How to read in 2d array from function in main - c

Question

Alright so I'm sure this is pretty simple and all but I have no idea how to use functions. Up until now I was able to get by with everything in main but now I'm required to use functions for just about anything I do. So from my code below, how do I read in (or w.e. the proper terminology is) a function from main?
EDIT: To clarify to everyone, my question is how can I access the array I returned in main?
Code below takes in the test scores from different amount of students specified by user input.
#include <stdio.h>
#include <stdlib.h>
int** getTestData();
int main (){
///this is where I'm lost..
int (*a)[];
a = getTestData();
}
int** getTestData(){
int students, numberOfTests, testScores, i, j;
int** testScoreBank;
// reads in studens
scanf("%i", &students);
testScoreBank = (int**) malloc (sizeof(int)*students);
for(i=0; i<students;i++){
//how many number of tests there are
scanf("%i", &numberOfTests);
testScoreBank = (int*) malloc (sizeof(int)*numberOfTests);
for(j=0; j<numberOfTests; j++){
//the tests themselves
scanf("%i", &testScores);
testScoreBank[i][j] = testScores;
}
}
return testScoreBank;
}

Answer 1

Ok, here is an example with global variables, how to fill your array inside a function, and access it in main()
#include <stdio.h>
#include <stdlib.h>
int** getTestData();
int numberOfStudents;
int* studentTestSizes;
int main (){
int** testScoresBank = getTestData();
int i, j;
for (i = 0; i < numberOfStudents; i++) {
for (j = 0; j < studentTestSizes[i]; j++) {
printf("%d", testScoresBank[i][j]);
}
}
return 0;
}
int** getTestData() {
int** testScoreBank;
// reads in studens
scanf("%d", &numberOfStudents);
testScoreBank = malloc(sizeof(int*) * numberOfStudents);
studentTestSizes = malloc(sizeof(int) * numberOfStudents);
int i;
for (i = 0; i < numberOfStudents; i++) {
//how many number of tests there are
scanf("%d", studentTestSizes + i);
testScoreBank[i] = malloc(sizeof(int) * studentTestSizes[i]);
int j;
for (j = 0; j < studentTestSizes[i]; j++) {
//the tests themselves
int testScore;
scanf("%d", &testScore);
testScoreBank[i][j] = testScore;
}
}
return testScoreBank;
}
alternative for global variables is to make global variables local and pass pointers to them to getTestData function, example is here:
#include <stdio.h>
#include <stdlib.h>
int** getTestData();
int main (){
int numberOfStudents; // those variables are now here
int* studentTestSizes;
int** testScoresBank = getTestData(&numberOfStudents, &studentTestSizes); // passing the pointers so we can change values that are pointed to
int i, j;
for (i = 0; i < numberOfStudents; i++) {
for (j = 0; j < studentTestSizes[i]; j++) {
printf("%d", testScoresBank[i][j]);
}
}
return 0;
}
int** getTestData(int* numberOfStudentsPtr, int** studentTestSizesPtr) {
int** testScoreBank;
// reads in studens
scanf("%d", numberOfStudentsPtr); // it's already a pointer so we must omit &
int numberOfStudents = *numberOfStudentsPtr; // will be constant from now on
testScoreBank = malloc(sizeof(int*) * numberOfStudents);
*studentTestSizesPtr = malloc(sizeof(int) * numberOfStudents);
int* studentTestSizes = *studentTestSizesPtr;
int i;
for (i = 0; i < numberOfStudents; i++) {
//how many number of tests there are
scanf("%d", studentTestSizes + i);
testScoreBank[i] = malloc(sizeof(int) * studentTestSizes[i]);
int j;
for (j = 0; j < studentTestSizes[i]; j++) {
//the tests themselves
int testScore;
scanf("%d", &testScore);
testScoreBank[i][j] = testScore;
}
}
return testScoreBank;
}

