Course Content

Introduction to Python

1. First Acquaintance

The Very First Steps Comments Some Simple Math More Challenging Math Challenge Bringing It All Together

2. Variables and Types

Storing Numbers Naming Rules Using Variables Data Types Numbers Challenge Storing Text String Indexing Negative Indexing and Length String Slicing Challenge Concatenation

3. Conditional Statements

Boolean Data Type Challenge: Comparison Operators Combining Conditions Challenge: Logical Operators Membership Operators and Types Comparison Simple if/else Expressions Challenge: Grocery Store Challenge: Odd and Even if/elif/else Expressions Challenge: Grocery Store 2 Challenge: Positive, Negative or Zero

4. Other Data Types

What Is a List?List Methods Challenge Nested Lists Challenge What Is a Tuple?Tuple Methods Challenge Nested Tuples Challenge What Is a Dictionary?Challenge Dictionary Methods Challenge

5. Loops

while Loop Challenge for Loop Challenge Range Challenge Iterating Over Indexes Nested Loops

6. Functions

Built-in Functions Challenge Defining Your First Function Challenge if/else Statements in Functions Challenge Functions With No Return Challenge Modifying Functions Challenge Lambda Functions Challenge

Introduction to Python

Built-in Functions

What if you were asked to find the largest number in a list? With the knowledge you've gained, you could write a loop to check if the current element is larger/smaller than the previous biggest one and update it; if not, you'd continue. But for long lists, this method can be quite time-consuming. Thankfully, there are built-in functions that can make this task more efficient. Here are a few:

min(x, y, ...) - Returns the smallest value among x, y, ...;
max(x, y, ...) - Returns the largest value among x, y, ...;
abs(x) - Gives the absolute value of x;
round(x, n) - Rounds the number x to n decimal places;
pow(x, n) - Raises x to the power of n.

For instance, suppose we want to calculate the population density for a set of countries in the countries list. To do this, we'd divide the population by the area. Here's how it's done:

Code Description

countries = ...: This line creates a list named countries. Each item in this list is another list containing three elements: the name of a country, its area in square kilometers, and its population.

for i in range(len(countries)):: This starts a loop. len(countries) counts how many items are in the countries list, and range(len(countries)) creates a sequence of numbers from 0 up to (but not including) the number of items in the countries list. The variable i takes each of these numbers in turn as the loop runs.

if type(countries[i]) is list:: This line checks if each item (indexed by i) in the countries list is a list. In this code, every item will indeed be a list (containing the country details), so this condition will always be true.

pop_dens = countries[i][2]/countries[i][1]: Inside the loop, this line calculates the population density of each country. countries[i] accesses the i-th item in the countries list (which is one of the inner lists representing a country). countries[i][2] gets the population of the country (the third element in its list), and countries[i][1] gets the area of the country (the second element in its list). Dividing the population by the area gives the population density.

print(countries[i][0], pop_dens, 'people per km²'): This line prints three things:

1. countries[i][0]: The first element in the i-th list of countries, which is the name of the country.
2. pop_dens: The population density calculated in the previous step.
3. 'people per km²': A string that just adds some context to the printed population density value.

In the example above, our list had 5 nested sub-lists. We looped through the main list and checked if each item was a list. If it was, we divided the third item (population) by the second item (area).

However, the results were not very reader-friendly since they had more than 10 decimal places. To make them more readable, we can use the round() function to reduce them to just 2 decimal places. Remember, this function takes two arguments: the first is the number you want to round, and the second specifies how many decimal places you want to keep.

Code Description

countries = ...: This line initializes a list named countries. Each element within this list is another list that contains information about a country: the country's name, its land area in square kilometers, and its population.

The for loop, for i in range(len(countries)):: This line sets up a loop that will execute once for each element in the countries list. len(countries) counts how many elements are in the countries list, and range(len(countries)) generates a sequence of numbers from 0 to one less than the number of elements in the countries list. The variable i sequentially takes on each value in this range.

Inside the loop, if type(countries[i]) is list: checks whether each element accessed by countries[i] is a list. Given the structure of our countries list, this condition will always be true.

pop_dens = round(countries[i][2]/countries[i][1], 2): This line calculates the population density for the country at the current index i by dividing the population (third element, index 2) by the land area (second element, index 1). The round function is used to round the resulting population density to two decimal places.

print(countries[i][0], pop_dens, 'people per km²'): This command prints the name of the country (first element, index 0 of countries[i]), followed by the calculated population density (pop_dens), and the string 'people per km²'. This output gives a clear and formatted display of the population density for each country listed in the countries array.

As you can see, the revised result is much clearer and easier to understand.

Everything was clear?

Section 6. Chapter 1