String Indexing and Slicing

Indexing and slicing are crucial parts of programming. In data analysis, indexing and slicing DataFrames is essential to keep track of rows and columns, something we will practice in Chapter 10, Data Analytics with pandas and NumPy. The mechanics behind indexing and slicing dataFrames is the same as indexing and slicing strings, which we will learn in this chapter.

Indexing

The characters of Python strings exist in specific locations; in other words, their order counts. The index is a numerical representation of where each character is located. The first character is at index 0, the second character is at index 1; the third character is at index 2, and so on.

Note

We always start at 0 when indexing.

Consider the following string:

destination = 'San Francisco'

'S' is in the 0th index, 'a' is in the 1st index, 'n' is in the 2nd index, and so on. The characters of each index are accessed using bracket notation as follows:

destination[0]

You should get the following output:

'S'

To access the data from the first index, enter the following:

destination[1]

You should get the following output:

'a'

To access the data from the second index, enter the following:

destination[2]

You should get the following output:

'n'

The character value for San Francisco and the corresponding index count is shown in Figure 1.15:

Figure 1.15: Diagrammatic representation of the character values and the corresponding positive index values

Now, try adding a -1 as the index value and observe the output:

destination[-1]

You should get the following output:

'o'

Note

Negative numbers start at the end of the string. (It makes sense to start with -1 since -0 is the same as 0.)

To access the data from the rear of San Francisco, we use the negative sign in this case -2:

destination[-2]

You should get the following output:

'c'

The following figure 1.16 mentions the characters sco from the word Francisco, and the corresponding index count:

Figure 1.16: Index value for the negative values for San Francisco

Here is one more example:

bridge = 'Golden Gate'
bridge[6]

You should get the following output:

' '

You may be wondering whether you did something wrong because no letter is displayed. On the contrary, it's perfectly fine to have an empty string. In fact, an empty string is one of the most common strings in programming.