Day 4: Lists¶

Day 4: Lists
- 1. List
- 2. References

1. List¶

Till now we are using list to store data and plot them. Now today we will learn about it extensively. List is a data structure, means a way to store and access data. So today we will learn how to store and access data in list data structure.

a list is a collection of data in a finite sequence.
it can hold values of multiple datatype, i.e., one entry can be integer, other can float or string.

1.1 Creating lists¶

A list is created using a square bracket as follows,

a = [] # an empty list
b = [1,2,3,4,5] # list of numbers
c = ["sandeep", "suman", 2020, 7] # mix datatypes
d = [["sandeep", "suman"], 2020, []] # list inside list

print(c)

['sandeep', 'suman', 2020, 7]

1.2 Accessing data inside list¶

Each entry has an index or position in a list, which can be used to access them. As counting starts in python from 0, So the first index is 0 and second index is 1 and so on.

c = ['sandeep', 'suman', 2020, 7]
print(c[0]) # to get the 0th element (count with 0)
print(c[2]) # to get the 2nd element (count with 0)
print(c[-1]) # count from last

sandeep
2020
7

1.3 Basic list operations¶

Length: The length of a list can be computed as follows
```
len(c)
```
```
4
```

Addition: Two or more lists can be added with + operation as follows

a = [1, 2]
b = ["a", "b"]
c = a + b
print(c)

[1, 2, 'a', 'b']

Multiplication: Repetition can be achieved with * operation as follows
```
a = ['Hi'] * 5
print(a)
```
```
['Hi', 'Hi', 'Hi', 'Hi', 'Hi']
```
Slicing: A part of of list starting with i index and end before j can be obtained by List[i:j], for example
```
c = ["sandeep", "suman", 2020, 7]
print(c[1:3])
```
```
['suman', 2020]
```

1.4 List as Stack¶

Stack is a data structure based on LIFO(Last in first out).

Append: The append method on a list will add element in the end of the list.

L = []
L.append(1) # 1 will be added to L
L.append(2) # 2 will be added to L
print(L)

[1, 2]

Pop: The pop method on list will remove the element from the last.
```
L = [1, 2]
L.pop()
print(L)
```
```
[1]
```
The method pop also takes addition argument to remove element from any given index.
```
L = [1, 2]
L.pop(0) # removes the element with index 0
print(L)
```
```
[2]
```

1.5 Other method on list¶

Index: This method is used to get the index of an element.

L = ["apple", "mango", "orange"]
i = L.index("mango")
print(i)

Remove: This method is used to remove an element in the list.

L = ["apple", "mango", "orange"]
L.remove("mango")
print(L)

['apple', 'orange']

Reverse: This method will reverse the elements of the given list.

L = ["apple", "mango", "orange"]
L.reverse()
print(L)

['orange', 'mango', 'apple']

Sort: This method is used to sort the elements of a list.

L = [4, 6, 2, 7, 1, 3]
L.sort()
print(L)

[1, 2, 3, 4, 6, 7]

Tip

The maximum element of a list can be obtained using sort taking in the last element. i.e.,

L = [4, 6, 2, 7, 1, 3]
L.sort()
max = L[-1]
print(max)

1.6 Membership and Iterating of a List¶

Like the membership in the set. We can tell if something is inside a list or not as follows

L = ["apple", "mango", "orange"]
print("apple" in L)
print("guava" in L)

True
False

Often in programming we want look for each element of a list one by one from start to finish. The process is generally known as iteration in programming.

A simple program to iterate a list is as follows
```
fruits = ["apple", "mango", "orange"]
for i in fruits: # semicolon is used to make code block
     print(i)    # 4 space is used here !important
```
```
apple
mango
orange
```

2. References¶

W3School on List