Lecture 3

Variables, assignments, input

MCS 260 Fall 2020
David Dumas

Reminders

• Complete worksheet 1 this week
  • Not collected; solution recently posted
• Quiz 1 due Monday 6pm Central
  • Contact staff about install problems
  • Can request to be excused once per calendar month by writing to TA (see syllabus)
  • Upload separate images or single PDF with BIG screenshot

Comments

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

The ignored text is a comment.

Comments should be added where they will make code easier to understand (for others, or for you in the future). They can also be reminders about known problems or future plans, if these are not recorded systematically elsewhere.

Variables and assignments

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

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

"foo" = 50    # FAILS: can't assign to string
foo = 50      # Works
foo = thing   # FAILS: thing is seen as variable name
foo = "thing" # Works
bar = "foo"   # Works, bar is now "foo"
bar = foo     # Works, bar is now "thing"

print(Hello world) # FAILS: Hello and world are unknown names
                   # and space between var names not allowed

How to think about assignment

        x = 15
    

• the value 15
• the name $x$ (i.e. create a new name if needed)
• that $x$ currently refers to 15

A diagram is often used to summarize this situation:

$ \boxed{x} \longrightarrow \boxed{15} $

• 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 are complicated and refer to a number of other documents and standards. Ultimately, there are about 120,000 characters allowed.)

Input

It then returns the text that was read, meaning that the code behaves as though that instance of $\texttt{input()}$ has been replaced by the string the user typed.

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

Greeting the user

The script greeting.py will greet you by name.

# Greet the user by name
# MCS 260 Fall 2020 Lecture 3 - David Dumas
name = input("Enter your name: ")
print("Nice to meet you,", name)

Usage:

$ python greeting.py
Enter your name: David Dumas
Nice to meet you, David Dumas
$ 

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:

complex("1+2j")   # Works
complex("(1+2j)") # Works
complex("2j+1")  # Fails

Warning: Conversion from int to float or int to complex may be destructive; ints are exact, but float and complex may replace input with an approximation.

>>> float(9_007_199_254_740_992)
9007199254740992.0
>>> float(9_007_199_254_740_993)
9007199254740992.0
    

Sum and product script

Here is a script sumprod.py that reads two floats from the user and prints their sum and product.

# Read two floats and print their sum and product
# MCS 260 Fall 2020 Lecture 3 - David Dumas
x = float(input("First number: "))
y = float(input("Second number: "))
print("Sum:    ",x,"+",y,"=",x+y)
print("Product:",x,"*",y,"=",x*y)

Usage:

$ python sumprod.py
First number: 1.2
Second number: -3.45
Sum:     1.2 + -3.45 = -2.25
Product: 1.2 * -3.45 = -4.14