String Methods and Transformations

A string is more than text displayed on a screen. It is a sequence: an ordered set of values. In a string, the values are characters, and each character has an integer index beginning at 0. This structure lets you select one character, select a range of characters, move through the text, or search for a target.

Consider the string hello. It contains five characters arranged in order. The first character is at index 0, the second at index 1, and so on. An index selects a single item from the sequence, so an index gives you one character rather than a range.

A slice selects a contiguous range instead of one character. It has a start index and an end index. The start is included, while the end is excluded. Therefore, a slice from index 1 to index 4 selects the characters at indices 1, 2, and 3. In hello, those characters are ell; the character at index 4 is not part of the slice.

Strings have methods that perform operations such as converting text to uppercase, finding the position of a character, or replacing one substring with another. A method is invoked by writing the string or a variable that holds it, then a dot, then the method name with parentheses. The notation connects the operation to the specific string object on which it should be performed.

Strings are immutable. That means an individual character in the original string cannot be changed in place. Immutability does not prevent string processing. Instead, concatenation or a string method can create a new string, while the original remains unchanged.

Traversal means visiting every item in a sequence systematically. For a string, traversal moves through its characters in order. This pattern is useful when you need to examine every character, count occurrences, or apply a transformation to all items.

Counters and flags often support traversal. A counter records how many matching characters have been encountered. A flag records whether a search condition has been satisfied. These variables help preserve information gathered while the traversal progresses.

Search is a specialized traversal. The process examines characters one at a time and checks each against a target character or substring. When the target is found, the search stops and reports the result. If the target is not found, the search reaches the end without a match.

A search can avoid unnecessary work when the target appears early because it stops at the first match. A flag can preserve the information that a match was found, while an index can identify where the match occurred.

The empty string is a valid string value. It has length 0 and contains no items. It can appear when a user provides no input or when a search produces an empty result. Treating it as a legitimate value helps you distinguish an empty result from an error or undefined state.