Passing a two dimensional array by reference in C - c

Question

I need help with a homework problem.
The description of the homework is:
Design a program in C, using the method of call by reference, which
generates statistics on the grades in a class consisting of M students
and N quizzes given during the course of a semester. The quizzes all
carry equal weighting, with the grade in each quiz being out of 10
points. The program is to do the following in the indicated sequence
when the program is executed:
Ask the user to type on the keyboard the number of number of students M and the number of quizzes N, with M representing the number
of rows and N representing the number of columns in a two-dimensional
array x[M][N] of floating-point numbers.
Ask the user to type on the keyboard the values of all the elements of the array x[M][N].
Display on the console the M rows and N columns of inputted array element values.
Generate and display on the console the elements of a one-dimensional array p[M] where each element of this array p[M]
represents the average grade over the semester of each of the M
students.
Generate and display on the console the elements of a one-dimensional array q[N] where each element of q[N] represents the
average grade in the class in each of the N quizzes .
Generate and display on the console the average grade z for the entire class over the entire semester.
This is the code that I've done so far:
#include <stdio.h>
#define students 300
#define quizzes 20
void averageGradeOfStudents(int M, int N, float p[], float *xPtr);
float averageGradeOfClass();
float averageGradeOfWholeClass();
int main() {
int M, N;
float x[students][quizzes];
printf("Enter the number students (Maximum: 300): ");
scanf_s("%d", &M);
printf("\nEnter the number of Quizzes (Maximum: 20): ");
scanf_s("%d", &N);
for (int i = 0; i < M; i++) {
for (int k = 0; k < N; k++) {
printf("Enter the grade for student %d, Quiz %d: ", i, k);
scanf_s("%f", &x[i][k]);
}
}
printf("\n--------------------------------------------------------------------------------\n\n");
for (int k = 1; k <= N; k++) {
printf("\t\tQuiz%d", k);
}
for (int i = 0; i < M; i++) {
printf("\nStudent%d Grades: ", i);
for (int k = 0; k < N; k++) {
printf("%.2f\t\t", x[i][k]);
}
}
printf("\n\n\n--------------------------------------------------------------------------------\n\n");
float p[students];
averageGradeOfStudents(M, N, p, &x[0][0]);
float q[quizzes];
float z;
getchar();
return 0;
}
void averageGradeOfStudents(int M, int N, float p[], float *xPtr) {
float grade[students];
for (int i = 0; i < M; i++) {
for (int k = 0; k < N; k++) {
grade[i] += *(xPtr + i);
}
}
for (int k = 0; k < M; k++) {
p[k] = grade[k] / N;
printf("The average grade of student%d is: %.2f\n\n", k, p[k]);
}
}
float averageGradeOfClass() {
}
float averageGradeOfWholeClass() {
}
The problem I'm having is that I can't figure out a way to sum each row of the array individually.

