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17.4: Substrings

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    The substring method returns a new string that copies letters from an existing string, starting at the given index.

    • fruit.substring(0) returns "banana"
    • fruit.substring(2) returns "nana"
    • fruit.substring(6) returns ""

    The first example returns a copy of the entire string. The second example returns all but the first two characters. As the last example shows, substring returns the empty string if the argument is the length of the string.

    To visualize how the substring method works, it helps to draw a picture like Figure 9.4.1.

    State diagram for a String of six characters.
    Figure \(\PageIndex{1}\): State diagram for a String of six characters.

    Like most string methods, substring is overloaded. That is, there are other versions of substring that have different parameters. If it’s invoked with two arguments, they are treated as a start and end index:

    • fruit.substring(0, 3) returns "ban"
    • fruit.substring(2, 5) returns "nan"
    • fruit.substring(6, 6) returns ""

    Notice that the character indicated by the end index is not included. Defining substring this way simplifies some common operations. For example, to select a substring with length len, starting at index i, you could write fruit.substring(i, i + len).

    This page titled 17.4: Substrings is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Allen B. Downey (Green Tea Press) .

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