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C program to demonstrate usage of variable-length arrays
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
- if p[i] is not null, then−
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
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