Why am I not getting the desired output? - c

Question

The problem is to write the asked fibonacci word. For example if input is 0, then f(0) = a, if 1, f(1) = b and similarly f(2) = ba, f(3) = bab, f(4) = babba and so on. I wrote the following code to find the output on Ubuntu 18.04 LTS Terminal. I am getting the right output for n=0,1,2,3. But for n=4 I am getting babb instead of babba. I have tried debugging also but could not find where the code is going wrong. Kindly help me in finding the error.
#include <stdio.h>
#include <string.h>
void fibonacci(int n);
int main()
{
int x;
printf("Enter the fibonacci nword number you want to see:(f(x), f(0) is the starting element.):\n");
scanf("%d",&x);
printf("Required Word is:\n");
fibonacci(x);
return 0;
}
void fibonacci(int n)
{
int i,j=0;
char *p,*q,*r;
if(n==0)
{
printf("a\n");
}
else if(n==1)
{
printf("b\n");
}
else
{
char str1[100] = "a";
char str2[100] = "b";
char str3[100];
p = str1;
q = str2;
r = str3;
for(i=0;i<n-1;i++)
{
*r = *q;
strcat(str2,str1);
*p = *r;
}
printf("%s\n",str2);
}
}

Answer 1

First answering the main question, as of "Why I'm not getting the desired output":
Because you don't know what you're doing.
You are declaring 3 char[] variables statically, assigning them to pointers of type char*, and not even using them correctly.
Let's annalyze a part of your code:
for(i=0;i<n-1;i++)
{
*r = *q;
strcat(str2,str1);
*p = *r;
}
What you're doing is basically:
assign str3[0] = 'b' (in *r = *q)
copy the contents of str1 into str2, so, "ba" in the first run
assign str1[0] = 'b' (in *p = *r)
And then, repeatedly concatenate "b" into str2, because both str1 will only contain a single "b" for now on.
Doing that, for anything above 4, you'll only get "babbbbbbbbbb"...
My advice: If you're going to statically declare some variables, stop using pointers to access them. Try accessing the str1/str2 as vectors.

Answer 2

Your code is obfuscated. I modified it so:
#include <stdio.h>
#include <string.h>
char*fib(int n)
{
if (0==n)
return "a";
else if (1==n)
return "b";
else
{
char static out[2000]={'b', 'a'};
int idx=2, prev=1, tmp;
n-=2;
while(n--)
{
/* invariant: all values start at the beginning of `out`.
idx: keep the length of the current object
prev: keep the size of previous object
*/
memcpy(out+idx, out, prev);
tmp=prev;
prev=idx;
idx+=tmp;
}
return out;
}
}
int main()
{
int x;
printf("Enter the fibonacci nword number you want to see:"
"(f(x), f(0) is the starting element.):\n");
scanf("%d",&x);
printf("Required Word is:\n");
printf("~~%s\n", fib(x));
return 0;
}