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What can you do with a tuple ?

You can do just about anything with a tuple that you can do with a list, taking into account the fact that the tuple is immutable . Therefore, those list operations that change the value of a list in place cannotbe performed on a tuple. For example, you cannot call the append method on a tuple as you can on a list.

As with strings and lists, items in a tuple are accessed using a numeric index. The first item in a tuple is at index value0.

Why do tuples exist ?

Tuples provide some degree of integrity to the data stored in them. You can pass a tuple around through a program and be confident that its value can'tbe accidentally changed. As mentioned earlier, however, the values stored in the items referred to in a tuple can be changed.

In addition, in a future module, you will see some sample programs that require the use of tuples.

A sample program

Indexing and slicing tuples

Listing 1 shows the beginning of a Python program that first creates, and then manipulates a simple tuple.

Listing 1 . Beginning of the program.
# Illustrates indexing and slicing a simple tuple #------------------------------------------------print("Create a simple tuple") aTuple = (3.14,59,"A string",1024)

The remainder of this program will be shown and discussed as code fragments in subsequent Listings. The entire program is shown in Listing 6 near the end of the module.

The program output

At this point, I am going to show you the output produced by executing the program so that youwill have it available for reference during the discussion that follows. The output from the code in Listing 6 is shown in Figure 2 .

Figure 2 . Output from code in Listing 6.
Create a simple tuple Print index value 2A string Print a short slice(3.14, 59, 'A string') Print the entire tuple(3.14, 59, 'A string', 1024)

Tuple syntax

From a syntax viewpoint, you create a tuple by placing a sequence of items inside a pair of enclosing parentheses and separating them by commas.

Note that the parentheses can be omitted when such omission will not lead toambiguity.

The fragment in Listing 1 creates a simple four-item tuple and assigns it to the variable named aTuple .

Note that the items in a tuple can be different types. This simple tuple contains a float, an integer, a string, and another integer.

In this case, the parentheses could be omitted from the tuple syntax, because such omission would not lead to ambiguity. Listing 2 shows what this code fragment would look like if the parentheses were omitted.

Listing 2 . Tuple code with parentheses removed.
# Illustrates indexing and slicing a simple tuple #------------------------------------------------print("Create a simple tuple") aTuple = 3.14,59,"A string",1024

The code fragments in Listing 1 and Listing 2 are operationally identical and produce the same output, as shown in Figure 2 .

Indexing items in a tuple

The items in a tuple can be accessed using an index enclosed in square brackets as shown in Listing 3 . (Earlier modules showed how to use an index in square brackets to access the items in a list.)

Listing 3 . Indexing items in a tuple.
print("Print index value 2") print(aTuple[2])

The third item in the tuple is accessed and printed in Listing 3 . (Remember, index values begin with the value 0, so index value 2 points to the third item in thetuple.)

The second and third lines of text in Figure 2 were produced by the code in Listing 3 .

Tuples can be sliced

Tuples can be sliced just like lists and strings. This is illustrated by the code in Listing 4 .

Listing 4 . Printing a short slice from a tuple.
print("Print a short slice") print(aTuple[0:3])

The code in Listing 4 uses a slice to access and print the first three items in the tuple. (Remember, a slice begins with the index shown by the first specified value and ends with the index whose value is one less than the secondspecified value.)

The output produced by the code in Listing 4 is shown by the fourth and fifth lines of text in Figure 2 .

Finally, the code fragment in Listing 5 causes the entire tuple to be accessed and printed, as shown by the last twolines of text in Figure 2 .

Listing 5 . Print the entire tuple.
print("Print the entire tuple") print(aTuple[:100])

Complete program listing

A complete listing of the program is shown in Listing 6 .

Listing 6 . Complete program listing.
# Illustrates indexing and slicing a simple tuple #------------------------------------------------print("Create a simple tuple") aTuple = (3.14,59,"A string",1024)print("Print index value 2") print(aTuple[2]) print("Print a short slice")print(aTuple[0:3])print("Print the entire tuple") print(aTuple[:100])

Run the program

I encourage you to copy the code from Listing 6 . Execute the code and confirm that you get the same results as those shown in Figure 2 . 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: Itse1359-1100-Indexing and Slicing Tuples
  • File: Itse1359-1100.htm
  • Published: 10/19/14
  • 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 freelyavailable on cnx.org and that it was made and published without my 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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