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Following that, an if...else statement is used to decide whether to visit the museum or to go to the beach based on the weather. Note the use of the equality operator ( == ) in the conditional clause of the if statement. Also note that the corresponding working variable ( rain or sunshine ) is set to True to confirm the visit for the benefit of the conditional clause of the while loop.

When the museum and the beach have each been visited at least once, the while loop will terminate causing the "go home" print statement to be executed.

Because of the use of a random number generator to control the weather, this program will produce a different output each time you run it. Figure 5 shows the output from one such run.

Figure 5 . Output from the code in Listing 2.
Sunshine, go to the beach. Sunshine, go to the beach.Sunshine, go to the beach. It's raining, visit the museum.Vacation is over, go home.

In this case, the random number generator produced the following sequence of numbers: 0,0,0,1 causing the beach to be visited three times before thevisit to the museum.

Visualization of the code in Listing 2

I recommend that you create a visualization on your own for the code in Listing 2 and step through the program one instruction at a time. Once again, as you do that, pay attention to themovements of the red and green arrows on the left, the diagram on the right, and the printed material at the bottom. That should help you to better understandthe behavior of an if...else statement.

Nested if statements

There are three keywords that can be used with nested if statements:

  • if
  • else
  • elif

You already know about if and else . The keyword elif is short for "else if" . Nested if statements can contain zero or more elif parts and the else part is optional.

The if...else construct allows the program to choose between two options. The use of elif makes it possible to write code that can choose among three or more options.

The program shown in Listing 3 extends the vacation analogy to three options:

  • rain -- visit the museum
  • sunshine -- go to the beach
  • snow -- go skiing

Once again, you won't go home until you have at least one opportunity to do each of the three activities in the above list.

Listing 3 . Example of nested if statements.
# Illustrates nested if statements #---------------------------------import random weather = ["sunshine","rain","snow"]rain = False sunshine = Falsesnow = False # Loop until rain and sunshine and snow are all truewhile (rain == False or sunshine == False or snow == False): # Get today's weatherweatherToday = weather[random.randint(0,2)] if weatherToday == "rain":rain = True print("It's raining, visit the museum.")elif weatherToday == "sunshine": sunshine = Trueprint("Sunshine, go to the beach.") else:snow = True print("It's snowing, go skiing.")print("Vacation is over, go home.")

The key thing to note in Listing 3 is the elif after the if and before the else . This extends the choices from two as shown in Listing 2 to three. The number of choices could be extended to more than three by inserting more elif options.

As before, the output will be different each time you run the program. One such output is shown in Figure 6 .

Figure 6 . Output from the code in Listing 3.
It's snowing, go skiing. It's raining, visit the museum.It's snowing, go skiing. It's raining, visit the museum.It's raining, visit the museum. It's raining, visit the museum.It's snowing, go skiing. Sunshine, go to the beach.Vacation is over, go home.

You should be able to compare the output with the code in Listing 3 and determine the sequence of eight random numbers produced by the random numbergenerator in this case.

Visualization of the code in Listing 3

Once again I recommend that you create a visualization on your own for the code in Listing 3 and step through the program one instruction at a time. As you do that, pay attention to themovements of the red and green arrows on the left, the diagram on the right, and the printed material at the bottom. That should help you to better understandthe behavior of nested if statements.

Run the programs

I encourage you to copy the code from Listing 1 , Listing 2 , and Listing 3 . Execute the code and confirm that you get the same results as those shown in Figure 2 , Figure 5 , and Figure 6 . Experiment with the code, making changes, and observing the results of your changes. Make certain that youcan explain why your changes behave as they do.

Miscellaneous

This section contains a variety of miscellaneous information.

Housekeeping material
  • Module name: Itse1359-1230-The if Statement
  • File: Itse1359-1230.htm
  • Published: 10/21/14
  • Revised: 01/31/16
Disclaimers:

Financial : Although the Connexions site makes it possible for you to download a PDF file for thismodule at no charge, and also makes it possible for you to purchase a pre-printed version of the PDF file, you should beaware that some of the HTML elements in this module may not translate well into PDF.

I also want you to know that, I receive no financial compensation from the Connexions website even if you purchase the PDF version of the module.

In the past, unknown individuals have copied my modules from cnx.org, converted them to Kindle books, and placed them for sale on Amazon.com showing me as the author. Ineither receive compensation for those sales nor do I know who does receive compensation. If you purchase such a book, please beaware that it is a copy of a module that is freely available on cnx.org and that it was made and published withoutmy prior knowledge.

Affiliation : I am a professor of Computer Information Technology at Austin Community College in Austin, TX.

-end-

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Source:  OpenStax, Itse 1359 introduction to scripting languages: python. OpenStax CNX. Jan 22, 2016 Download for free at https://legacy.cnx.org/content/col11713/1.32
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