Letter frequencies - Example

-- tabulate the frequency of occurrence of each letter in a
-- file of text

with TEXT_IO;
procedure frequencies is

    package int_io is new TEXT_IO.INTEGER_IO(INTEGER);
    use TEXT_IO, int_io;

    letter : CHARACTER;
    freq   : array ('A'..'Z') of NATURAL := (others => 0);

begin

    -- process each line in the file
    while not END_OF_FILE loop

        -- process each character of the line
        while not END_OF_LINE loop
            GET (letter);

            -- convert lower case letters to upper case
            if letter in 'a'..'z' then
                letter := CHARACTER'VAL (
                              CHARACTER'POS (letter)
                            - CHARACTER'POS ('a')
                            + CHARACTER'POS ('A') );
            end if;
            -- tally the character (if it is a letter)
            if letter in 'A'..'Z' then
                freq(letter) := freq(letter) + 1;
            end if;
        end loop;
        SKIP_LINE;

    end loop;

    -- print the frequencies
    for letter in 'A'..'Z' loop
        PUT (letter & " : ");
        PUT (freq(letter), width => 6);
        NEW_LINE;
    end loop;
    
end frequencies;