Lecture 3

Knowledge Check

>>> total = round(19.973, 2)
>>> a = "The total is $" + total + "."
>>> print(a) #what does this display?
>>>
>>> b = "My name is Zeus!"
>>> b.lower().capitalize()
>>> print(b) #what does this display?
>>>
>>> dir(b) #what does this display?

How might you do a case-insensitive search through text? In other words, how can you search through a string for another string, ignoring capitalization (e.g. finding 'zeus' in 'My name is Zeus!')?

Engaging the User: Getting Input

Programs are often written by a programmer and used by someone else. So far, we have been using the interpreter as a way of performing operations for ourselves and not writing programs to be used by others. We can prompt the user to enter information using the input function. The input function takes one optional argument, a prompt to display to the user. In the example below (and in all future examples), user input looks like this: input from the user.

>>> name = input("Please enter your name: ")
Please enter your name: Zeus
>>> age = input("Please enter your age: ")
Please enter your age: 2500
>>> print(name+", you are "+age+" years old.")
Zeus, you are 2500 years old.
>>> age_next_year = age + 1
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: Can't convert 'int' object to str implicitly

Note that input always returns a string. If you want a number from the user, you must convert it to a number.

>>> name = input("Please enter your name: ")
Please enter your name: Zeus
>>> age = input("Please enter your age: ")
Please enter your age: 2500
>>> age = int(age)
>>> print(name+", you are "+str(age)+" years old.")
Zeus, you are 2500 years old.
>>> age_next_year = age + 1
>>> print("Next year, you will be", age_next_year)

Practice

Write a short program that:

Asks the user for their name.
Asks the user for their savings account balance.
Asks the user for their monthly interest rate.
Calculates the interest they will receive after one month: balance * interest_rate
Displays the output to the user, using their name and formatting the interest as a dollar amount.

An example run is:

Please enter your name: Zeus
Please enter your balance: 100
Please enter your monthly interet rate: .02
Your interest after one month is $2.00.

Modules

Python comes with a lot of pre-written code for us to use. This course will cover only a very small portion of this pre-written code. This pre-written code is refered to as the "Python Standard Library" and can be found on Python's website: https://docs.python.org/3/library/index.html.

To gain access to a module, you must import it. Imports generally occur at the top of a source code file. To import the math module:

>>> import math

Once a module is imported, you can call a function by using moduleName.functionName. For example, to calculate the base-10 logarithm of 1000, we can use the log10 function:

>>> math.log10(1000)
3.0

If you will be using a function a lot, it might be good to just import that function from the module. For example, if a source code file may want to end the program, it could import just the exit function from the sys module:

>>> from sys import exit
>>> exit()

From IDLE, the exit function does not do anything. It only works when run from a source code file or from the command line.

Boolean Expressions

Boolean expressions are expressions that evaluate to either True or False values. These are very useful for if statements, while loops, and other common tasks. Before we get to these though, we first need to discuss Boolean expressions. Boolean expressions can be:

Boolean literals
Result of relational operator
Returned from method call
Result of logical operator

Boolean Literal

These are the values True or False.

Relational Operators

Relational operators evaluate the relationship between two values. There are six operators:

Operator	Meaning	Example
<	less than	>>> x = 5 >>> y = 9 >>> x < y True
<=	less than or equal to	>>> x = 5 >>> y = 9 >>> x <= y True
>	greater than	>>> x = 5 >>> y = 9 >>> x > y False
>=	greater than or equal to	>>> x = 5 >>> y = 9 >>> x >= y False
==	equal	>>> x = 5 >>> y = 9 >>> x == y False
!=	not equal	>>> x = 5 >>> y = 9 >>> x != y True

Note that the equal operator (==) is different from the assignment operator (=).

You can only compare compatible types. Comparing a string and a float will not work, even if the string looks like a number. Comparing an int and a float will work though.

It is possible to compare strings using these relational operators. However, Python cares about capitalization. So, 'cat' and 'Cat' are not equal. Additionally, 'Cat' comes before 'cat' because of the underlying Unicode representation of the characters. For this course, just know that string comparisons are possible, but capitalization makes things a little inconvenient. How can you do comparisons without worrying about capitalization?

>>> a = 'cat'
>>> b = 'Cat'
>>> a < b
False
>>> a > b
True
>>> a == b
False
>>> a == a
True

Comparing floats needs to be done carefully. You should rarely (never) use the equality operator because two numbers are equal only if their underlying binary representations are equal. Mathematically, 9.3/7.8 is equal to (10*9.3)/(10*7.8). However, in Python (and most other programming languages), they are not:

>>> x = 9.3/7.8
>>> y = (10*9.3) / (10*7.8)
>>> x == y
False
>>> x #Here's what x looks like
1.1923076923076925 >>> y #Here's what y looks like
1.1923076923076923 >>> #Notice the very last digit is different!

To compare floats, decide on a tolerance/threshold and see if the absolute value of the difference between the numbers is below that threshold:

>>> THRESHOLD = 1e-9
>>> x = 9.3/7.8
>>> y = (10*9.3) / (10*7.8)
>>> abs(x - y) < THRESHOLD #this is the better way to compare two floats for equality
True

Returned from Method Call

Certain methods return a boolean value. Some examples are shown below.

>>> a = 'Here is a sentence.'
>>> a.endswith('horse') #is the end of the string equal to "horse"?
False
>>> a.endswith('tence.') #is the end of the string equal to "tence."?
True
>>> a.isalnum() #is the string entirely alphanumeric characters?
False
>>> #it contains spaces and a period, so it is not just alphabet and numbers
>>>
>>> import math
>>> math.isfinite(9) #is the provided number finite (i.e. a non-infinite number)?

Of course, other methods can also be part of a boolean expression, but you'll need to use the relational operators to determine a True or False value.

Result of Logical Operator

Logical operators manipulate the result of boolean expressions. Python provides the standard three operators:

Operator	Effect	Example
and	Connects two (or more) boolean expressions. Both expressions must be true for result to be true	>>> a = 'It is raining cats and dogs.' >>> 'cat' in a and a[0].isupper() and a.endswith('.') True >>> #this looks like a sentence about a cat
or	Connects two (or more) boolean expressions. At least one expression must be true for result to be true	>>> option = 5 >>> option == 3 or option == 5 or option == 7 True >>> #option is one of the prime numbers less than 10
not	Reverses the truth of a boolean expression	>>> a = 'Here are a list of options:' >>> not a.endswith('.') False >>> #a does not end with a period

<< Previous Notes

Next Notes >>