Python - Basics

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Variables

A variable is a reserved space in a computers memory that can hold a specific value which is referenced by a specific name in the program. Variables can be declared with a unique name as a place holder to store a specific value.  Unlike other languages you do not need to declare the variable type.  

Variable Types

• String

• Integer

• Float

• Boolean

• Lists (similar to arrays in other languages)

See examples below:

name = "john smith"      #String

address = " 34 red st"   #String

age = 10                 #integer / number

height - 1.82            #float / number with a decimal

alive = True             #boolean (true or false)

Variable Representation

Naming Variable Rules

• each variable must a unique name - different to other variable names

• case sensitive

• no spaces allowed - use underscore _ to join words

• some names are not allowed to be used as they control other functions - 

• make variable name clear of what they are

• Must start with a letter - not allowed to start with a number

Arithmetic Operators

Addition  (+)

answer = 10 + 12  >>> 22

Subtraction (-)

answer = 10 - 2    >>> 8

Multiplication (*)

answer = 10 * 2    >>> 20

Division  ( / )

answer = 50 / 2   >>> 25

Modulus (%) - returns the remainder of a division of two numbers

answer = 3/2     >>> 1

Comparison Operators

(==)  EQUAL -  If the value of the two operands are equal (the same) the condition returns true

a = 1

b = 1

a == b    >>> true

(!=)   NOT EQUAL - If the two values are not equal the condition returns true

a = 1

b = 2

a != b   >>> true

(>) GREATER THEN - If the value on the left is greater then the value on the right the condition will return true

a = 10

b = 20

a > b  >>> false

b > a  >>> true

(<) LESS THEN - if the value on the left is smaller then the value on the right the condition will return true

a  = 10

b = 20

a <= b  >>> true

b <= a  >>> false

(>=) GREATER THEN OR EQUAL - If the value on the left is greater then or equal to the value on the right the condition will return true

a = 10

b = 20

a >= b  >>> false

(<=) LESS THEN OR EQUAL- if the value on the left is smaller or equal to the value on the right the condition will return true

a = 10

b = 20

c = 10

a <= b  >>> false

a <= c  >>> true

Input Values from a User

To allow the user to input values into the program you need to use a input command. See examples below.

Gathering String Input:

player_name = input("please enter a name)

Gather Number Input: This is important if you want to do a arithmetic calculations on the input 

player_age = int(input("please enter your age"))

Example Code

name = input("please enter a name)

age = int(input("please enter your age"))

print("Hi, I am " + name + " and I am + str(age) + " years old") 

CASTING - Converting Data Types

Casting is where we can convert a type of variable to another type of variable. The most common is from a string to an integer. Remember when writing an integer to a string through the print function, you need to cast variable type to a string with the function called  str() - see above example

You can cast a string to an integer using int()

IF Statements

In a program we can make decisions based on asking questions or testing certain conditions, then we can choose the correct output based on if the condition is true or false.

Single Statement - If the question is true we perform an action, if not true the program does nothing.

IF condition is true:

   perform this action

If Else Statement. If the first condition is NOT true the program runs the second part of the code after else at each instance

IF condition is true:

   perform this action 1

ELSE:

  perform this action 2

Else If Statement - we can test against multiple conditions, if neither of the first two conditions are true, the program will perform the action after else. Its important to remember that logical operation is that if condition 1 is true, then the code beneath will NOT be executed.

IF condition_1 is true:

   perform this action 1

ELIF condition_2 is true:

   perform this action 2

ELSE:

   perform this action 3

Code Examples 

if Example

answer = 12

if answer == 12:

   print("Correct")

If Else Example - don't for the colan after each if statement

answer = 5

if answer == 5:

   print("Correct")

else:

   print("incorrect")

If / Else If /Else Example

answer = int(input("Enter a number"))

if answer > 10:

 print("too large")

elif answer <10:

 print("too small")

else:

 print("you entered the number 10")

To make programs more efficient we can loop code (iteration) to repeat code sections over and over again. This saves lines of code and makes life a lot easier and the program more efficient.

While Loops

These loops continue repeating until the condition in the while statement is false. It then exits from the loop and continues the code below.

