String Algorithms

Scanning a String Character by Character

A for loop is the standard way to visit every character of a string — start at 0, stop before length():

String autoName = "BlueAlliance-Left";
int dashCount = 0;
for (int i = 0; i < autoName.length(); i++)
{
    if (autoName.substring(i, i + 1).equals("-"))
    {
        dashCount++;
    }
}

Use a for loop when you know you want to check every character. Use a while loop instead when you're searching for something and don't know in advance how many characters you'll need to check — see the find-and-replace pattern below.

Find and Replace with indexOf + substring

A while loop paired with indexOf repeatedly finds and processes one match at a time, until none remain:

String name = "A11iance";
int i = 0;
while (name.indexOf("1") >= 0)
{
    i = name.indexOf("1");
    String firstPart = name.substring(0, i);
    String lastPart = name.substring(i + 1);
    name = firstPart + "l" + lastPart; // replace the "1" with "l"
}

This loop runs twice: the first pass turns "A11iance" into "Al1iance", and the second turns that into "Alliance". Each pass finds the next occurrence, because the string was already reassigned — the search always runs against the current (updated) value of name.

Building a New String

Since strings are immutable, "building" one really means starting with an empty string and concatenating onto it inside a loop:

String reversed = "";
for (int i = 0; i < autoName.length(); i++)
{
    String letter = autoName.substring(i, i + 1);
    reversed = letter + reversed; // prepend — builds the string backwards
}

In real code, two shortcuts save you from hand-writing these patterns: name.replace("1", "l") does the entire find-and-replace loop above in one call (every occurrence, no manual indexOf loop), and StringBuilder is built for exactly this kind of repeated concatenation — new StringBuilder(autoName).reverse().toString() reverses a string in one line, and append(...) avoids creating a new String object on every pass the way += does.

Common Pitfalls

• Starting a string-scanning loop at 1 instead of 0. The first character is always at index 0 — starting at 1 skips it.
• Using <= instead of < against length(). i <= autoName.length() reads one index past the end and throws StringIndexOutOfBoundsException.
• An infinite loop in a find-and-replace pattern. If the replacement text itself still contains the thing you're searching for, indexOf finds it again forever — make sure each pass genuinely reduces what's left to find.

Key Takeaways

• for loops fit scanning every character; while loops fit searching for an unknown number of matches.
• A find-and-replace loop combines indexOf (to locate) with substring (to rebuild around the match), reassigning the string each pass.
• Building a new string from scratch means starting empty and concatenating inside a loop — strings can't be modified in place.

Derived from csawesome-2026: unit-2-selection-and-iteration/2.10-implementing-string-algorithms.md (CSA 2.10)