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I'm afraid to research too much because I don't want spoilers, I just want pointers. I'm getting a lot of errors, in particular atoi errors. I tried saving an array element as a char, but atoi still won't work on it...? Thanks for any help.

// Helper functions for music

#include <cs50.h>

#include "helpers.h"

#include "math.h"

// Converts a fraction formatted as X/Y to eighths
int duration(string fraction)
{
    // TODO
    //prompt user for the fraction
    fraction = get_string("Please enter a fraction in the format X/Y");
    //convert divisor to an integer
    // int atoi(string str);
    char f1 = fraction[0];
    int divisor = atoi(char f1);
    //convert divisor to an integer
    int dividend = atoi(fraction[2]);

    //calculate how many eights
    if(dividend == 8)
    {
        //get the eights from divisor
        duration = divisor;
    }
    else
    {
        //convert other fractions into eights with a common denominator
        int common = 8 / dividend;
        duration =  divisor * common;
    }
}

// Calculates frequency (in Hz) of a note
// The frequency, f, of some note is 2n/12 × 440, where n is the number of semitones from that note to A4
// (2 to the power of n/12) * 440
int frequency(string note)
{
    // TODO
    // take as input as a string a note (e.g., A4)
    note = get_string("Please enter a letter and number to represent your note. A flat or sharp may be placed after the letter.");
    //define the letter character
    char letter= note[0];

    //validate a letter from A to G, ascii 65-71
    for (int ascii=65; ascii < 72; ascii++)
    {
        if letter == ascii)
        {
            ascii++;
        }
        else
        {
            printf("Please enter a capital letter between A and G.");
        }
    }

    //determine whether the number of the octave is the 2nd or 3rd character
    //check if the 2nd character is a number
    // int isdigit(int c);
    if (isdigit(note[1]))
    {
        int octave = atoi(note[1]);
    }
    else
    {
        octave = atoi(note[2]);
    }

    // validate the octave is a number from 0 - 8
    for (int octaves = 0; octaves < 9; octaves++)
    {
        if (octave == octaves)
        {
            return 0;
        }
        else
        {
            printf("Please enter a note with a number between 0 and 8.")
        }
    }

    // The frequency, f, of some note is 2n/12 × 440, where n is the number of semitones from that note to A4
    // (2 to the power of n/12) * 440
    // frequency = 2 ** (n / 12) * 440

    //start with the A's
    if (letter == 'A')
    {
        //check for octaves below 4 - for A0 to A3
        if (octave >= 0 && octave <= 3)
        {
            //substract octave from 4 and store in tones
            int tones = 4 - octave;
        }
        // check for octaves above 4 - for A5 to A8
        else if (octave >= 5 && octave <= 8)
        {
            tones = 4 + octave;
        } else
        {
            tones = 0;
        }
    }

    //test A
    printf("%i\n", tones);

    //define semitones
    int n = tones * 2;
     printf("%i\n", n);

    // frequency = 2 ** (n / 12) * 440
    frequency = round((2**(n / 12)) * 440);
}

printf("%i\n", frequency);

// Determines whether a string represents a rest
bool is_rest(string s)
{
    // TODO
    s = get_string("Please enter a note.");
    if (s == "")
    {
        frequency = 0;
    }
}
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Simple issues. In the first case, atoi(char f1); the code is trying to declare a var as the parameter to pass to atoi. The keyword char doesn't belong there. (It's actually re-declaring f1.)

Next, and more important, most (if not all) uses of atoi() are incorrect. atoi() converts a string to an integer, not a single char. atoi() depends on the presence of the end of string marker at the end of the string. The code is trying to submit single chars to the atoi function, not strings, so it doesn't work.

If you're trying to deal with single chars, there are other ways to do so. But then, that's the point of the exercise! ;-)

If this answers your question, please click on the check mark to accept. Let's keep up on forum maintenance. ;-)

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