Selection Sort for Integer Arrays

This implementation sorts a sequence of $N$ integers in descending order using the selection sort algorithm. The program identifies largest remaining element in each iteration and swaps it into its correct position.

#include <stdio.h>

int main() {
    int count, list[10];
    if (scanf("%d", &count) != 1) return 0;
    for (int i = 0; i < count; i++) {
        scanf("%d", &list[i]);
    }
    for (int i = 0; i < count - 1; i++) {
        int pivot = i;
        for (int j = i + 1; j < count; j++) {
            if (list[j] > list[pivot]) {
                pivot = j;
            }
        }
        int temp = list[i];
        list[i] = list[pivot];
        list[pivot] = temp;
    }
    for (int i = 0; i < count; i++) {
        printf("%d%s", list[i], (i == count - 1) ? "" : " ");
    }
    return 0;
}

Frequency Analysis of Individual Digits

This logic processes a set of integers to determine which digit occurs most frequently across all input numbers. It outputs the highest frequency followed by the digits that achieved that frequency.

#include <stdio.h>

int main() {
    int n, digit_counts[10] = {0}, peak = 0;
    scanf("%d", &n);
    while (n--) {
        int val;
        scanf("%d", &val);
        if (val == 0) digit_counts[0]++;
        while (val > 0) {
            digit_counts[val % 10]++;
            val /= 10;
        }
    }
    for (int i = 0; i < 10; i++) {
        if (digit_counts[i] > peak) peak = digit_counts[i];
    }
    printf("%d:", peak);
    for (int i = 0; i < 10; i++) {
        if (digit_counts[i] == peak) printf(" %d", i);
    }
    return 0;
}

Validating Upper Triangular Matrices

A square matrix is upper triangular if all elements below the main diagonal are zero. This program checks multiple test cases for this property.

#include <stdio.h>

int main() {
    int tests;
    scanf("%d", &tests);
    while (tests--) {
        int size, flag = 1;
        scanf("%d", &size);
        for (int r = 0; r < size; r++) {
            for (int c = 0; c < size; c++) {
                int val;
                scanf("%d", &val);
                if (r > c && val != 0) flag = 0;
            }
        }
        printf("%s\n", flag ? "YES" : "NO");
    }
    return 0;
}

Computing Row Sums in a Matrix

This utility reads an $M \times N$ matrix and calculates the sum of each individual row, printing the results sequentially.

#include <stdio.h>

int main() {
    int rows, cols;
    scanf("%d %d", &rows, &cols);
    for (int i = 0; i < rows; i++) {
        int current_row_sum = 0;
        for (int j = 0; j < cols; j++) {
            int element;
            scanf("%d", &element);
            current_row_sum += element;
        }
        printf("%d\n", current_row_sum);
    }
    return 0;
}

Locating Matrix Saddle Points

A saddle point is defined as an element that is the maximum in its row and the minimum in its column. This program finds and outputs the coordinates of such a point if it exists.

#include <stdio.h>

int main() {
    int n;
    scanf("%d", &n);
    int grid[n][n];
    for (int i = 0; i < n; i++)
        for (int j = 0; j < n; j++)
            scanf("%d", &grid[i][j]);

    int found = 0;
    for (int i = 0; i < n; i++) {
        int max_col = 0;
        for (int j = 1; j < n; j++) {
            if (grid[i][j] >= grid[i][max_col]) max_col = j;
        }
        int is_saddle = 1;
        for (int k = 0; k < n; k++) {
            if (grid[k][max_col] < grid[i][max_col]) {
                is_saddle = 0;
                break;
            }
        }
        if (is_saddle) {
            printf("%d %d", i, max_col);
            found = 1;
            break;
        }
    }
    if (!found) printf("NONE");
    return 0;
}

Counting Uppercase Consonants

This program reads a string and counts how many characters are uppercase consonants (uppercase letters excluding A, E, I, O, U).

#include <stdio.h>

int main() {
    char c;
    int consonants = 0;
    while ((c = getchar()) != '\n' && c != EOF) {
        if (c >= 'A' && c <= 'Z') {
            if (c != 'A' && c != 'E' && c != 'I' && c != 'O' && c != 'U') {
                consonants++;
            }
        }
    }
    printf("%d", consonants);
    return 0;
}

Alphabetical Character Replacement

This script performs a substitution cipher where each uppercase letter is replaced by its opposite in the alphabet (e.g., 'A' to 'Z', 'B' to 'Y').

#include <stdio.h>

int main() {
    char input_char;
    while ((input_char = getchar()) != '\n' && input_char != EOF) {
        if (input_char >= 'A' && input_char <= 'Z') {
            input_char = 'A' + 'Z' - input_char;
        }
        putchar(input_char);
    }
    return 0;
}

Converting Hexadecimal Strings to Decimal

Extracts hexadecimal digits from a string (terminated by '#') and converts the sequence into a signed decimal integer.

