Several issues right away:

• The array is 7x7, you want it to be int my_data[8][8];
• Instead of using the number for the size of the array over and over, define a constant #define FOO 8
• The loops are not properly bounded, should be for (x=0;x<8;x++). (again, see my note on using a defined constant for the size)
• You are not saving the value to the array, you are saving it to the iterator variable.
• With two separate for loops you will not be able to fill the entire table, reconsider this structure because you will likely have to use nested loops.

scanf takes the address of the variable that is being read and returns the number of items read. It does not return the value read.

Replace

int i = scanf("%d",&i);
int y = scanf("%d",&y);

by

scanf("%d",&i);
scanf("%d",&y);

and

int r[a][b] = scanf("%d",&a,&b);

by

scanf("%d",&r[a][b]);

EDIT:

You are using variable length array (VLA) in your program:

int r[i][y];

as i and y are not constants and are variables. VLA are a C99 standard feature.

You need 2 nested loops, to traverse through rows and columns. The content of the field is the minimum of the control variables and and the minimum of the difference of a control variable and the size of the array dimension, incremented by 1.

N = 5

0: min(0, N-0-1) + 1 = 1
1: min(1, N-1-1) + 1 = 2
2: min(2, N-2-1) + 1 = 3
3: min(3, N-3-1) + 1 = 2
4: min(4, N-4-1) + 1 = 1

#include <stdio.h>

#define N 5

#define MIN(a,b) (((a)<(b))?(a):(b))

int main()
{
    int a[N][N];
    for ( int i = 0; i < N; ++ i )
    {
        for ( int j = 0; j < N; ++ j)
        {
            int minI = MIN(i, N-i-1);
            int minJ = MIN(j, N-j-1);
            a[i][j] = MIN(minI, minJ) + 1; 
            printf("%d ", a[i][j]);
        }
        printf("\n");
    }
    return 0;
}

Output:

1 1 1 1 1 
1 2 2 2 1 
1 2 3 2 1 
1 2 2 2 1 
1 1 1 1 1

See the live example

As for the code structure, given that the entire 2D array is only local to print_tables and the only purpose is to print the array, you might just as well remove the array altogether and print the values directly instead of storing them temporarily only to re-iterate over the array printing them.

I'd also handle the “header” row and column separately from the data rows; this simplifies the code by removing the ifs, so it's easier to track what's going on. For example:

#include <stdio.h>
#include <math.h>

const static float MAX_ELAS = 1.0;

void print_tables(unsigned minDimples, unsigned maxDimples, float minElasticity) {
    const unsigned elasticityLength = (unsigned) ceilf(MAX_ELAS / minElasticity);
    float elasticity = minElasticity;

    // Header row
    (void) printf(" "); // Empty (0,0) cell
    for (unsigned col = 0; col < elasticityLength; ++col) {
        (void) printf("\t%.02f", elasticity);
        elasticity += minElasticity;
    }

    for (unsigned dimples = minDimples; dimples <= maxDimples; ++dimples) {
        float dimpleMultiplier = 120.0f - dimples;
        dimpleMultiplier = 800.0f - (dimpleMultiplier * dimpleMultiplier);
        elasticity = minElasticity;

        (void) printf("\n%u", dimples); // Header column
        for (unsigned col = 0; col < elasticityLength; ++col) {
            float distance = elasticity * dimpleMultiplier;
            (void) printf("\t%.1f", distance);
            elasticity += minElasticity;
        }
    }
    (void) printf("\n"); // Terminate last row
}

int main (void) {
    // TODO: Read input instead of hard-coded values (or use command-line?)
    print_tables(121, 125, 0.14);
    return 0;
}

Outputs:

         0.14    0.28    0.42    0.56    0.70    0.84    0.98    1.12
121     111.9   223.7   335.6   447.4   559.3   671.2   783.0   894.9
122     111.4   222.9   334.3   445.8   557.2   668.6   780.1   891.5
123     110.7   221.5   332.2   443.0   553.7   664.4   775.2   885.9
124     109.8   219.5   329.3   439.0   548.8   658.6   768.3   878.1
125     108.5   217.0   325.5   434.0   542.5   651.0   759.5   868.0

As a suggestion for further cleaning up, perhaps specify elasticityStep which is the increment of elasticity between columns, and separate minElasticity, maxElasticity instead of elasticityLength. Then the column loops would be like:

for (elasticity = minElasticity; elasticity <= maxElasticity; elasticity += step)

First, you will need to allocate memory to this 2d array. For example, you getting the user input with variable "a" and "b" so you could declare the the array after the user input like this

   int a, b, c, d;

   printf("Number of rows and cumluns?\n");
   scanf("%d%d", &a, &b);
   int mat[a][b];

After you done allocate the memory you needed for the array, you will need to allocate the value you want in each position ( depends on c & d variables ), for example I used d variable values and assigned them to the 2nd array:

for (c = 0; c <= a; c++)
  for (d = 0; d <= b; d++)
     mat[c][d]=d;

Then you will probably want to print the array so, same For loops, just with print

for (c = 0; c < a; c++){
  for (d = 0; d < b; d++)
     printf("%d ",mat[c][d]);
printf("\n");  

Output if a = 2 and b = 4

Also in your original code, you forgot a ";" after the mat[c][d] aswell.

Hopefully this will answer your question.

Just declare your function as:

void fill_matrix(char matrix[MATRIX_SIZE][MATRIX_SIZE], char c);

and then call it like this

fill_matrix(matrix, ' ');