TheAlgorithms/Go · error

%c does not exist in characters

Error message

%c does not exist in characters

What it means

decipher resolves the row/column via strings.IndexRune in the cipher's character alphabet; a rune not in 'characters' (e.g. text not produced by this cipher) yields -1 and triggers this error naming the character.

Source

Thrown at cipher/polybius/polybius.go:95

func (p *Polybius) encipher(char rune) (string, error) {
	index := strings.IndexRune(p.key, char)
	if index < 0 {
		return "", fmt.Errorf("%q does not exist in keys", char)
	}
	row := index / p.size
	col := index % p.size
	chars := []rune(p.characters)
	return string([]rune{chars[row], chars[col]}), nil
}

func (p *Polybius) decipher(chars []rune) (string, error) {
	if len(chars) != 2 {
		return "", fmt.Errorf("the size of \"chars\" must be even")
	}
	row := strings.IndexRune(p.characters, chars[0])
	if row < 0 {
		return "", fmt.Errorf("%c does not exist in characters", chars[0])
	}
	col := strings.IndexRune(p.characters, chars[1])
	if col < 0 {
		return "", fmt.Errorf("%c does not exist in characters", chars[1])
	}
	return string(p.key[row*p.size+col]), nil
}

View on GitHub (pinned to 5ba447ec5f)

Solutions

1. Validate ciphertext characters against the cipher alphabet before decrypting
2. Ensure encrypt/decrypt use the same size and characters configuration
3. Strip or reject foreign characters in the input

When it happens

Trigger: Thrown at cipher/polybius/polybius.go:95 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of TheAlgorithms/Go@5ba447ec5f (2026-09-02). Data as JSON: /api/errors/f66f5c33cecdd7c9. Report an issue: GitHub.