#include <stdio.h>

int get_hex_digit(char c) {
    if (c >= '0' && c <= '9') return c - '0';
    if (c >= 'a' && c <= 'f') return c - 'a' + 10;
    if (c >= 'A' && c <= 'F') return c - 'A' + 10;
    return -1;
}

int main() {
    char raw[100];
    long long result = 0;
    int sign = 1, started = 0, minus_pending = 0;
    scanf("%[^#]", raw);
    for (int i = 0; raw[i]; i++) {
        int val = get_hex_digit(raw[i]);
        if (raw[i] == '-' && !started) minus_pending = 1;
        if (val != -1) {
            if (!started && minus_pending) sign = -1;
            result = result * 16 + val;
            started = 1;
        }
    }
    printf("%lld", result * sign);
    return 0;
}

Sorting a List of Strings

Accepts five strings and sorts them in lexicographical order using a standard string comparison and swapping method.

#include <stdio.h>
#include <string.h>

int main() {
    char items[5][80], buffer[80];
    for (int i = 0; i < 5; i++) scanf("%s", items[i]);
    for (int i = 0; i < 4; i++) {
        for (int j = i + 1; j < 5; j++) {
            if (strcmp(items[i], items[j]) > 0) {
                strcpy(buffer, items[i]);
                strcpy(items[i], items[j]);
                strcpy(items[j], buffer);
            }
        }
    }
    printf("After sorted:\n");
    for (int i = 0; i < 5; i++) printf("%s\n", items[i]);
    return 0;
}

Student Score Statistics

Uses dynamic memory allocation to store student scores and calculates the average, maximum, and minimum values.

#include <stdio.h>
#include <stdlib.h>

int main() {
    int n;
    double *grades, sum = 0, high = -1.0, low = 101.0;
    if (scanf("%d", &n) != 1 || n <= 0) return 0;
    grades = (double*)malloc(n * sizeof(double));
    for (int i = 0; i < n; i++) {
        scanf("%lf", &grades[i]);
        sum += grades[i];
        if (grades[i] > high) high = grades[i];
        if (grades[i] < low) low = grades[i];
    }
    printf("average = %.2f\nmax = %.2f\nmin = %.2f", sum / n, high, low);
    free(grades);
    return 0;
}

Time Increment Calculator

Given a time in HH:MM:SS format and an integer $n$ (seconds), this program calculates the new time after adding $n$ seconds, handling wrap-around for minutes, hours, and days.

#include <stdio.h>

struct Clock {
    int h, m, s;
};

int main() {
    struct Clock t;
    int offset;
    scanf("%d:%d:%d %d", &t.h, &t.m, &t.s, &offset);
    long total_sec = t.h * 3600 + t.m * 60 + t.s + offset;
    total_sec %= 86400;
    printf("%02ld:%02ld:%02ld", total_sec / 3600, (total_sec % 3600) / 60, total_sec % 60);
    return 0;
}

Adding 2D Vectors

Computes the sum of two 2D vectors and formats the output. If a coordinate's absolute value is less than 0.05, it is rounded to 0.0 to avoid displaying negative zero.

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

int main() {
    double x1, y1, x2, y2;
    scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
    double res_x = x1 + x2, res_y = y1 + y2;
    if (fabs(res_x) < 0.05) res_x = 0.0;
    if (fabs(res_y) < 0.05) res_y = 0.0;
    printf("(%.1f, %.1f)\n", res_x, res_y);
    return 0;
}

Book Search by Price

Stores book information in a structure and identifies the books with the highest and lowest prices from a list of $N$ entries.

#include <stdio.h>
#include <string.h>

struct Catalog {
    char title[35];
    double cost;
};

int main() {
    int count;
    scanf("%d", &count);
    struct Catalog list[10];
    int high_idx = 0, low_idx = 0;
    for (int i = 0; i < count; i++) {
        getchar();
        fgets(list[i].title, 35, stdin);
        list[i].title[strcspn(list[i].title, "\n")] = 0;
        scanf("%lf", &list[i].cost);
        if (list[i].cost > list[high_idx].cost) high_idx = i;
        if (list[i].cost < list[low_idx].cost) low_idx = i;
    }
    printf("%.2f, %s\n", list[high_idx].cost, list[high_idx].title);
    printf("%.2f, %s", list[low_idx].cost, list[low_idx].title);
    return 0;
}

Sorting Contacts by Birth Date

Maintains a list of contacts and sorts them chronologically based on their birth dates.

#include <stdio.h>

struct User {
    char name[15];
    int dob;
    char phone[20];
};

int main() {
    int n;
    scanf("%d", &n);
    struct User group[10], swap;
    for (int i = 0; i < n; i++) {
        scanf("%s %d %s", group[i].name, &group[i].dob, group[i].phone);
    }
    for (int i = 0; i < n - 1; i++) {
        for (int j = 0; j < n - i - 1; j++) {
            if (group[j].dob > group[j + 1].dob) {
                swap = group[j];
                group[j] = group[j + 1];
                group[j + 1] = swap;
            }
        }
    }
    for (int i = 0; i < n; i++) {
        printf("%s %d %s\n", group[i].name, group[i].dob, group[i].phone);
    }
    return 0;
}