Python Programming Lesson #32
August 1, 2000
Something for everyone
Beginners start at the beginning, and experienced programmers jump in further along. Lesson 1 provides an overall description of this online programming course.
Plus some other structures
It also introduced you to subscriptions, sequences, mutable sequences, mappings, slicings, and tuples.
That lesson showed you some of the ways that you can manipulate lists. The discussion was illustrated using sample programs.
Other ways to manipulate lists
This lesson carries that discussion forward by using sample programs to teach you other ways to manipulate lists.
You can replace a slice in a list with the elements from another list through assignment.
Can change the length of the list
Note that this operation can change the length of the original list.
Replacing a slice in a list with the elements from another list is illustrated in Figure 1.
The function named len()
This program also illustrates the use of the function named len() to get and print the length of the list.
Replaces a three-element slice with a six-element list
In this program, a slice of an original five-element list, consisting of the elements from 1 through 3 inclusive, is replaced by the elements of a new list consisting of six new elements.
Length of the list is increased by 3
Since three existing elements are replaced by six new elements, the overall length of the list is increased by three elements.
The output from this program is shown in Figure 2 (boldface added for emphasis).
Six new elements replaced three original elements
As you can see, the six new elements replaced the three original elements to increase the length of the list from 5 to 8 elements.
Replacing an element with a list
It is also possible to replace an element in an existing list with a new list, as illustrated in the following program.
Behavior is different from above
In this case, the behavior is different from that shown above where a slice from the original list was replaced with the elements from a different list (even though the right operand of the assignment operator is the same in both cases).
Produces nested lists
When a single element is replaced by a list, the result is that a new list is nested inside the original list.
Length is unchanged
It is also interesting to note that the length of the list is unchanged by this operation since the list that replaces the element is itself considered to be a single element. Therefore, the number of elements is not changed.
This is illustrated in Figure 3, where the element at index 2 of an original list is replaced with a new list having six elements.
The output from this program is shown in Figure 4 (boldface added for emphasis).
One element is itself a list
As you can see, the result is that one of the elements in the original five-element list is replaced by a new list containing six elements. However, the length of the list is unchanged.
Again, it is important to note that this results in one list being nested inside of another list.
Extracting elements from a nested list
Now I am going to illustrate the syntax for extracting elements from a nested list using pairs of matching square brackets. Figure 5 is an expansion of Figure 3.
Note the boldface statements
The most interesting statements in this program are highlighted in boldface.
Display the nested list combination
After nesting a list as element 2 in another list, the program displays the value of each of the elements of the original list. When element 2 is displayed, it can be seen to be another list.
Then the program uses double-square-bracket notation (listA) to extract and display each of the elements in the nested inner list that comprises element 2 of the outer list.
The output from this program is shown in Figure 6 (boldface added for emphasis).
More on nested elements
The program in Figure 7 illustrates some additional aspects of nested elements.
Create a list of lists
This program defines a three-element list containing three nested lists.
In other words, each of the elements in the outer list is itself a list.
Inner lists are different lengths
Furthermore, the lengths of the inner nested lists are not the same. The lengths of the inner nested lists are 2, 3, and 4 elements each, respectively.
Program behavior and output
The output from the program is shown in Figure 8 (boldface added for emphasis). The program displays the entire list, and then gets and displays the lengths of each of the nested lists.
Getting the length of a nested list
Note in particular the syntax used to pass a parameter to the len() method in order to get the length of a nested list ( len(listA) ).
Arrays -- Not for beginners
If you are a beginning programmer, just skip this section.
If you are an experienced programmer, you may have observed that a Python lists bear a striking resemblance to arrays in other programming environments.
Python lists are more powerful
However, Python lists are more powerful than the arrays I am aware of in other programming environments, including Java.
Sub-arrays can be different sizes
For example as in Java, when a Python list is used to construct a multidimensional array, the sub arrays don't have to be of the same size.
Types can also be different
However, unlike Java, the elements in the array don't even have to be of the same type (granted that the elements in a Java array can be of different types so long as there is an inheritance or Interface relationship among them).
A three-dimensional array program
In any event, the program in Figure 9 might represent what an experienced programmer would consider to be a three-dimensional array of integer data in some other programming environment.
Pay particular attention to the triple square bracket notation that is used to access and print each element in the array.
The output from the program is shown in Figure 10. The triple-square-bracket notation in the program of Figure 9 is essentially the same notation that would be used to access the individual elements in a three-dimensional array in C, C++, or Java.
Ans: See Figure 11.
2. Show how to replace an element in a list with a nested list.
Ans: See Figure 12.
3. Show how to extract the elements from one list that is nested in another list.
Ans: See Figure 13.
4. Show how to create a list of nested lists where the nested lists have different lengths.
Ans: See Figure 14.
Richard has participated in numerous consulting projects involving Java, XML, or a combination of the two. He frequently provides onsite Java and/or XML training at the high-tech companies located in and around Austin, Texas. He is the author of Baldwin's Java Programming Tutorials, which has gained a worldwide following among experienced and aspiring Java programmers. He has also published articles on Java Programming in Java Pro magazine.
Richard holds an MSEE degree from Southern Methodist University and has many years of experience in the application of computer technology to real-world problems.