Difference between revisions of "Python-3.4.3/C2/Getting-started-with-Lists/English"

Objectives

• Create lists
• Access list elements
• Append elements to lists
• Delete elements from lists

System Specifications

• Ubuntu Linux 14.04 operating system
• Python 3.4.3
• IPython 5.1.0

Pre-requisites

• run basic Python commands on the ipython console

If not, see the relevant Python tutorials on this website.

List

• A List can store a sequence of elements.
• All elements need not be of the same data types

ipython3

Now, type ipython3 and press Enter.

We are now in the ipython prompt.

Let us clear the terminal by pressing Ctrl + L for better view.

mylist = ['spam', 'eggs', 100, 1.234]

Highlight spam and eggs

Type

mylist equal to inside square brackets inside quotes spam comma inside quotes eggs comma hundred comma one point two three four and press Enter.

In Python, strings are always placed inside single or double quotes.

Highlight the command

Here type is a function which will return the datatype of the variable.

mylist is a list datatype.

In mylist,

• spam and eggs are strings,
• whereas hundred and one point two three four are integer and float respectively.

What is a variable?

• variable must either start with an alphabet or an underscore.

• They cannot start with numbers and cannot be the same as Python keywords.

In Python, a variable must either start with an alphabet or an underscore.

They cannot start with numbers and cannot be the same as Python keywords.

• for, if, else, elif, while, in, def, or, and
• A variable name cannot have spaces or punctuation characters or any arithmetic characters.

Few examples of keywords are “for, if, else, elif, while, in, def, or, and”.

A variable name cannot have spaces or punctuation characters or any arithmetic characters.

Valid and invalid variables

Valid variable names:

• x, y, xx, abc, a_b_c, a1, variable1, mylist, _x
• foreign, spiff, while_true, or_else, and_od_else_1

Invalid names:

• 1_x, 2_var, x-, x+, x;, x:
• x y, x+y x*y x&^ae
• for, in, elif

Highlight mylist = ['spam', 'eggs', 100, 1.234]

num underscore list is equal to inside square brackets '''one comma two' and press Enter

or

x equal to inside square brackets inside quotes a comma inside quotes b and press Enter

Any valid variable name could be used.

myemptylist = []

Type myemptylist is equal to open and close square brackets and press Enter.

This is an empty list without any element

list index

Index value of the elements starts from 0, 1, 2 and so on.

Negative indices are used to access elements from the end.

Which starts from -1, -2, and so on

The syntax to get a specific list element is

variable inside square brackets elements index value

mylist[0]

mylist[1]

mylist[3]

As we can see mylist[0] gives the first element spam

Type mylist inside square brackets one and press Enter.

mylist[1] gives the second element.

Exercise1

What happens when you type mylist[-1].

mylist[-1]

Type mylist inside square brackets minus one and press Enter.

As you can see you get the last element which is one point two three four.

List in list

This property makes lists heterogeneous data structures.

This is the syntax for list inside a list.

doublelist=[‘a’, [‘b’,’c’,’d’], 'and', 5, 6, 7, 8]

Highlight [‘b’,’c’,’d’]

Type the code as shown here:

Here b c d is a list in list which is represented within square brackets inside a list.

Type: doublelist inside square brackets one and press Enter

We can see the output as b c d within square brackets.

This is because the index value one contains the list inside a list as its element.

For this type: doublelist inside square brackets one inside square brackets zero and press Enter.

Here the 1st parameter represents the index of the list in the main list.

The second parameter represents the index value of b in the list inside the list.

Exercise2

doublelist=[‘a’, [‘b’,’c’,’d’], 'and', 5, 6, 7, 8]

1. What is the command to get the element 'and' in the list doublelist?
2. How would you get 'and' using negative indices?
3. How would you get element 'd' from the list doublelist?

Solution 2

len function is used to check the number of elements in the list.

Syntax:

len(variable)

The syntax is: len inside parentheses variable

len(mylist)

Switch to the terminal and clear the screen.

Recall mylist by pressing up arrow key and press Enter.

Type len inside parentheses mylist and press Enter.

We get the length of mylist as four.

This function will add the element to the end of the list.

The syntax is: variable dot append within parentheses element to be added.

mylist.append('sharp')

mylist.append(6)

Now type

mylist dot append inside parentheses within quotes sharp and press Enter.

Type mylist dot append inside parentheses six and press Enter.

Let us check the updated mylist.

Type mylist and press Enter

We can see mylist is appended with sharp and six at the end.

There are two ways of doing it.

• One is by using the index with del keyword.
• Syntax is del variable inside square brackets element’s index value.

• The other way is removing element by the value using remove function
• Syntax is variable dot remove inside parentheses element to be removed

[IPython Terminal]

del mylist[1]

mylist

Type mylist and press Enter to see the updated list.

Type del space mylist inside square brackets one and press Enter.

The keyword del deletes the element at index one,

i.e the second element of the list, eggs.

Again type mylist and press Enter.

We can see that the element eggs has been deleted from the list.

mylist.remove(100)

mylist

For this we can use the function remove.

Type mylist dot remove within parentheses hundred and press Enter.

Let us check the updated mylist.

mylist.append('spam')

Highlight spam

To check that, let’s do a small experiment.

Type

mylist dot append within parentheses within quotes spam and press Enter

Type mylist and press Enter

Here is the updated mylist.

It contains the element spam twice.

One is at the start and the next is at the end of the list.

Type mylist

mylist dot remove inside parentheses inside quotes spam and press Enter.

Let us check updated mylist.

The function remove removes the first occurrence of the element in the sequence.

Remember, the del keyword removes the element by index value.

remove function removes on the basis of element being passed on.

Let us try this with an example.

k = [1,2,1,3]

k.remove(k[2])

k

Then type k dot remove inside parentheses k inside square brackets two and press Enter

Type k and press Enter

We can see the output as [2,1,3].

Since it deletes the first occurrence of what is returned by k inside square brackets two which is one.

Exercise 3

1. Delete the fourth element from the list doublelist.

2. Remove 'and' from the list doublelist.

Exercise 3

Summary slide

In this tutorial, we have learnt to create:

• List with elements
• Empty list
• List within a list

• Find out the list length using len function.
• Access elements using their index numbers.
• Append elements to list using the function append.
• Delete element from list using the del and remove function

Evaluation

1. How do you create an empty list?
2. Can you have a list inside a list?
3. How would you access the last element of a list without finding its length?

Solutions

1. We create an empty list just by leaving the space inside the square brackets empty.

   myemptylist equal to open and close square brackets.

2. Yes, list can contain all the other data types, including list.
3. Using negative indices, we can access the last element from the list.

For more details, visit this website.

Thank You

Thank you.