Python

Lecture 17 - February 21st, 2017

Python Programming

You can install python on your own computer.
To execute a program:
- python

print: is a function and strings are surrounded by quotes ""

Function: is a block of code that is grouped together

The name makes it easy to execute
- Otherwise we'd have to copy and paste the same code multiple times
Functions help organize
Name Flowchart
Functions can input values and return values
We can write our own
"def" to signal we are about to define a function
Input parameters are in parenthesis, followed by colon
All code in the function is indented.

Variables: name for a value, can change, variable is like a box

ex.

x = 5

Comments: Allows us to document our code

Leave notes to you and others (not the computer)

ex.

# Here's a comment
x = 5
# Hey! Comment

Collatz:

Collatz

Syntax:

After every decision you must ":" and indent (This tells python which code belongs to the yes/no part if the decision)

ex.

if x % 2 == 0:
    x = x/2
else:
    x = x*3+1

Check equality: "==" Assign value: "="

Decision Loop

ex.

while x > 1
    print("The number is %i" % x)

    if x % 2 == 0:
        x = x/2
    else:
        x = x*3+1

Nesting ^^

ex. Collatz Example

# This tells python to load a library called "time"
# Which we will use for pauses in the loop
import time

x = 5

# Keep track of the number of times through the list
loop_count = 0
# "while" is like "repeat until" in scratch, but the while loop continues until the condition is FALSE, repeat until ends once the condition is TRUE.
while x > 1:
    print("The number is %i" % x)
    time.sleep(1) # this waits 1 second to slow down the prints
    if x % 2 == 0:
        x = x/2
    else:
        x = x*3+1
    loop_count = loop_count + 1

print("Done!  It took me %i loops" % loop_count)

ex. Errors

print(test)

5 = x

print("thry this)

print("try test"

myVar = 5
print(myvar)

Google the error, and most code will return where the error is found.

ex.

def myFunction(my_number):
    if my_number % 2 == 0:
        print("Even!")
    else:
        print("Odd!")

Call the function by calling it's name

ex. myFunction(10)

ex. Functions can call other functions

def isEven(my_number):
    result = False
    if my_number % 2 == 0:
        return = True
    return result

def callFunction(my_number2):
    if isEven(my_number2):
        print("Even!")
    else:
        print("Odd!")

Tutorial
http://cscircles.cemc.uwaterloo.ca/

Lecture 18 - February 22nd, 2017

print("number is " + str(x))

str: is a function that turns a number into a string

+: concatenates the two strings

Arrays: A[j] They are list of numbers, first index of an array is 0

Selection Sort Pseudocode:

Ex. In Python

def selection_sort(A):
    m = len(A)
    cur_ind = m-1

    while cur_ind > 0:
        cur_max = -99999 
        #or 
        #cur_max = A[0]
        i = 0
        max_ind = 0
        while i <= cur_ind:
            if A[i] > cur_max:
                cur_max = A[i]
                max_ind = i
            i = i + 1
        tmp = A[cur_ind]
        A[cur_ind] = A[max_ind]
        A[max_ind] = tmp
        cur_ind = cur_ind - 1

Demo:

A = [11,22,3,12,34,9]
print("A= "+str(A))
selection_sort(A)
print("A= "+str(A))

For Loop:

Combine the decision and index incrementing.
Takes care of:
- Creating a variable named i
- Incrementing it's value by one every time the loop is executed
- Compares i to N

for i in range(1,N):

Ex.

for i in range(1,5):
print(i)

1
2
3
4

---

for i in range(1, 0.5):
print(i)

Error = Expected integer, got float

Bubble Sort:

Ex. Python

def bubble_sort(A):
    m = len(A)
    swapped = True

    while swapped == True:
        swapped = False
        for i in range(1,m):
            if A[i-1] > A[i]
                tmp = A[i]
                A[i] = A[i-1]
                A[i-1] = tmp
                swapped = True