Writing code from prose

Writing code from prose — the specification is a paragraph, the answer is a method. The workflow:

1. Restate the postcondition as a formula or loop invariant.
2. Choose the traversal (array? string? list? which direction?).
3. Write the accumulator skeleton FIRST, fill the body SECOND.
4. Check the preconditions for shortcuts you may take.

Example spec: "Return the number of times the character ch appears in the string s, ignoring case." Restated: loop over every index of s, compare case-folded characters to ch, count matches.

public static int countIgnoreCase(String s, char ch) {
    int count = 0;
    for (int i = 0; i < s.length(); i++) {
        if (Character.toLowerCase(s.charAt(i)) == Character.toLowerCase(ch)) {
            count++;
        }
    }
    return count;
}

Common spec-reading failures, in order of frequency:

• missing the "ignoring case" / "ignoring order" / "at most" qualifier;
• returning the wrong shape (index vs value, count vs list);
• handling the empty input wrong (or crashing on it);
• shadowing the parameter (String s then int s = 0).