Add user input to array of unknown size

Question

I still pretty new to programming and my only prior experience before C was Javascript. I'm doing the CS50 Introduction to Computer Science and in one of the lectures there's an example code that computes the average of some user input. It looks like this:

#include <cs50.h>
#include <stdio.h>

const int TOTAL = 3;

float average(int length, int array[])
int main(void)
{
    int scores[TOTAL];
    for (int i = 0; i < TOTAL; i++)
    {
      scores[i] = get_int("Score: ");
    }

    printf("Average: %f\n", average(TOTAL, scores);
}

float average(int length, int array[])
{
    int sum = 0;
    for (int i = 0; i < length; i++)
    {
        sum += array[i];
    }
    return sum / (float) length;
}

The feature that I'm trying to add is to dynamically store the size of the array depending of the user input, instead of having one variable (TOTAL in this case). For example: I need to have a loop that is always asking the user for a score (instead of just 3 times like the code above), and when the user types zero(0), the loops breaks and the size of the array is defined by how many times the user has typed some score.

This is what I've done:

int main(void)
{
    int score;
    // start count for size of array
    int count = - 1;

    do
    {
        score = get_int("Score: ");
        // add one to the count for each score
        count++;
    }
    while (score != 0);
    
    // now the size of the array is defined by how many times the user has typed.
    int scores[count];

    for (int i = 0; i < count; i++)
    {
        // how do I add each score to the array???
    }
}

My problem is how to add each score that the user types to the array. In advance thanks!!!

Answer 1

You need a "dynamic data structure" that can expand as required. These are two ways:

1. Allocate an array of initial size with malloc and realloc when there is not enough room.

2. Use a linked list

(there are more ways, but these are quite common for this problem)

Answer 2

You need to have data structure which will keep track of the size and stores the data.

Here you have a simple implementation:

typedef struct
{
    size_t size;
    int result[];
}SCORES_t;

SCORES_t *addScore(SCORES_t *scores, int score)
{
    size_t newsize = scores ? scores -> size + 1 : 1;
    scores = realloc(scores, newsize * sizeof(scores -> result[0]) + sizeof(*scores));
    if(scores)
    {
        scores -> size = newsize;
        scores -> result[scores -> size - 1] = score;
    }
    return scores;
}

double getAverage(const SCORES_t *scores)
{
    double average = 0;
    if(scores)
    {
        for(size_t index = 0; index < scores -> size; average += scores -> result[index], index++);
        average /= scores -> size;
    }
    return average;
}

int main(void)
{
    int x;
    SCORES_t *scores = NULL;

    while(scanf("%d", &x) == 1 && x >= 0)
    {
        SCORES_t *temp = addScore(scores, x);
        if(temp)
        {
            scores = temp;
        }
        else
        {
            printf("Memery allocation error\n");
            free(scores);
        }
    }
    if(scores) printf("Number of results: %zu Average %f\n", scores -> size, getAverage(scores));
    free(scores);
}

https://godbolt.org/z/5oPesn

Answer 3

regarding:

int main(void)
{
    int score;
    // start count for size of array
    int count = - 1;

    do
    {
        score = get_int("Score: ");
        // add one to the count for each score
        count++;
    }
    while (score != 0);
    
    // now the size of the array is defined by how many times the user has typed.
    int scores[count];

    for (int i = 0; i < count; i++)
    {
        // how do I add each score to the array???
    }
}

this does not compile and contains several logic errors

it is missing the statements: #include <cs50.h> and #include <stdio.h>

regarding:

    int score;
    // start count for size of array
    int count = - 1;

    do
    {
        score = get_int("Score: ");
        // add one to the count for each score
        count++;
    }
    while (score != 0);

This only defines a single variable: score and each time through the loop that single variable is overlayed. Also, the first time through the loop, the counter: count will be incremented to 0, not 1

on each following time through the loop, the variable score will be overlayed (I.E. all prior values entered by the user will be lost)

suggest using dynamic memory. Note: to use dynamic memory, will need the header file: stdlib.h for the prototypes: malloc() and free(). Suggest:

#include <cs50.h>
#include <stdio.h>
#include <stdlib.h>
    
int main( void )
{
    // pointer to array of scores
    int * score = NULL;

    // start count for size of array
    int count = 0;

    while( 1 )
    {
        int score = get_int("Score: ");

        if( score != 0 )
        {  // then user has entered another score to put into array
            count++;
            int * temp = realloc( scores, count * sizeof( int ) )
            if( ! temp )
            { // realloc failed
                // output error info to `stderr`
                // note: `perror()` from `stdio.h`
                perror( "realloc failed" );

                // cleanup
                free( scores );

                // `exit()` and `EXIT_FAILURE` from `stdlib.h`
                exit( EXIT_FAILURE );
            }

            // implied else, 'realloc()' successful, so update the target pointer
            scores = temp;

            // insert the new score into the array
            scores[ count ] = score;
        }

        else
        { // user entered 0 so exit the loop
            break;
        }
    }

note: before exiting the program, pass scores to free() so no memory leak.