Peaks

Part of the Fundamentals section of Coddy's C journey — lesson 62 of 63.

Challenge

You’re given a 2D grid of integer elevations representing a terrain map. A cell is called a peak if its value is strictly greater than all of its adjacent neighbors (up to 8: north, south, east, west, and the four diagonals).

Rules

8 Neighbors: You must check the north, south, east, west, and all four diagonals.
Boundary Cells: If a cell is on the edge or corner, it has fewer than 8 neighbors. It is still a peak if it is strictly greater than all of its existing neighbors.
Strictly Greater: If a neighbor has the same elevation as the current cell, the current cell is not a peak.

Input

Two integers N and M representing rows and columns.
An N*M grid of integers

Output

An integer representing the total number of peaks.
The coordinates (row and column, 0‑indexed) of each peak, sorted first by row then by column.

For example for this input:

This is the output:

Explanation The peaks occur at the following locations:

(0,4) with elevation 4: Greater than neighbors (3, 1, 1).
(1,0) with elevation 5: Greater than neighbors (1, 2, 1, 1, 1).
(2,2) with elevation 9: Greater than all 8 surrounding 1s.
(4,1) with elevation 3: Greater than neighbors (2, 1, 1, 1, 1).
(4,3) with elevation 4: Greater than neighbors (1, 1, 1, 2, 1).

Try it yourself

#include <stdio.h>

#define MAXN 100
#define MAXM 100

int N, M;
int grid[MAXN][MAXM];

/**
 * Returns 1 if the cell at (r, c) is a peak, 0 otherwise.
 * A peak must be strictly greater than all existing neighbors.
 */
int is_peak(int r, int c) {
    // TODO: Define the relative positions of the 8 neighbors
    // (North, South, East, West, and the 4 diagonals).
    
    // TODO: Loop through each neighbor.
    // CRITICAL HINT: Before checking grid[nr][nc], you MUST ensure 
    // that nr and nc are within the bounds (0 to N-1 and 0 to M-1).
    
    return 1; // Placeholder
}

int main() {
    // Read dimensions
    if (scanf("%d %d", &N, &M) != 2) {
        return 1;
    }

    // TODO: Read the grid values from stdin using nested loops.
    
    // TODO: Iterate over every cell (i, j) in the grid.
    // If is_peak(i, j) is true, store the coordinates.
    // You can use two arrays like peak_r[MAXN*MAXM] and peak_c[MAXN*MAXM] 
    // to store the results before printing.

    // TODO: Print the total count of peaks.
    
    // TODO: Print each stored (row, col) pair. 
    // Since you iterate by row then column, they will already be sorted!

    return 0;
}

All lessons in Fundamentals

1Introduction

C Language Hello World!Comments Basic Program Structure

4Control Flow

If Statement If - Else Else-If Switch Case Ternary Conditional Operator Recap Challenge Nested If - Else

7Functions

Declare a Function Return Types Parameters Recap Challenge #1 Recursion Basics Function Prototypes Recap Challenge #2