Storing integers as an array by reading size of array and the elemnts of the array in a single line

Question

I am trying to create an array from the integers that a user inputs. They must be inputted on the same line, e.g. 5 1 2 3 1 6. The number of integers can be any amount.

However, the first number + 1 determines the size of the array. E.g. if the user inputs the number 5 first, it must be followed by 5 other random integers thus making the length of the array 6. I'm getting confused on how to do this because there is so much user input. Thanks in advance!

Here is my code so far, though i don't think it'll be much help:

#include <stdio.h>

int main(void) {
    int arr_days[13];
    int i = 0;
    printf(" ");
    while(i < 13) {
        scanf("%d", &arr_days[i]);
        i = i + 1;
    }
    printf("%d", arr_days[0]);
    return 0;
}

Answer 1

You weren't too far off, you just need to allocate the buffer dynamically and read the first number separately so that you know how much space and how many loops to perform.

You might also want to check if arr_days is NULL after the malloc to protect against 'out of memory' problems.

#include <stdio.h>

int main(void) {
    int* arr_days;
    int i = 0;
    int n;
    printf(" ");
    // get first number to find out how many more to expect
    scanf("%d", &n);
    // create memory space to store them
    arr_days = malloc(n * sizeof(int));
    while(i < n) {
        scanf("%d", &arr_days[i]);
        i = i + 1;
    }
    printf("%d", arr_days[0]);
    return 0;
}

EDIT: I see from some comments further down that there is some confusion about scanf. This command will trigger an input request if there is no input pending, then it will process any pending input. So when you type in a bunch of numbers in response to a scanf input prompt (or pipe a text file with those numbers in to the program) each invocation of scanf("%d",...) will only extract the next single integer, leaving the rest of the input still available. If you think of it like the string is a file, scanf is reading from that file and leaving the file pointer at the end of the bit it just read ready for the next invocation to read the next bit... (which is exactly what happens if you pipe a text file!)

Answer 2

First you would need to get the number so that you know how many numbers to read and how big to allocate the array.

int num;
scanf("%d", &num);

int *arr = malloc(num * sizeof(int));

Second, read each of the numbers one by one.

int I;
for (I = 0; I < num; I++) {
    scanf("%d", &arr[I]);
}

This shows as an example of how to do the task, you will need to take care of check scanf's result to make sure the input was successful, and check malloc for allocation too.

Answer 3

If you want to set the array size based on an input then the input has to be read first. What you want is to input all numbers in a line(including size of the array), which means the numbers will be read after the end of line only(after pressing [Enter]). This case is not possible with static arrays.

You can't read all the inputs i.e. array size & integers to be put in it on the same line & build an static array. But you may build a dynamic array by implementing a dynamic array and taking all inputs in the same line.

A dynamic array is a re-sizable array or array list with random access, variable-size list data structure that allows elements to be added or removed.

A static array required the size of the array to be created so that that memory could be allocated before array elements can be added to it.

All this being said, if your array size has an upper-bound size(i.e. the maximum number of elements which you would accept is set) then you may create a static array of the maximum size and read you inputs into it.

int arr[MAX_ARRAY_SIZE ASSUMED];

This allocates an array of size MAX_SIZE_ASSUMED which means all your inputs maybe stored in the static array. But this is a bad way to implement as MAX_SIZE_ASSUMED is always the size of the array even when you may just enter one element into the array.