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I will illustrate indexing and slicing of strings using a sample program contained in a script file named String01.py .
A complete listing of the program, (along with the output produced by the program) , is provided in Listing 9 near the end of the module.
I will discuss the program in fragments, illustrating particular aspects of indexing and slicing in each fragment. This is a scheme that I will usefrequently in this set of tutorial modules.
A single character can be extracted from a string by referring to the string and enclosing the index of the character in square brackets, as shown in thecode fragment in Listing 1 .
| Listing 1 . Extract a single character. |
|---|
aStr = "This is a string"
print(aStr[0]) #print T
print(aStr[3]) #print s |
(Note that this is a fragment from a script file, not from an interactive Python program.)
The fragment in Listing 1 creates a string object and assigns it to a variable named aStr . From this point forward, the contents of the string can be accessed by referring to the variable.
Figure 1 shows a visualization of the string object. This visualization was produced using the Online Python Tutor .
Figure 1. Visualizing the string object.
The green arrow in Figure 1 indicates that the first statement in the program is the most recently executed statement in this visualization. (This is the statement that created the string object.) The diagram on the right shows that the variable named aStr contains a reference to an object of type str , which contains the value: "This is a string" .
This is a very simple diagram but it shows a pattern that you need to remember. Variables in Python containreferences to objects. Those objects may be "dumb" objects intended solely to encapsulate a value (such as type int ) or they may be "smart" objects (such as type str ) that can not only encapsulate data values but can also encapsulate methods (such as capitalize() ) to manipulate those data values in various ways.
(Note that even the dumb objects are not completely dumb. For example,all of the numeric types such as int know how to participate in arithmetic operations, how to negate themselves, and how to do a few otherthings that might be expected of a numeric.)
Unlike eggs and Air Force enlisted men, the first character in a string is always located at index 0, as in aStr[0] .
Thus, the second statement in the fragment extracts and prints the T , which is the first character in the word This .
Similarly, the last statement in the fragment in Listing 1 extracts and prints the s from index position 3, as in aStr[3] . The character at this index position is the s that ends the word This .
The last statement is equivalent to the following request, "Will the character at index position 3 please display yourself on the screen."
You would probably refer to this as the fourth character, and you would be correct if you did. The character at index 3 is the fourth character in thestring. The character at index 0 is the first character in the string. First does not equate to index 1. You need to think about this, because this can be avery confusing topic for new programmers.
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