How to input numbers into a vector until I press enter - arrays

Question

I want to have a versatile code for which I don't have to input the number of numbers.
Instead of doing something like this
#include <stdio.h>
void main(){
int n,a[100];
scanf("%d", &n);
for(int i=0;i<n;i++)
scanf("%d ", a[i]); }
Here I would first input n and then input n numbers in my array.
To give a more concrete example, I want to code the union and reunion of two sets. Instead of thinking of the number of elements of each sets, I would just enter them as I please without limiting to the "n" I mentioned above.
Is there any way in which I could skip inputting n? And how would I declare my array as to not lose memory.

Answer 1

You'll need to use a pointer to a block of memory which you keep increasing in size each time you go get a new number:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void main()
{
int *a = NULL;
int n = 0;
char str[10000];
while(fgets(str, sizeof str, stdin) != NULL &&
strlen(str) > 1)
{
n += 1;
a = realloc(a, n * sizeof(int));
a[n-1] = atoi(str);
}
for(int i = 0 ; i < n ; ++i)
printf("a[%d] = %d\n", i, a[i]);
}
OnlineGDB here

Answer 2

You need to grow your memory buffer dynamically. Here's a method that reads the first array as space separated integers, and looks for a semi-colon in the input to tell when the second array starts:
/* Read two sets of integers */
#include <stdio.h>
#include <stddef.h>
#include <ctype.h>
#include <stdlib.h>
void * xrealloc(void *buf, size_t nmemb, size_t size, void *iterator);
struct buffer {
int *start;
int *end;
size_t cap;
};
void push(int c, struct buffer *);
int
main(void)
{
int c;
struct buffer a = {0};
struct buffer b = {0};
while( scanf("%d", &c) == 1 ){
push(c, &a);
}
if( (c = getchar()) != ';' ){
fprintf(stderr, "Unexpected input\n");
return EXIT_FAILURE;
}
while( scanf("%d", &c) == 1 ){
push(c, &b);
}
printf("set a: ");
for( int *p = a.start; p < a.end; p++ ){
printf("%s%d", p > a.start ? ", " : "", *p);
}
printf("\nset b: ");
for( int *p = b.start; p < b.end; p++ ){
printf("%s%d", p > b.start ? ", " : "", *p);
}
putchar('\n');
return 0;
}
void
push(int c, struct buffer *b)
{
if( b->start == NULL || b->end >= b->start + b->cap ) {
b->start = xrealloc(b->start, b->cap += BUFSIZ, 1, &b->end);
}
*b->end++ = c;
}
void *
xrealloc(void *buf, size_t num, size_t siz, void *endvp)
{
void **endp = endvp;
ptrdiff_t offset = buf && endp && *endp ? *endp - buf : 0;
buf = realloc(buf, num * siz);
if( buf == NULL ){
perror("realloc");
exit(EXIT_FAILURE);
}
if( endp != NULL ){
*endp = buf + offset;
}
return buf;
}
A simple run looks like:
$ echo '1 3 5; 3 5 9 3 5' | ./a.out
set a: 1, 3, 5
set b: 3, 5, 9, 3, 5

Answer 3

The usual way to do this is to use fgets to read a line, then use sscanf to parse that line. Something like:
char buffer[2048];
int a[100], n;
if (fgets(buffer, sizeof(buffer), stdin)) {
n = 0;
char *p = buffer;
int len;
while(n < 100 && sscanf("%d%n", &a[n], &len) > 0) {
++n;
p += len; } }
You might also want to use malloc/realloc for the array a so that it can start small and grow as large as you need based on the input, but that is a different question (with many answers here).