| << Chapter < Page | Chapter >> Page > |
| Figure 1 . Output from the script in Listing 1. |
|---|
Create and print a list
[100, 200, 300, 400, 500]Original length is:
5Replace an element
Print the modified list[100, 200, [2, 4, 8, 16, 32, 64], 400, 500]Modified length is:
5Extract and display each
element in the list100
200[2, 4, 8, 16, 32, 64]
400500
Extract and display eachelement in nested list
24
816
3264 |
While the code in Listing 1 does some interesting and useful things, it is missing one major component of modern computer programming -- control flow .
Control flow is generally considered to include the following features plus some others that I will get into in future modules:
Repetition is the ability to cause specified portions of the code to be executed repeatedly under tightly controlled conditions. Repetition almostalways embeds decision logic in some form. This module will introduce you to one of the mechanisms for implementing repetition in Python -- the while loop. Other mechanisms will be discussed in future modules.
Modularity is the ability to subdivide the code into separate reusable units often called functions and methods . The design of functions and methods will be discussed in future modules.
Decision logic is the ability to make decisions on the basis of the program state and to determine which code will be executed as well as when and how itwill be executed. As mentioned above, repetition almost always embeds decision logic in some form. In addition, Python supports a stand-alone decisioncapability -- the if statement, which will be discussed in a future module.
The while loop is the most fundamental and the most general of all the repetition mechanisms in most programming languages. Most programminglanguages provide other more specialized looping mechanisms for convenience, and Python is no exception to that rule. However, the fundamental behavior of mostand perhaps all of those more specialized looping mechanisms can be replicated with a properly-configured while loop.
The code in Listing 2 illustrates the use of a while loop. It also illustrates one of the features of lists -- the append method. Finally, it illustrates a decision structure that is embedded in the while loop.
| Listing 2 . Use a while loop to manipulate a list. |
|---|
# Illustrates the use of a while loop to manipulate a list
aList = ["a","b","c"]print(aList)
count = 0while count<= 5:
print(count)aList.append(count)
count = count + 1# count += 1
print(aList) |
The output produced by this program is shown in Figure 2 .
| Figure 2 . Output from the script in Listing 2. |
|---|
['a', 'b', 'c']
01
23
45
['a', 'b', 'c', 0, 1, 2, 3, 4, 5] |
Figure 3 shows shows a visualization of the program after stepping through several iterations of the while loop. This static image doesn't do justice to the visualization. You really needto run it yourself and step through the program to appreciate it.
Figure 3. Visualization of the program.
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