Answer 2

In addition to the other answers, you have a number of subtle issues to consider. Two are just general helpful tips for writing/debugging your beginning applications (1) if you are taking user input, prompt for it - you can strip the prompts later, but it is much easier to enter data in response to an informative prompt than it is to wonder what the blinking cursor is doing down there -- did to program freeze?; (2) provide adequate spacing in your code - you can always delete blank lines later, but separating your code into functional blocks of logic will help you keep your logic straight.
When allocating space for numeric arrays, it is good practice to initialize all values to 0 as part of (or after) allocation. This will absolutely prevent the inadvertent read from uninitialized space. You can allocate and initialize all at once by using calloc instead of malloc. This also provides benefits when allocating arrays of pointers to char* as well.
While you may have fixed your allocation, you still have the glaring problem of knowing "How many students and tests do I have?". Here is where passing an additional pointer for the number of students and storing the number of tests in an additional array index is required. (there are other ways to do it, this is just efficient). You can declare the the number of students in main passing its pointer to getTestData. Updates to the value of students in getTestData are then available in main. But, "What if the students have a different number of scores? What then?" You are already filling an array of integers, if you just store the number of tests for the student as the first integer, you make that value available no matter where you pass the array.
Finally, you need to pay closer attention to your choice of variable types. For indexes and lengths that can never be negative, unsigned or size_t is a better choice and will allow the compiler to point out instances where you may be using that value incorrectly.
You need to validate each time you allocate memory with malloc or calloc, and you are responsible to keeping track of the address to the start of each allocation and freeing it when it is no longer needed.
That said, here is one approach to making it all work. Try it. Let me know if you have any questions:
#include <stdio.h>
#include <stdlib.h>
int **getTestData (size_t *students);
int main (void) {
int **a = NULL;
int j;
size_t i, s = 0;
if (!(a = getTestData (&s))) {
fprintf (stderr, "error: getTestData failed to return student data.\n");
return 1;
}
/* print student data */
putchar ('\n');
for (i = 0; i < s; i++)
{
printf (" Student[%2zu] scores : ", i+1);
/* adjust indexes to read no. of tests */
for (j = 1; j < a[i][0]; j++)
printf (" %3d", a[i][j]);
putchar ('\n');
}
putchar ('\n');
/* free allocated memory */
for (i = 0; i < s; i++)
free (a[i]);
free (a);
return 0;
}
int **getTestData (size_t *students)
{
size_t tests, testScores, s, t;
int **testScoreBank;
/* reads in students */
printf ("\n No. of students: ");
scanf ("%zu", students);
if (!(testScoreBank = calloc (*students, sizeof *testScoreBank))) {
fprintf (stderr, "%s() error: virtual memory exhausted.\n", __func__);
return NULL;
}
for (s = 0; s < *students; s++)
{
/* how many number of tests there are */
printf ("\n No. of scores for student[%2zu]: ", s+1);
scanf ("%zu", &tests);
tests += 1; /* allow space for number of tests as [s][0] */
testScoreBank[s] = calloc (tests, sizeof **testScoreBank);
testScoreBank[s][0] = tests;
for (t = 1; t < tests; t++)
{
/* the tests themselves */
printf (" student[%2zu]-test[%2zu] score: ", s+1, t);
scanf ("%zu", &testScores);
testScoreBank[s][t] = testScores;
}
}
return testScoreBank;
}
Use/Output
$ ./bin/testdata
No. of students: 2
No. of scores for student[ 1]: 3
student[ 1]-test[ 1] score: 88
student[ 1]-test[ 2] score: 91
student[ 1]-test[ 3] score: 82
No. of scores for student[ 2]: 4
student[ 2]-test[ 1] score: 93
student[ 2]-test[ 2] score: 95
student[ 2]-test[ 3] score: 96
student[ 2]-test[ 4] score: 91
Student[ 1] scores : 88 91 82
Student[ 2] scores : 93 95 96 91

Answer 3

(I assume this is just a snippet of your overall code and there's more to it or else you might have other problems)
Anyway you should be able to set up your 2D array with the following modifications:
scanf("%i", &students);
testScoreBank = malloc (sizeof(int*)*students); //sizeof int* instead of sizeof int.
for(i=0; i<students;i++)
{
scanf("%i", &numberOfTests);
*(testScoreBank + i)= malloc (sizeof(int)*numberOfTests); //You forgot the * operator on testScoreBank and to iterate through it.
for(j=0; j<numberOfTests; j++)
{
scanf("%i", &testScores);
testScoreBank[i][j] = testScores;
}
}