Answer 1

Very good progress on your assignment. It is refreshing to get full code and effort. There are a couple of things that will help before we get to an average. First, and always good practice, initialize your variables. This is especially true with arrays as it prevents an accidental attempt to read from an uninitialized value, and in the case of character arrays can provide automatic null-termination for the first copy to the array (as long as you only copy size-1 chars).
int M = 0, N = 0;
float x[students][quizzes] = {{ 0 }};
float p[students] = {0};
float q[quizzes] = {0};
float z = 0;
Passing arrays is always a problem area. When passing an array declared as a 2D array, you must pass the width of the array as part of the function argument:
void averageGradeOfStudents (int M, int N, float p[], float xPtr[][quizzes]);
and then you can simply call the function by passing the array itself.
averageGradeOfStudents (M, N, p, x);
(note: the first level of indirection of any array is converted to a pointer when passed as a function argument, but that is a topic for a later question. However, for your reference, your function declaration could also be properly written as):
void averageGradeOfStudents (int M, int N, float p[], float (*xPtr)[quizzes]);
Now on to your average, you were close, you only needed to sum the elements per-student by changing the way you were indexing xPtr (and initialize the values of grade):
void averageGradeOfStudents (int M, int N, float p[], float xPtr[][quizzes])
{
float grade[M];
/* initialize variable length array to 0 */
for (int i = 0; i < M; i++) grade[i] = 0;
/* sum of grades for each student */
for (int i = 0; i < M; i++) {
for (int k = 0; k < N; k++) {
grade[i] += xPtr[i][k];
}
}
printf ("The average grades for the students are:\n\n");
for (int k = 0; k < M; k++) {
p[k] = grade[k] / N;
printf(" student[%3d] is: %6.2f\n", k, p[k]);
}
}
A quick example of its use in the full-code could be:
#include <stdio.h>
#define students 300
#define quizzes 20
void averageGradeOfStudents (int M, int N, float p[], float (*xPtr)[quizzes]);
int main (void) {
/* good habit -- always initialize your variables */
int M = 0, N = 0;
float x[students][quizzes] = {{ 0 }};
float p[students] = {0};
printf("Enter the number students (Maximum: 300): ");
scanf ("%d", &M);
printf("\nEnter the number of Quizzes (Maximum: 20): ");
scanf ("%d", &N);
for (int i = 0; i < M; i++) {
for (int k = 0; k < N; k++) {
printf("Enter the grade for student %d, Quiz %d: ", i, k);
scanf("%f", &x[i][k]);
}
}
printf("\n--------------------------------------------------------------------------------\n\n");
printf (" ");
for (int k = 1; k <= N; k++) {
printf(" Quiz%-2d", k);
}
putchar ('\n');
for (int i = 0; i < M; i++) {
printf("\n Student[%3d] Grades: ", i);
for (int k = 0; k < N; k++) {
printf(" %6.2f", x[i][k]);
}
}
printf("\n\n--------------------------------------------------------------------------------\n\n");
averageGradeOfStudents (M, N, p, x);
/* getchar(); */
return 0;
}
void averageGradeOfStudents (int M, int N, float p[], float (*xPtr)[quizzes])
{
float grade[M];
/* initialize variable length array to 0 */
for (int i = 0; i < M; i++) grade[i] = 0;
/* sum of grades for each student */
for (int i = 0; i < M; i++) {
for (int k = 0; k < N; k++) {
grade[i] += xPtr[i][k];
}
}
printf ("The average grades for the students are:\n\n");
for (int k = 0; k < M; k++) {
p[k] = grade[k] / N;
printf(" student[%3d] is: %6.2f\n", k, p[k]);
}
putchar ('\n');
}
Use/Output
$ ./bin/studentavg
Enter the number students (Maximum: 300): 3
Enter the number of Quizzes (Maximum: 20): 3
Enter the grade for student 0, Quiz 0: 8
Enter the grade for student 0, Quiz 1: 9
Enter the grade for student 0, Quiz 2: 8
Enter the grade for student 1, Quiz 0: 9
Enter the grade for student 1, Quiz 1: 9
Enter the grade for student 1, Quiz 2: 10
Enter the grade for student 2, Quiz 0: 7
Enter the grade for student 2, Quiz 1: 8
Enter the grade for student 2, Quiz 2: 9
--------------------------------------------------------------------------------
Quiz1 Quiz2 Quiz3
Student[ 0] Grades: 8.00 9.00 8.00
Student[ 1] Grades: 9.00 9.00 10.00
Student[ 2] Grades: 7.00 8.00 9.00
--------------------------------------------------------------------------------
The average grades for the students are:
student[ 0] is: 8.33
student[ 1] is: 9.33
student[ 2] is: 8.00
Note: I've changed scanf_s to scanf for my system, you will need to change it back. Let me know if you have further questions.

Answer 2

I would not declare the array until you know the values of M and N. So the first part of main would be as shown below. (This also allows you to dump the students and quizzes constants.) And always check the return values from library functions.
int main( void )
{
int M, N;
printf("Enter the number students (Maximum: 300): ");
if ( scanf_s("%d", &M) != 1 || M > 300 ) {
printf( "Whatever\n" );
exit( 1 );
}
printf("\nEnter the number of Quizzes (Maximum: 20): ");
if ( scanf_s("%d", &N) != 1 || N > 20 ) {
printf( "Sigh...\n" );
exit( 1 );
}
float x[M][N];
// ...
}
When passing an array to a function, you can use array notation. And you're allowed to omit the first dimension of the array. So the following two function declarations are both valid
void averageGradeOfStudents(int M, int N, float p[M], float x[M][N]);
void averageGradeOfStudents(int M, int N, float p[], float x[][N]);
Inside the function, there are some problems
grade[i] is not initialized before being used (this should have resulted in a compiler warning)
Each grade[i] should be the averages of N grades (indexed by k not i)
You don't really need the grade array since you already have the p array
Here's how I would write the function (I changed the index names to keep my rows and columns clear)
void averageGradeOfStudents(int M, int N, float p[], float x[][N])
{
for (int row = 0; row < M; row++) {
p[row] = 0; // initialize the sum to zero
for (int col = 0; col < N; col++) {
p[row] += x[row][col]; // update the sum with each test result
}
p[row] /= N; // convert the sum to the average
}
for (int row = 0; row < M; row++) {
printf("The average grade of student%d is: %.2f\n\n", row, p[row]);
}
}
And the function should be called with
averageGradeOfStudents(M, N, p, x);

Answer 3

Summing an array is a fairly common idiom:
double sumArray(double arr[], int arrLen)
{
int i;
double sum = 0.0;
for (i = 0; i < arrLen; i++)
sum += arr[i];
return sum;
}
This sums a one-dimension array, but keep in mind that each row of your array x is a one-dimension array.

Answer 4

int avg=0;
int sum=0;
for (int i = 0; i < M; i++) {
for (int k = 0; k < N; k++) {
printf("Enter the grade for student %d, Quiz %d: ", i, k);
scanf("%f", &x[i][k]);
sum=sum+x[i][k]
}
}avg=sum/M;
as to get avg you have to sum...try this may help you