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it looks like my file compiles but it doesn't work correctly, and I get this msg:

clang: error: linker command failed with exit code 1 (use -v to see invocation)

Any help much appreciated.

// Helper functions for music

#include <cs50.h>

#include "helpers.h"

// #include "math.h"
#include <math.h>
#include <stdio.h>
#include <ctype.h>
#include <string.h>

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

    //convert divisor to an integer
    int dividend = fraction[2];

    //calculate how many eights
    if(dividend == 8)
    {
        //get the eighths from divisor
        eighths = divisor * 1;
    }
    else
    {
        //convert other fractions into eights with a common denominator
        int common = 8 / dividend;
        eighths = divisor * common;
    }
    printf("%i\n", eighths);
    return eighths;
}

// 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.\n");
    //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

    //define octave (in array), octaves and tones
    int octaves;
    int octaven;
    int wavelength;
    //check if the 2nd character is a number
    if (!isalpha(note[1]))
    {
        octaven = note[1];
    }
    else
    {
        octaven = note[2];
    }

    // validate the octave is a number from 0 - 8
    for (octaves = 0; octaves < 9; octaves++)
    {
        if (octaven == 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
    int tones = 0;
    //start with the A's
    if (letter == 'A')
    {

        //check for octaves below 4 - for A0 to A3
        if (octaven >= 0 && octaven <= 3)
        {
            //substract octave from 4 and store in tones
            tones = 4 - octaven;
        }
        // check for octaves above 4 - for A5 to A8
        else if (octaven >= 5 && octaven <= 8)
        {
            tones = 4 + octaven;
        } else
        {
            tones = 0;
            //test A
             printf("%i\n", tones);
        }
    }
    else
    {
        return 1;
    }

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

    // frequency = 2 ** (n / 12) * 440
    // pow(10.00, 2.00)
    wavelength = round((pow(2.00, (n / 12.00)) * 440));
    // frequency = round((2**(n / 12)) * 440);
    printf("%i\n", wavelength);
    return wavelength;
}

// Determines whether a string represents a rest
bool is_rest(string s)
{
    int wavelength = 0;
    // TODO
    s = get_string("Please enter a note.");
    // int strcmp(const char * s1, const char * s2);
    if(strcmp(s, "") == 0)
    {
        wavelength = 0;
    }
    return wavelength;
}
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The lines above this error message would have been interesting, because they usually indicate what could not be found.

I can only guess you didn't compile by just calling make (equivalent to make all, as all is first target in makefile), or make notes, or make synthesize.

I think you got multiple things wrong with your code. You are not supposed to ask the user for a note or duration. That's the task of the code calling your helper functions, code you are not supposed to change.

Your for loops make no sense to me (did you mean something like if (octaves < 0 || octaves > 9)? Just assume all input is valid, we don't even have a way to signal an error)

Also, remember ASCII, which you learnt about in pset2. Digit character '4' is not the same as number 4 (in fact it's 52 or 0x34 as you can see in the ASCII table)

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