Part A: Advantages and Disadvantages of Global Variables

A global variable is a variable declared outside all functions, accessible from any function throughout the program.

Advantages:

• Universal Accessibility — Any function in the program can access and modify the global variable without passing it as a parameter.
• Reduced Parameter Passing — No need to pass variables as arguments between multiple functions, simplifying function calls.
• Data Sharing — Multiple functions can share data easily through global variables.
• Persistence — Global variables retain their value throughout the entire program execution.

Disadvantages:

• Uncontrolled Access — Any function can accidentally modify the global variable, making debugging difficult.
• Namespace Pollution — Too many global variables can cause naming conflicts in large programs.
• Reduced Modularity — Functions become dependent on global variables, making them harder to reuse in other programs.
• Security Risk — Data is exposed to all functions, violating the principle of data hiding.
• Difficult Debugging — Hard to track which function changed the variable's value when errors occur.

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Part B: Merge Two Sorted Files into a Third Sorted File

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

int main() {
    FILE *f1, *f2, *f3;
    int a[100], b[100], c[200];
    int i = 0, j = 0, k = 0, n1 = 0, n2 = 0, temp;

    // Read integers from num1.txt
    f1 = fopen("num1.txt", "r");
    if (f1 == NULL) {
        printf("Cannot open num1.txt\n");
        exit(1);
    }
    while (fscanf(f1, "%d", &a[n1]) != EOF) {
        n1++;
    }
    fclose(f1);

    // Read integers from num2.txt
    f2 = fopen("num2.txt", "r");
    if (f2 == NULL) {
        printf("Cannot open num2.txt\n");
        exit(1);
    }
    while (fscanf(f2, "%d", &b[n2]) != EOF) {
        n2++;
    }
    fclose(f2);

    // Copy all elements into array c
    for (i = 0; i < n1; i++)
        c[k++] = a[i];
    for (j = 0; j < n2; j++)
        c[k++] = b[j];

    // Sort array c using Bubble Sort
    for (i = 0; i < k - 1; i++) {
        for (j = 0; j < k - i - 1; j++) {
            if (c[j] > c[j + 1]) {
                temp = c[j];
                c[j] = c[j + 1];
                c[j + 1] = temp;
            }
        }
    }

    // Write sorted integers to file3.txt
    f3 = fopen("file3.txt", "w");
    if (f3 == NULL) {
        printf("Cannot open file3.txt\n");
        exit(1);
    }
    for (i = 0; i < k; i++) {
        fprintf(f3, "%d\n", c[i]);
    }
    fclose(f3);

    printf("Files merged and sorted successfully into file3.txt\n");
    return 0;
}

Logic Explanation:

• Read all integers from num1.txt and num2.txt into separate arrays.
• Combine both arrays into a single array c[].
• Sort the combined array using Bubble Sort in ascending order.
• Write the sorted integers into file3.txt using fprintf().

Conclusion: Global variables offer easy data sharing but compromise modularity and security. The file merging program demonstrates file handling with fopen(), fscanf(), fprintf(), and fclose() along with sorting logic to produce a sorted output file.