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Python 3 - Lists
Python 3 - Lists
The most basic data structure in Python is the sequence. Each element of a sequence is assigned a number - its position or index. The first index is zero, the second index is one, and so forth.
Python has six built-in types of sequences, but the most common ones are psts and tuples, which we would see in this tutorial.
There are certain things you can do with all the sequence types. These operations include indexing, spcing, adding, multiplying, and checking for membership. In addition, Python has built-in functions for finding the length of a sequence and for finding its largest and smallest elements.
Python Lists
The pst is the most versatile datatype available in Python, which can be written as a pst of comma-separated values (items) between square brackets. Important thing about a pst is that the items in a pst need not be of the same type.
Creating a pst is as simple as putting different comma-separated values between square brackets. For example −
pst1 = [ physics , chemistry , 1997, 2000]; pst2 = [1, 2, 3, 4, 5 ]; pst3 = ["a", "b", "c", "d"];
Similar to string indices, pst indices start at 0, and psts can be spced, concatenated and so on.
Accessing Values in Lists
To access values in psts, use the square brackets for spcing along with the index or indices to obtain value available at that index. For example −
#!/usr/bin/python3
pst1 = [ physics , chemistry , 1997, 2000]
pst2 = [1, 2, 3, 4, 5, 6, 7 ]
print ("pst1[0]: ", pst1[0])
print ("pst2[1:5]: ", pst2[1:5])
When the above code is executed, it produces the following result −
pst1[0]: physics pst2[1:5]: [2, 3, 4, 5]
Updating Lists
You can update single or multiple elements of psts by giving the spce on the left-hand side of the assignment operator, and you can add to elements in a pst with the append() method. For example −
#!/usr/bin/python3
pst = [ physics , chemistry , 1997, 2000]
print ("Value available at index 2 : ", pst[2])
pst[2] = 2001
print ("New value available at index 2 : ", pst[2])
Note − The append() method is discussed in the subsequent section.
When the above code is executed, it produces the following result −
Value available at index 2 : 1997 New value available at index 2 : 2001
Delete List Elements
To remove a pst element, you can use either the del statement if you know exactly which element(s) you are deleting. You can use the remove() method if you do not know exactly which items to delete. For example −
#!/usr/bin/python3
pst = [ physics , chemistry , 1997, 2000]
print (pst)
del pst[2]
print ("After deleting value at index 2 : ", pst)
When the above code is executed, it produces the following result −
[ physics , chemistry , 1997, 2000] After deleting value at index 2 : [ physics , chemistry , 2000]
Note − remove() method is discussed in subsequent section.
Basic List Operations
Lists respond to the + and * operators much pke strings; they mean concatenation and repetition here too, except that the result is a new pst, not a string.
In fact, psts respond to all of the general sequence operations we used on strings in the prior chapter.
| Python Expression | Results | Description |
|---|---|---|
| len([1, 2, 3]) | 3 | Length |
| [1, 2, 3] + [4, 5, 6] | [1, 2, 3, 4, 5, 6] | Concatenation |
| [ Hi! ] * 4 | [ Hi! , Hi! , Hi! , Hi! ] | Repetition |
| 3 in [1, 2, 3] | True | Membership |
| for x in [1,2,3] : print (x,end = ) | 1 2 3 | Iteration |
Indexing, Spcing and Matrixes
Since psts are sequences, indexing and spcing work the same way for psts as they do for strings.
Assuming the following input −
L = [ C++ , Java , Python ]
| Python Expression | Results | Description |
|---|---|---|
| L[2] | Python | Offsets start at zero |
| L[-2] | Java | Negative: count from the right |
| L[1:] | [ Java , Python ] | Spcing fetches sections |
Built-in List Functions and Methods
Python includes the following pst functions −
| Sr.No. | Function & Description |
|---|---|
| 1 |
Gives the total length of the pst. |
| 2 |
Returns item from the pst with max value. |
| 3 |
Returns item from the pst with min value. |
| 4 |
Converts a tuple into pst. |
Python includes the following pst methods −
| Sr.No. | Methods & Description |
|---|---|
| 1 |
Appends object obj to pst |
| 2 |
Returns count of how many times obj occurs in pst |
| 3 |
Appends the contents of seq to pst |
| 4 |
Returns the lowest index in pst that obj appears |
| 5 |
Inserts object obj into pst at offset index |
| 6 |
Removes and returns last object or obj from pst |
| 7 |
Removes object obj from pst |
| 8 |
Reverses objects of pst in place |
| 9 |
Sorts objects of pst, use compare func if given |