print the adjacent sum sequence of integers in C without array - c

Question

Is it in general possible to store a sequence of inputs and print it out later without using arrays?
I am required to take integer inputs sequentially with -1 marking as the end of input without taking it as an input (call it xn). Output is supposed to be sequence an where
an:= xn-1 + xn, where n>1
Following is my code, but the output happens along with the input and not at one go as the problem really requires. Here is the code:
#include <stdio.h>
int main() {
int xo,a;
int sum =0;
int previous=0;
scanf("%d",&xo);
previous = xo;
while ((!scanf("%d",&a)==0) && (!(a==-1))) {
sum = previous+a;
previous = a;
printf("%d",sum);
}
return 0;
}
Input:
1
2
3
4
-1
Output:
3 5 7

Answer 1

I'm not entirely sure, but the purpose of your task could be to familiarise yourself with lists. Then you don't need an array to store the input values:
struct list {
int val;
struct list * next;
};
Handling lists should be abstracted into general functions:
struct list * list_add(struct list * lst, int val) {
struct list * node =
(struct list *) malloc(sizeof(struct list));
node->val = val;
node->next = lst;
return node;
}
void list_free(struct list * lst) {
while (lst) {
struct list * temp = lst;
lst = lst->next;
free(temp);
}
}
struct list * list_nreverse(struct list * lst) {
struct list * nlst = lst;
if (lst) {
lst = lst->next;
nlst->next = NULL;
while (lst) {
struct list * temp = lst;
lst = lst->next;
temp->next = nlst;
nlst = temp;
}
}
return nlst;
}
To use the list, you can either store your input values in the list or, to stay close to the code that you already have, calculate the output (the adjacent sum) on the fly and store that in a list, printing the entire list
at the end (in reverse order):
int current;
int previous;
struct list * output = NULL;
scanf("%d", &previous);
while ((!scanf("%d", &current) == 0) && (current != -1)) {
sum = previous + current;
previous = current;
output = list_add(output, sum);
}
output = list_nreverse(output);
struct list * iterator = output;
while (iterator) {
printf("%d ", iterator->val);
iterator = iterator->next;
}
list_free(output);
Note: Code is currently untested, but should give you an idea of how you can accomplish this.

Answer 2

Just keep appending to a string. At the end of the input, print the string.
a string can be considered as an array. maybe this invalidates this answer
Otherwise, no. You need an array to store either the input or the calculations.
Oops ... as Daniel Jour says in his answer you can use a list instead of an array.

Answer 3

integer inputs are sequentially taken with 'scan'. To find end of the input, checking the scan return value with -1 so to scan the null element.
Here is my code as given below, output happens according to your requirement. output happens at one go that your problem really requires #subrat. The code is as follows:
#include <stdio.h>
int main()
{
int xo,a;
int sum =0;
int diff=0;
int previous=0;
scanf("%d",&xo); //dont need xo here :: use previous only
previous = xo;
while ((scanf("%d",&a)!=-1)) //important to use sum=0
{
sum = previous+a;
previous = a;
printf("%d ",sum);
sum=0;
}
return 0;
}
INPUT : 1 2 3 4 -1
OUTPUT : 3 5 7 3