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File: Python Pdf 185714 | L02 Variables 4sps
learning outcomes at the end of this lecture students should be able to perform calculations using standard arithmetic operators use variables to store values describe differences between int and float ...

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                                                                                                Learning Outcomes
                                                                                          At the end of this lecture, students should be able to:
                                                                                             perform calculations using standard arithmetic operators
                                                                                             use variables to store values
                                                                                             describe differences between int and float types
                                                                                             print numbers and strings to standard output
                                                 COMPSCI 101 
                                 Principles of Programming
                            Lecture 2 – Variables, program execution, 
                                                     calculations, print()
                                                                                          2                          COMPSCI 101 – S1, 2020
           Installing Python 3                                                                  A Program is a Sequence of Instructions
     Go to the resources page of the COMPSCI 101 website.  You will                       A program is a sequence of instructions which performs a 
     see the link to python.org where you will be able to download                        specific task
     the python installer.  Make sure you install Python 3.                                  Instructions are specified in a sequence 
                                                                                             Computers execute the instructions one after the other
                                                                                          Instructions are specified using a formal language
                                                                                             Natural languages are the languages spoken by people
                                                                                             Formal languages are designed by people for a specific purpose, 
                                                                                               e.g., mathematical notation, chemical structure of molecules, 
                                                                                               programming languages
                                                                                          We shall be writing our programs in the Python 3 programming 
             https://www.cs.auckland.ac.nz/courses/compsci101s1c/resources/               language
     3                         COMPSCI 101 – S1, 2020                                     4                          COMPSCI 101 – S1, 2020
                                                                                                                                                 IDLE – The program editor used in 
                The Python Interpreter                                                                                                           COMPSCI 101 
       Source code (programs) is written in a programming language                                                                      IDLE (Integrated DeveLopmentEnvironment) is an integrated 
       such as Python.                                                                                                                  development environmentfor Python.  This is the development 
                                                                                                                                        environment provided when you download Python.
       The Python interpreter translates and executes source code                                                                        This is the environment we will be using to write and execute our 
           One instruction at a time is converted into machine code and                                                                   Python programs.
              executed by the interpreter.
                                                                   1001 0011 1001 0100 0100 1110 
                                                                   1010 1011 1111 0000 0101 0000 
                                                                   1010 0000 1011 0101 0101 0101 
                                            INTERPRETER            0100 0010 0000 1010 1000 1000 
                                                                   1111 1100 1001 0010 1010 1010 
                                                                   0100 0001 0100 1100 1010 0000 
                                                                   1001 0100 1001 0001 0010 0010 
                                                                   0000 1010 1010 1010 0100 0100 
                                                                   1001 0110 0100 1110 0001 0101
       5                                       COMPSCI 101 – S1, 2020                                                                   6                                       COMPSCI 101 – S1, 2020
                Programs are Deterministic                                                                                                       Storing Information – Variables
       Programs are deterministic                                                                                                       Variables are names for storage locations
           the result of the program instructions is well defined                                                                          Almost any name will work – but there are some constraints
                                                                                                                                            A variable stores only one value at a time
           rules govern the results of instructions.  Once we learn the rules,                                                             Assign a value to a variable location using       (the assignment 
              we can govern what the computer does.                                                                                            operator)                                                            =
           then the output is completely predictable                                                                                       Refer to the value in a location using the variable name. 
                                                                                                                                                     Three variables used     name= "Damir"
                                                                                                                                                     to store three pieces    height = 183.0
                                                                                                                                                       of information.
                                                                                                                                                                              age= 25
       7                                       COMPSCI 101 – S1, 2020                                                                   8                                       COMPSCI 101 – S1, 2020
         Variable Names                                                        Variable Naming Conventions
    The following are valid variable names:    x                          Always choose variables with meaningful names:      name
                                               age                                                                            age
                                               age_of_child                                                                   course
                                               box1
                                               box_2                      The convention when using multiple words,
                                               _age                        is to join words with an underscore:          user_input
                                               age_                                                                      age_allowed
                                                                                                                         age_of_child
    The following are invalid variable names:  3                                                                         circle_area
                                               age of child               The convention is to use lower case letters for variable names.
                                               age-child
    What are the rules for naming variables?   1st                        Python is case sensitive.  For example, the variable, age, is not 
                                               2_box                      the same as the variable, Age.
    9                     COMPSCI 101 – S1, 2020                          10                    COMPSCI 101 – S1, 2020
         Variable Names Should Not Be Key Words                                Style Guide
    Variable names should not be keywords (also called reserved
    words):       and     elif      import       raise
                  as      else      in           return
                  assert  except    is           try
                  break   finally   lambda       while
                  class   for       nonlocal     with
                  continue from     not          yield
                  def     global    or
                  del     if        pass
    Look on page 11 of the reference book:
     'Think Python – How to think like a computer scientist’.
     https://www.cs.auckland.ac.nz/courses/compsci101s1c/resources/                   http://legacy.python.org/dev/peps/pep-0008/
    11                    COMPSCI 101 – S1, 2020                          12                    COMPSCI 101 – S1, 2020
           What Kind of Information Can Our 
           Programs Store?                                                                       Assigning to a Variable
     Information in a program is categorised into different types.  There are four        The assignment operator       is used to assign a value to a 
     basic types in Python:                                                                                               =
                                                                                          variable, i.e., to store some information in the program 
        integer                                                                          memory:
        floating point
        string                                                                                            result1 = 54
        boolean                                                                                           my_age = 21
     Integer values are numbers with no decimal point.  They can be positive, 0                            bank_balance = 2019
     or negative: 202
                   0
                   -32                                                                          The left hand side of the assignment operator is 
     Floating point numbers  are numbers with decimal points.  They can be                                          always a variable.
     positive, 0 or negative:  1.0                   Note that the precision 
                               -3.405                    of floating point 
                               0.0                     numbers is limited.
                               3.3333333
     13                         COMPSCI 101 – S1, 2020                                     14                        COMPSCI 101 – S1, 2020
           Doing Calculations                                                                    Expressions
      The following mathematical operators can be used with                              An expression always evaluates to a single value, e.g., 
       integers and with floating point numbers:                                              ‐3 + 5
                                                                                              6 * 2 + 1
        Addition             +                                                               52 –3 * 2.3
                                                                                              4.6
        Subtraction          ‐        result1 = 54 + 4                                   The right hand side of an assignment statement is always an expression.  
                                       result2 = 15 / 5
        Multiplication       *        bank_balance = 2019 * 2                                            result1 = 54 + 4 / 5
                                       solution = 4 ** 3                                                  result2 = 15 / 5
        Division             /                                                                           bank_balance = 2019 * 3 / 100
                                                                                                          age = 1 + 2 + 1 + 1
        Exponentiation       **                                                                          result3 = 7
                                                                                          Firstly, the expression on the right hand side of the assignment operator is 
                                                                                          evaluated, and then, the resulting value is assigned to the variable on the left hand 
                                                                                          side of the assignment operator.  
     15                         COMPSCI 101 – S1, 2020                                     16                        COMPSCI 101 – S1, 2020
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