C program example demonstrating the use of variable length arrays

Suppose we are responsible for building a library system for monitoring and querying various operations of the library. Now we are asked to implement three different commands that perform the following operations:

• Using command 1, we can record the insertion of a book with y pages on bookshelf x.

• Using command 2, we can print out the page number of the yth book on the bookshelf x.

• Using command 3, we can print out the number of books on bookshelf x.

These commands are given as a 2D array in the format {command type, x, y}. If there is no y value, the default value is 0. We print the result of the given command.

So, if the input is as follows: Number of bookshelfs = 4, number of queries = 4, input array = {{1, 3, 23}, {1, 4, 128}, {2, 3, 0}, {3, 4, 0}}; then the output will be

23
1

Command 1 inserts a book with 23 pages on shelf 3.
Command 2 inserts a book with 128 pages on shelf 4.
Command 3 prints the page number of book 0 on shelf 3.
Command 4 prints the number of books on shelf 3.

To solve this problem, we will follow the following steps:

• b := new array of size s
• p := New array of size s
• Initialize i := 0, when i
• b[i] := 0
• p[i] := New array

Initialize loopCount := 0, when loopCount

qtype := q_array[loopCount, 0]

If qtype is the same as 1, then −

• x := q_array[loopCount, 1]
• y := q_array[loopCount, 2]
• b[x] := b[x] 1
• p[x] := Release p[x ] object pointed to and returns size
• b[x]
• p[x, b[x] - 1] = y

else , when qtype is the same as 2, then −

• x := q_array[loopCount, 1]
• y := q_array[loopCount, 2]
• print p[x, y]

## Otherwise

x := q_array[loopCount, 1]
• Print b[x]

If b is not empty, then −

Release the memory obtained by b

Initialize i := 0, when i

If p[i] is not empty, then−
• Release the memory acquired by p[i]
If p is not empty, then −
• Release the memory obtained by p

Example

Let’s take a look at the following implementation for better understanding −

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

void solve(int s, int q, int q_array[][3])
{
      int* b;
      int** p;
   b = (int*)malloc(sizeof(int)*s);
   p = (int**)malloc(sizeof(int*)*s);
   for(int i = 0; i < s; i++)
   {
      b[i] = 0;
      p[i] = (int*)malloc(sizeof(int));
   }
   int loopCount;
   for(loopCount = 0; loopCount < q; loopCount++)
   {
      int qtype;
      qtype = q_array[loopCount][0];
      if (qtype == 1)
      {
         int x, y;
         x = q_array[loopCount][1];
            y = q_array[loopCount][2];
         b[x] += 1;
         p[x] = realloc(p[x], b[x]*sizeof(int));
         p[x][b[x] - 1] = y;
      }
      else if (qtype == 2)
      {
         int x, y;
         x = q_array[loopCount][1];
            y = q_array[loopCount][2];
         printf("%d</p><p>", p[x][y]);
      }
      else
      {
         int x;
         x = q_array[loopCount][1];
         printf("%d</p><p>", b[x]);
      }
   }
   if (b)
      free(b);
   for (int i = 0; i < s; i++)
      if (p[i])
         free(p[i]);
   if (p)
      free(p);
}
int main() {
   int input_arr[][3] = {{1, 3, 23}, {1, 4, 128}, {2, 3, 0}, {3, 4, 0}};
      solve(4, 4, input_arr);
}

Input

int input_arr[][3] = {{1, 3, 23}, {1, 4, 128}, {2, 3, 0}, {3, 4, 0}};
solve(4, 4, input_arr);

Output

23
1

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