Lecture 3

Comments, variables, input

MCS 260 Fall 2021
David Dumas

Course bulletins

• Ask for help ASAP if you don't have Python and Visual Studio Code working
• Worksheet 1 solutions posted
• Homework 1 available in Gradescope; due at 10am Tuesday August 31

Comments

        print("Hello world!")   # TODO: Choose a new greeting
    

The ignored text is a comment. Comments are used to explanatory text for use by humans.

A comment can take up an entire line, and this is often used to add a header at the top of a script.

        # Hello MCS 260 script by David Dumas
        # Written on 2021-08-26
        print("Hello world!")
    

Variables and assignments

To set the value of a variable we use an assignment statement. The basic syntax is

$\texttt{name} = \texttt{value}$

>>> side_length = 5
>>> side_length
5
>>> side_length**2
25
>>> side_length = 6
>>> side_length**2
36

"a" = 50    # FAILS: LHS is a value, not a variable name
a = 50      # Works
a = thing   # FAILS: thing is an unknown variable name
a = "thing" # Works
b = "uic"   # Works, b is now "uic"
b = a       # Works, b is now "thing"

print(b) # The current value of variable b
         # appears on the screen

>>> old_semester_tuition = 4763
>>> semester_tuition = old_semester_tuition * (1 + 11.1/100)
>>> semester_tuition
5291.693

Spaces around $\texttt{=}$ are optional.

• Must not start with a number
• Must not contain spaces
• Must not be a Python keyword ($\texttt{if}$, $\texttt{while}$, $\ldots$)

False      await      else       import     pass
None       break      except     in         raise
True       class      finally    is         return
and        continue   for        lambda     try
as         def        from       nonlocal   while
assert     del        global     not        with
async      elif       if         or         yield    

• Use only $\texttt{A}$-$\texttt{Z}$, $\texttt{a}$-$\texttt{z}$, $\texttt{0}$-$\texttt{9}$, and $\texttt{_}$ (underscore)
• Use $\texttt{_}$ as a word separator

    class_avg = 93.8    # Works
    260avg = 93.8       # FAILS: starts with a number
    secret code = 12345 # FAILS: spaces prohibited
    secret_code = 12345 # Works
    SecretCode = 12345  # Works, atypical style
    测试成绩 = "great"    # Works, not recommended

(The exact rules for which characters can appear in variable names are rather complicated.)

Types

Every object in Python (whether a variable or a literal) has a type. You can determine the type using the built-in function $\texttt{type()}$:

str means string, a sequence of characters

>>> type("Hello world!")
<class 'str'>

int means integer

>>> type(77)
<class 'int'>

float means floating-point number

>>> type(0.1)
<class 'float'>

Dynamic typing

In Python, you are free to change the type of a variable at any time.

Many languages don't allow this!

        x = 5          # x is an int
        x = 3.14159    # now it's a float
        x = "umbrella" # now it's a string
        

More about printing

The $\texttt{print()}$ function can accept any number of values, of any types, in a comma-separated list.

The basic syntax is $\texttt{print(val1, val2, val3, ...)}$.

    >>> print("The decimal value of binary 1001 is",0b1001)
    The decimal value of binary 1001 is 9
    >>> print("The sum of",99,"and",0b10,"is",99+0b10)
    The sum of 99 and 2 is 101
    >>> print(1,1.0,1+0j)
    1 1.0 (1+0j)
    >>>
    

In the output, values are separated by spaces.

Input

The $\texttt{input}()$ function waits for the user to type a line of text in the terminal, optionally showing a prompt.

Then, the place where $\texttt{input}()$ was called gets replaced with the string the user entered.

>>> s = input("Enter some text: ")
Enter some text: organizing heliotrope <--- keyboard input
>>> print("You entered:",s)
Your entered: organizing heliotrope
>>> input()
programming exercises  <--- keyboard input
'programming exercises'
>>>

Greeting the user

Let's write a program that will ask the user for their name, and then display a greeting.

Arithmetic on input?

We can't do arithmetic on input directly, because the input is always a string.

>>> 5 + input("Enter a number: ")
Enter a number: 10
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'str'

Instead we need to convert input to a numeric type, using $\texttt{int()}$, $\texttt{float()}$ or $\texttt{complex()}$.

>>> 5 + int(input("Enter a number: "))
Input: 10
15

>>> float(42)
42.0
>>> int(12.9)
12

Supported conversions:

Rectangle area and perimeter

Let's write a script to compute the area and perimeter of a rectangle.

It will ask the user for the dimensions using $\texttt{input()}$ and then print the results.