Quick Revision Table

Part 1: Multiple-Choice Quiz

Assume a 4-byte int. Open each answer after you have decided.

Question 1

For int a[2][3] = {1, 2, 3, 4}; what is the value of a[1][1]?

1. 4
2. 0
3. 2
4. Garbage value

Question 2

Which declaration is invalid in C?

1. int a[][3] = {{1,2,3},{4,5,6}};
2. int a[2][3] = {0};
3. int a[3][] = {{1,2},{3,4},{5,6}};
4. int a[2][2] = {{1},{2}};

Question 3

What is sizeof(int[4][5]) on a platform where int is 4 bytes?

1. 20
2. 40
3. 80
4. 9

Question 4

Matrix A is 3 x 2 and matrix B is 2 x 4. What is the size of A x B?

1. 2 x 2
2. 3 x 4
3. 4 x 3
4. The product is not defined

Question 5

How many multiplications does the standard triple loop perform for the product in Question 4?

1. 12
2. 14
3. 24
4. 6

Question 6

In an in-place transpose of a square matrix, what happens if the inner loop starts at j = 0 instead of j = i + 1?

1. The matrix is transposed twice as fast
2. The matrix ends unchanged, because each pair is swapped twice
3. The diagonal is lost
4. Compile error

Question 7

For int a[2][3][4], at what flat offset does a[1][2][1] sit?

1. 19
2. 20
3. 21
4. 23

Question 8

Which statement is true about an uninitialized local 2D array?

1. All elements are 0
2. All elements are -1
3. Elements have indeterminate values
4. The compiler refuses to compile it

Question 9

Which expression is element-wise multiplication, not matrix multiplication?

1. c[i][j] += a[i][k] * b[k][j];
2. c[i][j] = a[i][j] * b[i][j];
3. c[i][j] = a[i][k] + b[k][j];
4. c[j][i] = a[i][j];

Question 10

Why should a large 3D array in an MCU program usually be static or global?

1. Local arrays cannot be 3D
2. Large locals can overflow the small stack
3. Static arrays are faster to write
4. Global arrays cannot be overwritten

Part 2: Predict the Output

Problem 1

int m[2][3] = { {1, 2, 3}, {4, 5, 6} };
printf("%d %d\n", m[1][2], *(*(m + 1) + 1));

Problem 2

int d[3][3] = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} };
int t = 0;
for (int i = 0; i < 3; i++)
    t += d[i][i];
printf("%d\n", t);

Problem 3

int t[4][5];
printf("%zu %zu\n", sizeof(t) / sizeof(t[0]),
                    sizeof(t[0]) / sizeof(t[0][0]));

Problem 4

int m[2][3] = { {1, 2, 3}, {4, 5, 6} };
int row[2] = {0}, col[3] = {0};
for (int i = 0; i < 2; i++)
    for (int j = 0; j < 3; j++) {
        row[i] += m[i][j];
        col[j] += m[i][j];
    }
printf("%d %d | %d %d %d\n", row[0], row[1], col[0], col[1], col[2]);

Part 3: Programming Challenges

Worked Solution: Symmetric Matrix Check

A square matrix is symmetric when m[i][j] equals m[j][i] for every pair. We only need to test the cells above the diagonal. The function below uses a C99 variable-length array parameter, which lets the function accept any square size, and it returns 1 or 0.

/* symmetric.c */
#include <stdio.h>

static int is_symmetric(int n, int m[n][n])
{
    for (int i = 0; i < n; i++)
        for (int j = i + 1; j < n; j++)
            if (m[i][j] != m[j][i])
                return 0;
    return 1;
}

int main(void)
{
    int s1[3][3] = { {1, 7, 3}, {7, 4, -5}, {3, -5, 6} };
    int s2[3][3] = { {1, 7, 3}, {2, 4, -5}, {3, -5, 6} };
    printf("symmetric s1=%d s2=%d\n", is_symmetric(3, s1), is_symmetric(3, s2));
    return 0;
}

$ gcc -std=c11 -Wall -Wextra -o symmetric symmetric.c
$ ./symmetric
symmetric s1=1 s2=0

Try the other three challenges yourself before looking anything up. A good self-check: run them on a matrix whose answer you can compute by hand.

Part 4: Interview Questions on Arrays

These come up in embedded C programming interviews, from fresher campus rounds to firmware engineer screens. Try answering aloud in two or three sentences first.

Common Mistakes Checklist

Summary and Key Takeaways

• A 2D array is a contiguous block in row-major order, and the column count is the row stride.
• Matrix addition is cell by cell, multiplication is row times column, and transpose swaps the indices.
• 3D arrays add a third index with offset i*D2*D3 + j*D3 + k.
• Interviewers favour questions on layout, initialization, passing arrays to functions and bounds safety.

Conclusion

If you could answer most of the questions above without peeking, you have a solid base in multi-dimensional arrays and a good start for any embedded C programming interview. Rewrite the programs from this chapter from memory, change the sizes, and watch what breaks; that habit teaches more than any amount of reading. Keep going through the free embedded C course, because every later chapter builds on the array skills you practiced here.

Frequently Asked Questions