, that draws a square using turtle arg1 with arg2 as the length of a side.

Accumulators

• An accumulator is a variable that collects a desired value during a computation.

• Example: Consider Friday's in-class exercise.

  • What's needed is a variable for keeping track of how many invitations have been printed.

  • Example accumulator name count

  • Start count at 0

  • Add 1 to count every time another invitation is printed.

  • Then, the final value of count will equal the number of invitations sent.

  • Resulting code:

        count = 0
        for name in ["Joe", "Amy", "Brad", "Angelina", "Zuki"]:
    	print("Hi", name, "Please come to my party on Saturday!")
            count = count + 1
        print(count, "invitations were printed")
    

  • Notes:

    • count = count + 1 adds 1 to count

      Left side uses old value of count

      Right side says where to store the incremented value (i.e., into the same variable count)

    • meaning of count: At any point during the computation, count invitations have been issued.

• Exercise: Write a loop with an accumulator that returns a sum of a list of integers

• Exercise: Write a function sumInts() with one argument, a list of integers, that returns the sum of the integers in that list

Function specifications ('specs'), etc.

• local variables -- only accessible within a function's definition

  E.g., count in the function invite(); also the argument variables num1 and num2 in the function mean(); etc.

• Specs...

• Binding and evaluation

To study for quiz

This will be the first of about 10 weekly quizzes, with about 15 min of questions involving the following topics

• Python3 coding: input and print; arithmetic operators; using variables;

• for loops and range, similar to homework questions.

• Turtle graphics: import turtle; defining a screen and a turtle; methods forward(), left(), right(), color() or pencolor()

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