CS 110A Lecture Notes - Simple Python Data - Chapter 2

Chapter 2 of How to Think Like a Computer Scientist

Making You Own Notes for Useful Commands

• print

• input

• type

Data Types and Variables

Some of the most common data types:

• Floating Point - a term referring to real number approximations stored in computers
• str - string (text), any number of characters.
• float - floating point number, can be positive, negative or zero.
• int - integer (whole) number, can be positive, negative or zero.

Calculations and Assignments

• Arithmetic operators (they work on numeric datatypes only, not strings):
     *    for multiplication
     **    for exponentiation (powers), e.g. 2**3 is 2³ which equals 8.
     +    for addition
     -    for subtraction
     /    for division
     //    for integer division, e.g. 14//3 equals 4
     %    for modulus (the remainder from integer division), e.g. 14%3 equals 2
     =    for assignment (storing the result of expression at right in variable at left)
• The order of operations is just like in mathematics: exponentiation (**) is processed first, followed by multiplication (*), division (/, //), and modulus(%), then addition (+) and subtraction (-). After all of that, the assignment operator (=) is processed. Parentheses may be used to specify a different of order of operations.
• Note that variable assignments can be changed. For example:

  a = 5
  b = a    # b stores a copy of a's value: both are 5.
  print(a, b)
  a = 3    # a now stores 3, but b is still 5.
  print(a, b)  
    

Programming Style

• Programs are written to be read by other humans as well as the computer.
• You should always have Comments at the top of your program, stating your name, the name of your program source file, the name of the class and assignment, and a brief description of what the program does.
• Comments should also be used to explain hard-to-understand code, and to summarize big steps such as functions and sometimes loops.
• Use helpful variable names (strive for self-commenting code - clear without comments to explain it.)
• Provide a good user interface, including prompting for inputs and explaining outputs.

calcDemo.py: Here is an example demonstrating how variables and data types work in calculations, and with simple input.

In-class exercise: Write a program to calculate and output the average of three test grades input by the user. Here is some sample output:

Please enter the first test score 90
Please enter the second test score 81.2
Please enter the third test score 88.77
The average grade is 86.65666666666665