Usage of Variable Length Arrays in C

Suppose we are in charge of building a library system that monitors and queries various operations at the library. We are now asked to implement three different commands that perform the following −

• By using command 1, we can record the insertion of a book with y pages at shelf x.

• By using command 2, we can print the page number of the y-th book at shelf x.

• By using command 3, we can print the number of books on shelf x.

The commands are given to us as a 2D array in this format {command type, x, y}. If there is no y value, the value will default at 0. We print the results of the given commands.

So, if the input is like number of shelves = 4, queries = 4, input_arr = {{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, we will follow these steps −

• b := a new array of size s
• p := a new array of size s
• for initialize i := 0, when i < s, update (increase i by 1), do:
  • b[i] := 0
  • p[i] := a new array
• for initialize loopCount := 0, when loopCount < q, update (increase loopCount by 1), do −
  • qtype := q_array[loopCount, 0]
  • if qtype is same as 1, then −
    • x := q_array[loopCount, 1]
    • y := q_array[loopCount, 2]
    • b[x] := b[x] + 1
    • p[x] := deallocate the object pointed by p[x] and return a pointer of size
    • b[x]
    • p[x, b[x] - 1] = y
  • otherwise when qtype is 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 null, then −
  • deallocate the memory acquired by b
• for initialize i := 0, when i < s, update (increase i by 1), do−
  • if p[i] is not null, then−
    • deallocate the memory acquired by p[i]
  • if p is not null, then−
    • deallocate the memory acquired by p

Example

Let us see the following implementation to get 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[x][y]);
      }
      else
      {
         int x;
         x = q_array[loopCount][1];
         printf("%d
", 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