While condition is true

 perform this action/s

Example While loops 

count = 10

while count > 0:

 print(count)

 count -=1

Note: with while loops, ensure the indentation is correct

answer = 11

guess = int(input("Make a guess"))

while guess != answer:

 print ("Try again")

 guess = int(input("Make a guess"))

print("well done, you are correct")

Another while loop example

while qNum <= 5:

  num1 = randint(1,10)

  num2 = randint(1,10)

  qAns = num1 + num2

  print("Question: ", qNum)

  ans = input("What is " + str(num1) + " + " + str(num2) + " ?: ")

  if int(ans) == qAns:

    print("correct")

    score +=1

    qNum +=1

  elif int(ans) != qAns:

    print("incorrect")

    qNum +=1

FOR Loops

FOR Loops repeat code for a set number of iterations. They use a counter variable to trace the number of times the block of code has been executed. These are useful for running a loop of code to repeat like counting down to zero from a specific number . Its is also important to note the count will start at zero by default and count up

Examples

#Prints out 0,1,2,3,4

print("Example of a range"

for x in range(0,5):

  print(x)

#the first number in the brackets is the starting point of the loop

# the second number in the brackets is the number the counter will count to

#Prints out -2,-1,0,1,2,3

print("Example of negative -> positive")

for x in range(-2,4):

  print(x)

#this example shows how the break function stops a loop when a condition is met

print("Example of a break function")

fruits = ["apple", "tomato", "banana", "cherry"]

for x in fruits:

  print(x) 

  if x == "banana":

    break

Random Numbers - generate a random number in a set range

import random # must insert to work at top of code

random.randint(1, 100) # random number between 1 and 100

#print to screen

print("a random number" + str(random.randint(1,300))

Getting Current Date and Time

You can access any part of a date or time format using the following steps in python.

import datetime

now = datetime.datetime.now()

print (now.year, now.month, now.day, now.hour, now.minute, now.second)

>>> 2018 12 13 2 30 21

String Manipulation 

- http://www.pythonforbeginners.com/basics/string-manipulation-in-python

Accessing individual Characters from a variable

word = "Hello World"

letter=word[0] >>> print letter H

Other options:

word = "Hello World"

print word[0]          #get one char of the word

print word[0:1]        #get one char of the word (same as above)

print word[0:3]        #get the first three char

print word[:3]         #get the first three char

print word[-3:]        #get the last three char

print word[3:]         #get all but the three first char

print word[:-3]        #get all but the three last characters

Length of a String

len(variable or string)

x = input("Name?") >>> Bob

print(len(x)       >>> 3

Add Colour to Your Text

from termcolor import colored

print (colored("Hello World!","red"))

List Diagram Representation

A larger container of variables that are indexed starting at 0.

These are useful for storing multiple values at a time without creating multiple variables.

Lists Commands

Search and sort lists in Python: https://www.dummies.com/programming/python/how-to-search-and-sort-lists-in-python/

Search through lists in Python: https://thispointer.com/python-how-to-check-if-an-item-exists-in-list-search-by-value-or-condition/

Creation of a list

L = ['yellow', 'red', 'blue', 'green', 'black']

Print Lists

print(L)

>>> ['yellow', 'red', 'blue', 'green', 'black']

Accessing / Indexing Lists

L[0]  = returns 'yellow'

Slicing

L[1:4]  = returns ['red', 'blue', 'green']

L[2:]   = returns ['blue', 'green', 'black']

L[:2]   = returns ['yellow', 'red']

L[-1]   = returns 'black'

L[1:-1] = returns ['red', 'blue', 'green']

Looping through a list with an index

for i in list_name:

  print(i)

Adding an element to the list:

list_name.append(value)

Deleting an item from the list

list_name.pop()

#example - removing the number 7 from a list

for i in list_name:

 if i == 7

 list_name.pop()

Searching lists for a particular value:

list1 = ['cat', 'bat', 'mat', 'rat', 'pet'] 

print(list1)

# search for and then store the index of 'bat' in list1 in a variable called petCheck

petCheck = (list1.index('bat'))

print(petCheck)

print(list1[petCheck])

returns the values: 1, bat 

Searching lists for a value input by a user using index:

# ask a user for a search string in myList, 

# if the string is present it will return the index and value,

# if not it will throw an error and halt the program...

myList = ['Tom Thumb', 'Jenny Smith', 'Andrew Foster', 'Mary Johnson']

print(myList)

nameSearch = input("Search the list for a name: ")

nameCheck = (myList.index(nameSearch))

print(nameCheck)

print(myList[nameCheck])

Searching lists for a value input by a user using a manual approach:

# checks a list (this case namesList) for a user input string

# loops through until the word QUIT is input

# you could possibly use the index method above to return the value of the first instance

# of the value in the list...

checkNames = ""

while str.upper(checkNames) != "QUIT":

 checkNames = input("Please type a name: ")

 if (namesList.count(checkNames) >= 1):

  print("The name exists in the list!")

 elif (str.upper(checkNames) != "QUIT"):

  print("The list doesn't contain the name.")