String x;

void setup() {
  Serial.begin(9600);
}

void loop() {

if(Serial.available()){
  x = Serial.readString();
  Serial.print("Letters typed are : {");
  Serial.print(x);
  Serial.print("}");
  Serial.println("");
  countLetters(x);  
}
}

void countLetters(String inputString) {
    // Initialize an array to store letter counts (for 26 letters a-z)
    int letterCount[26] = {0};

    // Iterate through each character in the input string
    for (int i = 0; inputString[i] != '\0'; i++) {
        char ch = tolower(inputString[i]); // Convert to lowercase

        // Check if the character is a letter (a-z)
        if ('a' <= ch && ch <= 'z') {
            // Calculate index in the letterCount array (0 for 'a', 1 for 'b', ..., 25 for 'z')
            int index = ch - 'a';
            // Increment the count for this letter
            letterCount[index]++;
        }
    }

    // Print or display the results as needed
    for (int i = 0; i < 26; i++) {
        if (letterCount[i] > 0) {
            char letter = 'a' + i;
            Serial.print(letter);
            Serial.print(": ");
            Serial.println(letterCount[i]);
        }
    }
    Serial.println("");
}