Storing Fibonacci numbers in an array (C) - c

Question

Can't get my program to output the correct number. I feel like I am making a simple mistake. This is written in C.
#include <stdio.h>
#include <stdlib.h>
int main()
{
int n, i;
int list[n];
while(1)
{
scanf("%d", &n);
if(n == -1)
{
break;
}
else
{
for(i = 2; i < n; i++)
{
list[i] = list[i-1]+list[i-2];
}
printf("%d %d", i, list[i] );
}
}
}

Answer 1

(To make things simpler, I'm going to ignore dealing with input.)
First problem is turning on compiler warnings. Most C compilers don't give you warnings by default, you have to ask for them. Usually by compiling with -Wall. Once we do that, the basic problem is revealed.
test.c:6:14: warning: variable 'n' is uninitialized when used here [-Wuninitialized]
int list[n];
^
test.c:5:10: note: initialize the variable 'n' to silence this warning
int n, i;
^
= 0
1 warning generated.
int list[n] immediately creates a list of size n. Since n is uninitialized it will be garbage. You can printf("%d\n", n); and see, it'll be something like 1551959272.
So either n needs to be initialized, or you need to reallocate list dynamically as n changes. Dynamic allocation and reallocation gets complicated, so let's just make it a static size.
So we get this.
#include <stdio.h>
#include <stdlib.h>
int main() {
/* Allocate an array of MAX_N integers */
const int MAX_N = 10;
int list[MAX_N];
/* Do Fibonacci */
for(int i = 2; i < MAX_N; i++) {
list[i] = list[i-1]+list[i-2];
}
/* Print each element of the list and its index */
for( int i = 0; i < MAX_N; i++ ) {
printf("%d\n", list[i]);
}
}
That runs, but we get nothing but zeros (or garbage). You have a problem with your Fibonacci algorithm. It's f(n) = f(n-1) + f(n-2) with the initial conditions f(0) = 0 and f(1) = 1. You don't set those initial conditions. list is never initialized, so list[0] and list[1] will contain whatever garbage was in that hunk of memory.
#include <stdio.h>
#include <stdlib.h>
int main() {
/* Allocate an array of MAX_N integers */
const int MAX_N = 10;
int list[MAX_N];
/* Set the initial conditions */
list[0] = 0;
list[1] = 1;
/* Do Fibonacci */
for(int i = 2; i < MAX_N; i++) {
list[i] = list[i-1]+list[i-2];
}
/* Print each element of the list and its index */
for( int i = 0; i < MAX_N; i++ ) {
printf("%d\n", list[i]);
}
}
Now it works.
0 0
1 1
2 1
3 2
4 3
5 5
6 8
7 13
8 21
9 34

Answer 2

Here is code snippet,
#include <stdio.h>
int main()
{
int MAX_SIZE = 100; //Initial value
int n, i;
int list[MAX_SIZE];
printf("Enter value of 'n'");
scanf("%d",&n);
if(n < 0){
printf("'n' cannot be negative number");
return 0;
}else if (n==1){
list[0]=0;
}else if(n == 2){
list[0]=0;
list[1]=1;
}else{
list[0]=0;
list[1]=1;
for(i = 2; i <= n; i++)
{
list[i] = list[i-1]+list[i-2];
}
}
//To view array elements
for(int i=0;i<n;i++){
printf("%3d",list[i]);
}
}

Answer 3

You don't have return in main function.
n must be defined previous. Otherwise it took random value from memory.
So, your list array is created with unknown value.
int list[n];
Also, this will never happends, becous n is declared, but not defined.
i < n;
Is this what you need?
#include <stdio.h>
#include <stdlib.h>
int main()
{
int F[100];
F[0] = 0;
F[1] = 1;
int i = 2;
while(1)
{
if(i < 100)
{
F[i] = F[i-1] + F[i-2];
i++;
}
else
{
break;
}
}
i = 0;
while(1)
{
if(i < 100)
{
printf("%d ; ", F[i]);
i++;
}
else
{
break;
}
}
return 0;
}

Answer 4

You need to allocate memory on demand for each iteration. In your code, n is uninitalized which leads to unpredectiable behavior. Also you need to initialize list[0] and list[1] since this is the 'base' case.
#include <stdio.h>
#include <stdlib.h>
int main()
{
int n, i;
int* list; /* Declare a pointer to the list */
while(1)
{
scanf("%d", &n);
if(n == -1)
{
break;
}
else if ( n > 0 )
{
list = (int *) malloc( n * sizeof(int) );
list[0] = 1;
list[1] = 1;
for(i = 2; i < n; i++)
{
list[i] = list[i-1]+list[i-2];
}
printf("%d %d\n", i, list[i-1] );
free(list);
}
}
}