function for multiple occurrences in linear search using pointer - c

Question

This is the code i wrote for multiple occurrences in linear search.Can you please help me point out the mistake ? I want the function to store multiple values in the pointer array and then later to print the array
#include <stdio.h>
void linearsearch(int n,int a[n],int x,int count,int *b[count])
{
count=0;
int i;
for(i=0;i<n;i++)
{
if(a[i]==x)
{
count+=1;
}
}
int j=0;
for(i=0;i<n;i++)
{
if(a[i]==x)
{
b[j]=i;
j++;
}
}
}
int main()
{
int n;
scanf("%d",&n);
int a[n];
int i;
for(i=0;i<n;i++)
{
scanf("%d",&a[i]);
}
int x;
scanf("%d",&x);
int count;
int *b[count];
linearsearch(n,a,x,count,b);
for(i=0;i<count;i++)
{
printf("%d",*b[i]);
}
return 0;
}

Answer 1

I think there are several errors.
count must be initialized, like int count = 0;
count variable is not changed after linearsearch function.
b array should allocated dynamically.
suggested patch is:
#include <stdio.h>
void linearsearch(int n,int a[n],int x,int *count,int **b)
{
count=0;
int i;
for(i=0;i<n;i++)
{
if(a[i]==x)
{
count+=1;
}
}
int j=0;
for(i=0;i<n;i++)
{
if(a[i]==x)
{
*b = realloc(*b, sizeof(int) * (j + 1));
(*b)[j]=i;
j++;
}
}
}
int main()
{
int n;
scanf("%d",&n);
int a[n];
int i;
for(i=0;i<n;i++)
{
scanf("%d",&a[i]);
}
int x;
scanf("%d",&x);
int count = 0;
int *b = NULL;
linearsearch(n,a,x,&count,&b);
for(i=0;i<count;i++)
{
printf("%d",b[i]);
}
return 0;
}

Answer 2

Well the ideal way would be to allocate the memory dynamically based of the number of x found. But well let's look at the errors first. Maybe after this discussion you can write the code.
b[j]=i;
In the called function let's dissect the type of this.
i is of type int. And b[j] is of type int*. Then you assigned them (type mismatched). Then %d expects an int but you passed something of type int*. This is undefined behavior.
Now there was another flaw you had - you passed count and somehow you expect the value you changed in search will be there in main(). C is pass by value and you changed to a local variable. That change is lost when the called function ends.
#include <stdio.h>
#include <stdlib.h>
int* linearsearch(int n,int* a,int x,int* count)
{
*count=0;
if( n <= 0){
fprintf(stderr, "%s\n","Error" );
exit(1);
}
int *t = malloc(sizeof(*t)*n);
if( t == NULL){
perror("Malloc failed");
exit(1);
}
for(int i = 0; i < n; i++)
if(a[i] == x)
t[(*count)++] = i;
int *temp = realloc(t,sizeof(*temp)* (*count));
if( temp == NULL){
perror("Realloc failed");
exit(1);
}
t = temp;
return t;
}
int main(void)
{
int n;
if( scanf("%d",&n)!= 1){
fprintf(stderr, "%s\n", "Error in input");
exit(1);
}
if( n <= 0 ){
fprintf(stderr, "%s\n", "Error in input : must be greater than 0");
exit(1);
}
int a[n];
for(int i=0; i < n; i++)
if(scanf("%d",&a[i])!=1){
fprintf(stderr, "%s\n","Error in input." );
exit(1);
}
int elmt_to_find;
if( scanf("%d",&elmt_to_find)!= 1){
fprintf(stderr, "%s\n", "Error in input : Element to find(must be integer)");
}
int count;
int *b = linearsearch(n,a,elmt_to_find,&count);
for(int i = 0; i < count; i++)
printf("%d ",b[i]);
printf("%s","\n");
free(b);
return 0;
}

Answer 3

If you want to stick with using VLA for b, you can alter your linearsearch to return count only if b is NULL. Then, you can create b as VLA, and pass it back to linearsearch again to be populated.
int count = linearsearch(n, a, x, 0, 0);
int b[count];
linearsearch(n, a, x, count, b);
Then, your function could look like:
int linearsearch(int n,int a[n],int x,int count,int *b[count])
{
int i;
if(count==0)
{
for(i=0;i<n;i++)
{
if(a[i]==x)
{
count+=1;
}
}
}
if (b==0)
{
return count;
}
int j=0;
for(i=0;i<n;i++)
{
if(a[i]==x)
{
b[j]=i;
j++;
}
}
return count;
}