Course Info

Section: 003
Instructor: Lori Case
E-mail: lori.case@uwaterloo.ca
Office: DC 3103
Office Hours: See Office Hours or by appointment.

CS 116

Python only now (We are testing to see if CS 116 works as just Python)

Small note: They altered the CS 115 to accomodate for this change in CS 116. At the time of writing this in Spring 2016, I took CS 115 almost two years ago in Fall of 2014. There is some information that is familiar to people who took CS 115 last term (Winter 2016) but not to people like me due to the changes in CS 115.

###Some Python Basics:

• Written using regular mathematical notation 3+ 4 5 * (3 + 4) – 1
• Two numeric types (integers and floating point numbers) instead of one
• Strings, Booleans, lists
• No character or symbol type used in CS116.

###A very simple Python program:
x = 2 * (4 + 12) y = x +8
z = y * y
w = “hi”
u = w + w

##Basic Mathematical Operations

• Addition (+), Subtraction (-), Multiplication (*):
• If combining two Int values, the result is an Int
• If combining two Float values, or a Float and an Int, the result is a Float

Division:
x / y

• The result is a Float for any numerical values x and y (even if both are Int)

Integer division:
x // y

• The result is the integer part of the division
• If x and y are both Int, the result is an Int
• If either x or y is a Float, the result is an Float, with the decimal part being .0
• Usually used with x and y as Int

Remainder
x % y

• x and y should both be Int
• produces the Int remainder when x divided by y

Exponents
x ** y

• (anyof Int Float) (anyof Int Float) -> (anyof Int Float) – produces x raised to the power of y

###Last Tips:

• Python precedence operations are standard math precedence rules (BEDMAS)
• Use ## or # for comments (from beginning or middle of line)
• Do not use dash in variable names – Use underscore instead

###Functions in Python:

fn_name (arg1, arg2, …, argN)

• must have correct number of arguments
• separate arguments by single comma

abs(-3.8) => 3.8
type(5) => <type ‘int’>
max(3,5,9) => 9

###The Math Module

To use functions from math

• Import the math module into your program
• Use math.fn or math.const to reference the function or constant you want

import math

math.sqrt(25)  

math.log(32,2)  

math.log(32.0, 10)   

math.floor(math.log(32.0, math.e))   

math.factorial(10)  

math.cos(math.pi)    

Creating new functions in Python

def fname (p1, p2, ..., pN):  

  statement1  

   statement2  

• Indent each statement the same amount For function to return a value, include return answer
• where answer is the value the function produces
• If no return statement, the function produces None

Design Recipe

Purpose statement:

• Explicitly indicate what the function does, including how the parameters are used

Contract

• Types of consumed and produced values
• Include any needed requirements on consumed
• Most type names are the same as in Racket, except for Num; Use Nat, Int, Float as appropriate

Testing in Python

Download the file: check.py from the CS116 web pages.
Put a copy in the same folder as your assignment .py files for each assignment.

Add the following line to each assignment file:
import check
You do NOT need to submit check.py when you submit your assignment files.

check.expect

## Question 1, Test 1: description check.expect( “Q1T1”, expr, value_expected)

• This function performs the test: Does expr exactly equal value_expected?
• Use for checking exact values (integer or strings).

check.within

## Question 2, Test 2: description check.within( “Q2T2”, expr, value_expected, tolerance) • This function performs the test: abs(expr – value_expected) <= tolerance • Use for checking inexact values (floating point numbers only).

Example of check.expect and check.within:

## Test 1: middle is first value

check.expect("Q1T1",middle(3,10,1),3)  

Note: You should now include your examples with your tests.

## Q2, Test 2: positive radius (1.0)

check.within("Q2T2", area_circle(1.0), 3.14159, 0.00001)    

More Casting and Conversion Functions

float: Int -> Float
float(1) => 1.0
float(10) => 10.0

float: Str -> Float
float(“34.1”) => 34.1
float(“2.7.2”) => Error
float(“23”) => 23.0

int: (anyof Float Str Int)-> Int
int(4.7) => 4
int(3.0/4) => 0
int(-12.4) => -12

This is a truncation operation (not rounding)
int(“23”) => 23
int(“2.3”) => Error

str: (anyof Int Float Str) -> Str
str(3) => “3”
str(42.9) => “42.9”

Mutation:

Python allows us to change the values of variables
The following Python program is valid:
x = “a”
x = 100
x = 2 * x - 1

Local vs Global variables

Variables defined inside a function are called local variables
– Local variables only can be mutated inside the function they are defined in

Variables defined outside a function are called global variables
– Global variables cannot be mutated inside any functions in CS116.

###Correct Usage of Global Variables: tax_rate = 0.13
def total_owed(amount):
return amount * (1+tax_rate)

This Causes an Error:

grade = 87
def increase_grade(inc):
      grade = grade + inc increase_grade(5)

Mutating Parameters

Consider the program:

def add1(n):
      n = n + 1
return n

starter = 0
y = add1(starter)

The value of n is changed locally, but the value of starter is not changed.The mutation of n is a local mutation only.
Even if starter was called n, the same behaviour would be observed.

Tip: Python expects each line of code to be an entire statement
Can be a problem e.g. due to indentation
If a statement is not done, use a \ (backslash) character to show it continues on next line

Built-in type Bool:

• True, False
• Equality testing: ==
• Use for all atomic values (except for floats)
• Inequality testing:<,<=,>,>=
• != is shorthand for not equal

Very similar to Scheme

v1 and v2
True only if both v1, v2 are True
v1 or v2
False only if both v1, v2 are False
not v
True if v is False , otherwise False

Like Scheme, Python uses Short-Circuit evaluation
Stop evaluating as soon as answer is known
or: stop when one argument evaluates to True
and: stop when one argument evaluates to False

Example of an If Statement

if test: true_action_1 … true_action_K

def double_positive(x):
      result = x
      if x > 0:
            result = 2*x
      return result

“Chained” Conditional Statement

if test1:
      action1_block
elif test2:
      action2_block
elif test3:
      action3_block
else:
      else_action_block

def ticket_cost(age):
      if age < 3:
            cost = 0.0
      elif age < 18:
            cost = 5.50
      elif age < 65:             cost = 9.25
      else:
            cost = 8.00
      return cost

Factorial Function

## factorial(x) produces the product ## of all the integers from 1 to n ## factorial: Nat -> Nat
## example:
## factorial(5) => 120 ## factorial(0) => 1 def factorial (x):
      if x == 0:
            return 1
      else:
      return x * factorial( x - 1)

Limitations

factorial(1000) -> RuntimeError: maximum recursion depth exceeded

• There is a limit to how much recursion Python “can remember”

Important

If a function does not include a return statement, then the produced value (and type) is None

• The purpose statement does not need to include “Produces None” as this will be included in the contract.