Difference between revisions of "Python-3.4.3/C3/Manipulating-lists/English"

Objectives

• slicing and striding of lists
• Sort and reverse lists.

System Specifications

• Ubuntu Linux 16.04 operating system
• Python 3.4.3
• IPython 5.1.0

Pre-requisites

• run basic Python commands on the ipython console and
• use lists

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

Slicing

The syntax for slicing is p inside square brackets start colon stop.

It returns all the elements of p between start and stop values.

The element with the stop index value will not be included.

Open the terminal.

Press Enter

From here onwards, remember to press the Enter key after typing every command on the terminal.

primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]

Type, primes is equal to then type as shown.

primes[4:8]

Type, primes inside square brackets 4 colon 8

Recall that the start index value is 0.

Observe that the first element we want is 11 which has index 4 in the list.

So, start value is 4.

Also, 19 is the last element we require which has index 7 in the list.

The element with index equal to stop value will not be included.

So, end value is 8.

Try this exercise and then resume the video.

primes[0:4]

Observe that in this case, our start value is 0.

7 is the last element we require which has index 3 in the list.

So, end value is 4.

<<PAUSE>>

Type 0 to 13 in a list and assign it to a variable num as shown.

num[1:10:2]

Press Enter

We will specify the step value in which the slice must be obtained.

Type num inside square brackets 1 colon 10 colon 2

We have to start from element with index 1 upto index 10 in steps of 2.

It is called striding of list.

Press Enter to get the output.

We got the odd numbers less than 10.

num[:10]

Similarly, there are default values for start and stop as well.

Type, num inside square brackets colon 10

It gives the first 10 elements of the list.

If we don't specify the start value, the first element of the list is taken as starting.

num[10:]

This gives us all the elements from 10th element to the end.

If we don't specify the stop value, the elements till the last index of the list will be returned.

Start or end value can be negative to indicate that they are counted from the end of the list.

num[::2]

Observe that all the even numbers are at even index locations.

So, type num inside square brackets colon colon 2

We got all the even numbers.

This is called striding.

Try this exercise and then resume the video.

It gives us every third element from the list (i.e) multiples of 3.

<<PAUSE>>

Type, a is equal to inside square brackets 5, 1, 6, 7, 7, 10

Type, a.sort open and close brackets

a

We can see that the contents of the list a is sorted now.

Sorted()

• Python provides a built-in function called sorted.
• sorted function sorts the list which is passed as an argument to it.
• It returns a new sorted list.

Type, sa is equal to sorted inside brackets a

Type, r is equal to inside square brackets 1, 2, 3, 4, 5

We got the reverse of the list.

The original list r is changed now.

Again we will assign the same value to r as shown.

Type, ra is equal to r inside square brackets colon colon minus 1

Try this exercise and then resume the video.

Exercise 3

Obtain a list with marks in descending order.

sorted(marks)[::-1]

sorted inside brackets marks inside square brackets colon colon minus 1

sorted(marks, reverse = True)

sorted inside brackets marks, reverse equal to True

<<PAUSE>>

Summary slide

Let us summarize.

In this tutorial, we have learnt to,

• Obtain the parts of lists using slicing and striding.
• Sort lists using the sort method.
• Use the method reverse to reverse the lists.

Evaluation

1. Given the list primes. How do you obtain the last 4 prime numbers?
2. Given a list p, of unknown length. Obtain the first 3 characters of it.

Solutions

1. The last four primes can be obtained from the given list as,

primes inside square brackets minus 4 colon

1. The first 3 characters can be obtained as,

p inside square brackets colon 3

Acknowledgement

For more details, visit this website.