Lesson 2 - Lists and if statements¶
In the last lesson, we learned basic data types - numbers and strings. In this lesson, we will study a new data type called List. We will also learn about some more operations that Python provides for these data types.
As you have already seen, it is useful to store values in variables so that our code is more readable. Example:
>>> count = 10
Now, if we want to increment this counter or add a number to it, this is how we do it:
>>> count = count + 1
>>> count = count + 10
>>> print(counter)
21
In addition to print() function, Python supports many other
functions called built-in functions. These are called built-in
because they are provided by the language itself. As you will learn
later, you can also write functions and in fact, that is how you will
organize your code for the most part. For a full list of built-in
functions, see here.
Lists¶
Let us now get back to learning about data types supported by Python. Here we learn about “list” type. A list is a collection of other types. For example: a list of strings or a list of integers. A list is formed by enclosing the items with “square brackets”, like so:
>>> ["red", "green", "blue"]
Let us use a variable to contain a list and then, we can see some list operations.
>>> colors = ["red", "green", "blue"]
>>> print(colors)
['red', 'green', 'blue']
# To add items to a list at the end.
>>> colors.append("magenta")
>>> print(colors)
['red', 'green', 'blue', 'magenta']
# To remove an item
>>> colors.remove("red")
>>> print(colors)
['green', 'blue', 'magenta']
Notice how the variable “colors” contains a list and various list operations can be performed using the variable.
A list contains elements of different data types. For example, the following list contains both strings and integers.:
>>> testlist = ["abc", "def", 1, 2]
Even though this is possible, most of the times, a list contains elements all of the same type.
Here are some more operations possible on lists.
To sort the list alphabetically:
>> colors.sort()
>>> print(colors)
['blue', 'green', 'magenta']
To reverse a list::
>> colors.reverse()
>>> print(colors)
['magenta', 'blue', 'green']
Notice how “sort” and “reverse” operations changed the data stored in the variable “colors”. What if we don’t want to affect the data but want to get a new list that is sorted or reversed?
>>> print(colors)
['magenta', 'blue', 'green']
# This gives a new list, leaving the original list unaffected.
>>> sorted(colors)
['blue', 'green', 'magenta']
>>> print(colors)
['magenta', 'blue', 'green']
>>> reversed(colors)
['blue', 'green', 'magenta']
>>> print(colors)
['magenta', 'blue', 'green']
len()¶
You will use the built-in function len() to find the size of the
lists as well as that of strings. Examples:
>>> len("test")
4
>>> colors = ["red", "green", "blue"]
>>> len(colors)
3
# Empty string
>>> len("")
0
Boolean expressions¶
Many times in a program, you need to do something depending on whether some condition is true or false. For example, this is how you check if a number is even:
>>> i = 5
>>> if i % 2 == 0:
... print("even")
... else:
... print("odd")
As you can see, you use if-else statements to do this. This
statement contains a condition that can be true or false. In the
above code, the condition is i % 2 == 0. Here, we are checking if
the remainder when a number is divided by 2 is 0. Apart from the
condition itself, there is a block of code that is run when the
condition is true and another block of code that gets run when the
condition is false. Did you notice : after the condition and after
else? That is how we indicate a block of code to Python. Also
note that the code block is indented by few spaces. It is very
important to use same number of spaces for a code block. The
convention is to use 4 spaces.
Now, there are may operators that you can use as part of conditions. Some examples:
>>> i > 4
>>> i < 4
>>> i == 4 # to check if value of "i" is 4.
To check if two strings are equal:
>>> s == "test"
>>> test_string = "abc"
>>> len(test_string) < 5
Note that = is used to assign a value while == is used to
check for equality.
The operator in is used to see if an element is in a list:
>>> "red" in colors
BTW, “else” part of the “if” statement is optional. For example:
>>> i = 5
>>> if i % 2 == 0:
... print("even")
In this case, nothing happens if the number is not even.
Finally, it is possible to check for multiple conditions using a
different variation of if statement. In the following example, we
are checking if a number n is
- less than 10
- between 10 and 20
- or above 20
>>> n = 10
>>> if n < 10:
... print("less than 10")
... elif n < 20:
... print("less than 20")
... else:
... print("above 20")
...
less than 20
Accessing list elements¶
Sometimes, we need to access individual elements of a list.:
>>> colors = ["red", "green", "blue"]
>>> print(colors[0])
'red'
Here, we are accessing the first element of the list. Note that the counting of the elements in a list starts with “0”. So to print all the elements of this list, here is one way:
>>> print(colors[0])
>>> print(colors[1])
>>> print(colors[2])
The number we use to access an element is called index. The index starts from 0 and goes all the way up to the one less than the size of the list.
You will get an error if you try to use an invalid index.:
>>> print(colors[3])
IndexError: list index out of range
It is also possible to access elements from the end of the list instead of from the start. For example, the following will give you the last element of a list:
>>> colors[-1]
>>> # To access second element from the last:
>>> colors[-2]
Assignment¶
There is a new object in the solar system that is found to be traveling at 40000 miles per hour.
Write a program that calculates the number of days it takes this object to travel from Sun to Earth. Your program should print the following when run:
It takes N days
where N is the value your program should calculate.
Note. This object is named Oumuamua and in reality, it doesn’t travel from Sun to Earth in straight line.