Traversing a String Character by Character

Some problems about strings can't be answered by looking at the whole thing at once. Counting vowels, finding a space, checking if a string is a palindrome: each of these needs you to examine individual characters and build up an answer as you go. Traversal breaks a big string problem into a sequence of small, identical decisions, one per character.

The standard pattern uses a for loop with an index variable, usually i, starting at 0 and running while i is less than s.length(). On each pass, s.charAt(i) hands you the character sitting at position i. Since i takes every value from 0 up to length() - 1 exactly once, the loop visits every character exactly once, in order, with no repeats and no skips. The index variable isn't just a counter ticking along beside the string; it IS the position currently being examined, which is why a trace table showing i's value also shows you where the loop is looking.

This connects directly to the rest of the unit: the loop's condition (i < s.length()) is the same kind of boolean test you've been tracing in while and for loops, and finding a character often means pairing the loop with an if statement that checks s.charAt(i) against some target. Once a match is found, many traversals break out early or return, which is the same short-circuit instinct you use with boolean logic, just applied to stopping a loop instead of skipping a branch.