Branching & Iteration

Agenda

• Why Python?
• Objects and types
• String datatype
• Comparison operators
• Iteration and loops: for and while
• Indentation
• Control flow: if, elif, else

Python

Why Python?

• A general-purpose programming language
• First released in 1991

Python

Why Python?

• Open source & free
• English-like, readable syntax
• Very popular across domains

Python

Python for HSS

• Data cleaning and manipulation
• Handling vectors and matrices
• Web data collection
• Visualization
• Text-as-data/NLP/LLM, machine/deep learning
• Network analysis
• Statistical analysis (to a lesser degree)

Python

Programming environments

• Install Python (https://www.python.org/)
• Or use tools like Anaconda
  • Automatically installs Python and widely-used libraries
  • Makes Python library/package management easy
• A wide selection of environments: VS Code, Jupyter Notebook, etc.
• Google Colab
  • Browser-based (no need to install), and sharable with a link
  • Free (or subscribed) access to computing resources (CPU/RAM/GPU)

Data types

Python creates and manipulates data objects

• Objects have a type, which defines how they can be used
• Two (broad) object types
  • Scalar (cannot be subdivided)
  • Non-scalar (e.g., list, tuple, dictionary, set)

Data types

Scalar Objects

• int - integers, e.g. 3 or -142
• float - real numbers, e.g. 4.2 or -3.5
• bool - Boolean, also known as logical in some languages, True or False
• NoneType - a special type indicating “missing” with one possible value, None

Data types

Is str scalar or not?

• String (str): a sequences of characters
• Typically used to represent text
• Whether str is scalar or not depends on the context

Data types

Type conversions (casting)

• Objects of different types can be converted to one another

• Be careful, though

string data type

Concatenate with +

string data type

To create a string with double quotes in it

• Create it with single quotes
• And vice versa

string data type

If you need both, ''' can be used

Variables and values

Equal sign (=) is used for assignment of a value to a variable

• A variable is a name that refers to a value stored in memory (a label attached to an object)
• Easier to read and debug

Variables and values

Retrieve value by invoking the name of the variable

Variables and values

Rebinding

• One can rebind a variable name to a different value

Variables and values

If a value changes, it changes in just one place

Comparison operators

Used to compare to variables to one another

• Note the signs < or > precede =
• These evaluate to a Boolean
  • var1 > var2
  • var1 >= var2
  • var1 != var2

Comparison operators

This is widely useful, including

• Control flow
• And many many more

Control flow: branching

if is used to evaluate if a condition is True

if <condition>:
    <this is ran if condition is True>
    <this is ran if condition is True>

Control flow: branching

Indentation

• Be careful and meticulous when you indent multiple times (and multiple levels)
• The expressions should (by convention) be indented by 4 spaces or a tab (that’s how Python understands that those are the expressions to be run if the condition is True)

if <condition>:
    <this is ran if condition is True>
    <this is ran if condition is True>
    
<this does not depend on condition>

Control flow: branching

Best practices for indentation

• Either always use tabs or always spaces
• Python community mostly uses 4 spaces
• Many environments will allow you to automatically convert tabs to 4 spaces
• Applicable not only to branching but also to: for, while, with, def, class, try, except, etc.

For more discussion see here.

Control flow: branching

We can also use else with if

• Run a if condition is True, otherwise run b

if <condition>:
    <a>
else:
    <b>

Control flow: branching

Control flow: branching

elif stands for ‘else if’

• If condition1 is True, run a
• If condition1 is False but condition2 is True, run b

if <condition1>:
    <a>
elif <condition2>:
    <b>
elif <condition3>:
    <c>
    .
    .
else:
    <z>

Control flow: branching

Note that, in this setting, last code is only run only if everything above is False

• E.g. c will not be run if condition1 and condition2 are not both False

if <condition1>:
    <a>
elif <condition2>:
    <b>
elif <condition3>:
    <c>
    .
    .
else:
    <z>

Control flow: branching

Example

Control flow: while loops

Run codes as long as a condition is True

while <condition>:
    <a>
    <b>
    ...

Control flow: while loops

Control flow: while loops

Control flow: while loops

Control flow: while loops

Useful when the number of iterations is known

• Every time through the loop, <variable> takes a new value (iterating through <iterable>)

for <variable> in <iterable>:
    <a>
    <b>
    ...

Control flow: while loops

• Iterable is oftentimes range(<some_num>)

• Can also be a list, tuple, string, dictionary, Pandas Series, etc.

for <variable> in range(<some_num>):
    <expression>
    <expression>

Control flow: while loops

range(start, stop, step)

• By default, start = 0 and step = 1
• Only stop is required
• It will start at 0, loop until stop - 1

Control flow: while loops

range(start, stop, step)

• If we supply one argument, it’s understood as the stop argument

Control flow: while loops

range(start, stop, step)

• Starts at 11 and ends before 15

Control flow: while loops

range(start, stop, step)

• What will this return?

Control flow: while loops

Can iterate over many other things

Control flow: while loops

break and continue statements

break exits the loop entirely

• The remaining expressions are not evaluated

break and continue statements

continue only skips the current iteration

• “Continue”s with the remaining iterations

break and continue statements

break and continue can be used in both for and while loops