Category: Basic Python

Loops in Python: For Loop
Loops are used when one wants to repeat a set of commands several times (iterations). For example,when you want to apply a set of operations on elements of a list, by choosing one element at a time, you can use a python loop.

There are two types of loops:
- For loop
- While loop
For loop:

For loop is used when we want to run the set of commands for a known or defined number of times. Few of the ways in which we can use the “for loop” is as follows:
Python
```
for i in range(10):
    print(i)
range(start, stop, step)
```
Here the range(10) function will create number in sequence starting from 0 and ending at 9. The value of i will thus stat from 0 and increase by 1 in each iteration stopping at 9

The general format of range function is:
Python
```
range(start, stop, step)
```
By default the value of step is 1.

The other way of running for loop is:
Python
```
mylist = ["a", "b", "c"]
for i in mylist:
    print(i)
    
mylist = ["a", "b", "c"]
for i, element in enumerate(mylist):
    print(f'index: {i}, element: {element}')
```
```
a
b
c
```
Here, the for loop iterates over each element in the list (mylist). In the first loop the value of “i” is “a”; in the second loop the “i” has the value of “b”; and so on.

The enumerate function allows us to simultaneously iterate over the elements of a list as well as yielding the indices of the current element in the list in a particular iteration.
Python
```
mylist = ["a", "b", "c"]
for i, element in enumerate(mylist):
    print(f'index: {i}, element: {element}')
```
```
index: 0, element: a
index: 1, element: b
index: 2, element: c
```
This is useful when you might need to access the elements of other lists relative to this index in the loop.
List in Python
Lists in python are collection of objects. They can include any python objects such as numerals, strings, other lists and dictionaries. The list is defined by square bracket as follows:
Python
```
my_list = []
```
The above example shows an empty list. Following is an example of a list with numbers:
Python
```
my_list = [1,2,4,65,43]
my_list
```
```
my_list = [1,2,4,65,43]
my_list
```
Lists can also contain strings.
Python
```
list_of_str = ['abc', 'goku', 'vada_pav', 'chicken_rice']
list_of_str
```
```
['abc', 'goku', 'vada_pav', 'chicken_rice']
```
Single list can also contain different types of objects. Following is an example of a list containing number, string, another list and a dictionary.
Python
```
list_mix = [3, 4, 'burger', ['Chicago', 23, 'Nile'], {'city_names': ['Delhi', 'London'], 'food_items': ['idli', 'mala-hotpot', 'rojak']}]
list_mix
```
```
[3,
 4,
 'burger',
 ['Chicago', 23, 'Nile'],
 {'city_names': ['Delhi', 'London'],
  'food_items': ['idli', 'mala-hotpot', 'rojak']}]
```
When the command lines become too long we can break the line after a comma in the list.
Python
```
list_mix = [3,
            4,
            'burger',
            ['Chicago', 23, 'Nile'],
            {'city_names': ['Delhi', 'London'],
             'food_items': ['idli', 'mala-hotpot', 'rojak']}]
list_mix
```
```
[3,
 4,
 'burger',
 ['Chicago', 23, 'Nile'],
 {'city_names': ['Delhi', 'London'],
  'food_items': ['idli', 'mala-hotpot', 'rojak']}]
```
List indexing

List index starts with zero as all indexes in python. Following is the general format of indexing a list.

my_list[from_index:to_index:step]

The to_index is not inclusive. It means that the list item at to_index will not be included in the result of indexing. We will see this an example of a list of number from 0 to 20.
Python
```
a = list(range(20))
a
```
```
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
 
```
Python
```
a[2:18:3]
```
```
[2, 5, 8, 11, 14, 17]
```
Here, we indexed the list from 2nd index to the 18th index with a step size of 3. The default stepsize is 1.

We will see different ways of indexing/slicing a list not.

From a defined index (e.g. 5) to end of the list.
Python
```
a[5::]
```
```
[5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
```
From start of the list to a defined index (e.g. 15) including the item at that index.
Python
```
a[:15+1]
```
```
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
```
Python
```
a[:16]
```
```
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
```
The last element of a list can be indexed as -1, and the second last element is indexed as -2 and so on. Following are few example to index a list using negative numbers.
Python
```
a[-1]
```
```
19
```
Python
```
a[-10:-1]
```
```
[10, 11, 12, 13, 14, 15, 16, 17, 18]
```

Category: Basic Python

Loops in Python: For Loop

List in Python

List indexing