Delimiter Counting and Field Extraction

A delimiter is a character that separates fields — commas in CSV data, slashes in dates, hyphens in IDs. Counting them tells you how many fields you have; tracking their positions lets you extract each field.

The number of fields in a row equals commaCount + 1 (assuming no leading/trailing commas).

On the AP exam, delimiter-based parsing appears in CSV FRQs and date/format problems.

Counting commas

public int countCommas(String s) {
    int count = 0;
    for (int i = 0; i < s.length(); i++) {
        if (s.charAt(i) == ',') count++;
    }
    return count;
}

Field count from comma count

If a CSV row has 3 commas, it has 4 fields. (Edge cases: empty string has 1 empty field; leading/trailing commas add empty fields.)

Extracting one field at a time

public String firstField(String csv) {
    int commaIndex = csv.indexOf(',');
    if (commaIndex == -1) return csv;     // no comma; whole string is the only field
    return csv.substring(0, commaIndex);
}

public String afterFirstComma(String csv) {
    int commaIndex = csv.indexOf(',');
    if (commaIndex == -1) return "";
    return csv.substring(commaIndex + 1);
}

Repeated application of afterFirstComma peels off one field per call.

The "remaining" pattern

String remaining = csv;
while (remaining.indexOf(',') != -1) {
    int idx = remaining.indexOf(',');
    String field = remaining.substring(0, idx);
    // process field
    remaining = remaining.substring(idx + 1);
}
// last field has no comma
String lastField = remaining;

Each iteration takes one field and shrinks remaining by that field plus its trailing comma.