C Program to Print Matrix Boundary Elements
C Program to Print Matrix Boundary Elements
A small school table has two rows and two columns. The matrix boundary operation should preserve what each row and column means, rather than treating the table as four unrelated values.
A matrix boundary contains entries in the first or last row or first or last column; emit each selected position once.
Before this exercise, review array indexes and bounds and nested loops. The worked program below applies those ideas to this specific problem.
Explain
Explain why matrix boundary uses this sequence of operations.
Trace
Trace the sample values and predict the printed result.
Apply
Apply the boundary rule to a three-by-three table and exclude its center.
Matrix Boundary: follow the data
Given data
A small school table has two rows and two columns. The matrix boundary operation should preserve what each row and column means, rather than treating the table as four unrelated values.
Rule
A matrix boundary contains entries in the first or last row or first or last column; emit each selected position once.
Observed result
1 2 3 4
Understand the operation
An or-condition identifies membership. Printing the entire top row and then both side columns separately can duplicate corner entries unless those passes exclude overlaps.
The example fixes both dimensions at two so every index can be checked directly. A general matrix interface must carry dimensions, require compatible shapes for its operation and validate storage for each referenced element.
Read the complete C program
The input values are written into this complete C17 example so the calculation can be reproduced without an unavailable service. Change the named data, rebuild and compare the actual output with your prediction.
C17 · complete program
#include <stdio.h>
int main(void) {
int matrix[2][2] = {{1, 2}, {3, 4}};
for (int row = 0; row < 2; row++) {
for (int col = 0; col < 2; col++) { if (row == 0 || row == 1 || col == 0 || col == 1) printf("%d ", matrix[row][col]); }
}
putchar('\n');
return 0;
}
Expected output
1 2 3 4
Follow the values through the code
C17 · code used in this explanation
#include <stdio.h>
int main(void) {
int matrix[2][2] = {{1, 2}, {3, 4}};
for (int row = 0; row < 2; row++) {
for (int col = 0; col < 2; col++) { if (row == 0 || row == 1 || col == 0 || col == 1) printf("%d ", matrix[row][col]); }
}
putchar('\n');
return 0;
}
Every position in a two-by-two table is on the boundary. The larger worked table has one interior entry, five.
Read the resulting rows in order; moving a value to another row or column is part of the operation’s meaning, not merely an output-format choice.
Follow the worked calculation
Start with the sample
Every position in a two-by-two table is on the boundary. The larger worked table has one interior entry, five.
Apply the rule
Read the resulting rows in order; moving a value to another row or column is part of the operation’s meaning, not merely an output-format choice.
Check the result
1 2 3 4
1 2 3 4
Try a small variation
C17 · code used in this explanation
#include <stdio.h>
int main(void) {
int matrix[2][2] = {{1, 2}, {3, 4}};
for (int row = 0; row < 2; row++) {
for (int col = 0; col < 2; col++) { if (row == 0 || row == 1 || col == 0 || col == 1) printf("%d ", matrix[row][col]); }
}
putchar('\n');
return 0;
}
Apply the boundary rule to a three-by-three table and exclude its center.
Worked answer
The complete worked answer uses the same valid two-by-two input. Trace its index rule independently; compare each printed result with the source element or accumulation that produced it.
C17 · complete program
#include <stdio.h>
int main(void) {
int larger[3][3] = {{1,2,3},{4,5,6},{7,8,9}};
for (int row = 0; row < 3; row++) for (int col = 0; col < 3; col++) if (row == 0 || row == 2 || col == 0 || col == 2) printf("%d ", larger[row][col]);
putchar('\n');
return 0;
}
Expected output
1 2 3 4 6 7 8 9
Recognize the important boundaries
Define the desired traversal order. This example uses row-major selection, not clockwise perimeter order.
Larger or untrusted dimensions require checked storage-size calculations. Integer products and accumulated sums also need a representable-range contract.
Check your understanding
Choose the answer, then compare the reasoning for every option.
Use the idea independently
A matrix boundary contains entries in the first or last row or first or last column; emit each selected position once.
Continue with array boundaries.
Continue with function bounds contracts.
Language reference: WG14 C11 committee draft. These examples use the C17-compatible subset of the explained language features.
Reaching the recap and completing the quizzes are separate milestones.