Day 2 - Intro to Python Workshop

Wednesday 8B | Room 57

9/10/2025

Downloading Python

www.python.org/downloads/

Online python environment

online-python.com

Operators

• Symbols that specify a certain computation (ex. addition)
• Arithmetic: +, -, /, *, %, **
• Comparison: >, <, >=, <=, !=, ==
• Examples:

Variables

• Storage location for a piece of data
• Value can be changed at any time
• Assigned using the “=” operator
• Variable names:
• Must begin with letter or underscore (“_”)
• Case sensitive (capitalization counts!)

Examples of Variables

Examples of Variables

Examples of Variables

Examples of Variables

Strings

• A sequence of characters (letters, numbers, punctuation, etc.)
• Represented using quotes (ex. “abc”)
• Example:
• X = “hello” → the variable X now has the value hello
• len(string) function used to get length of string

If-Else

• If, elif, and else statements are used to implement conditional program behavior
• Syntax:

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• elif and else are not required (used to add additional conditional statements)

Examples of If-Else Clauses

Examples of If-Else Clauses

Lists

• A general purpose 1-dimensional container for variables
• Represented using square brackets (ex. [a, b, c])
• Can change in size
• Elements can be added or removed
• len(list) function used to get length of list

Examples of Lists

Examples of Lists

List Indices

• Elements in a list are accessed by an index number
• Index numbers start at 0
• List: x = [“a”, “b”, “c”, “d”, “e”]
• First element: x[0] → 'a'
• Nth element: x[2] → 'c'
• Last element: x[-1] → 'e'
• Next-to-last: x[-2] → ‘d’

List Slicing

• Taking only a section of a larger list
• Slice syntax: x[start : end : step]
• The start value is inclusive, and the end value is exclusive
• Step is optional and defaults to 1.
• Leaving out the end value means “go to the end”
• Slicing always returns a new list copied from the existing list

List Slicing

• List: x = [“a”, “b”, “c”, “d”, “e”]
• x[0 : 1] → ['a']
• x[0 : 2] → ['a','b']
• x[-3 :] → ['c', 'd', 'e']
• x[2 : 5 : 2] → ['c', 'e']

List Assignments and Deletion

• Lists can have their elements overwritten or deleted
• del command
• List: x = [“a”, “b”, “c”, “d”, “e”]
• x[0] = -3.14 → x is now [-3.14, “b”, “c”, “d”, “e”]
• del x[0] x is now [“b”, “c”, “d”, “e”]

Loops

• A sequence of instruction s that is continually repeated until a certain condition is reached
• Two types:
• While loops
• For loops

While Loops

• While loops have a condition and a code block
• Example:

For Loops

• Often used to loop through each element of list/string
• Example:

range() Function

• Auto-generates sequences of numbers that can be used for indices
• range() syntax: range(start, exclusive end, increment)
• Examples:
• range(3) → 0, 1, 2
• range(0,4) → 0, 1, 2, 3
• range(-3,15,3) → -3, 0, 3, 6, 9, 12

Examples of range()

Practice Problem Bio in CS

In bioinformatics, a common task is to calculate the GC content of a DNA sequence. GC content is the percentage of Guanine (G) and Cytosine (C) bases in a DNA or RNA sequence. This value can be important for a variety of reasons, such as predicting the stability of DNA.

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The Problem

Write a Python program that takes a DNA sequence as a string and calculates its GC content.

Answer

Kahoot Time!!

Get your computers out :D