how to scanf unknown amount of integer numbers into array in C?

Question

I know that I can scanf certain amount of numbers with scanf for example for 3 numbers

scanf("%d %d %d",array[0],array[1],array[2]);

but how can I scan it if I didn't know how many numbers (integer, not float) I would input into an array before enter (NOT EOF)? for example

input : 12 43 23(enter) --> array[0]=12, array[1]=43, array[2]=23
input : 10 20 30 40 50(enter) --> array[0]=10, array[1]=20, array[2]=30, array[3]=40, array[4]= 50
etc..

It's about how to input the numbers into an integer array.

And if it's possible, I want to save it into an 2 dimensions array, for example

input : 12 43 23(enter) --> array[0][0]=12, array[0][1]=43, array[0][2]=23
input : 10 20 30 40 50(enter) --> array[1][0]=10, array[1][1]=20, array[1][2]=30, array[1][3]=40, array[1][4]= 50

Read the whole line and parse it some other way? The fgets and strtol function can be used for this. — Some programmer dude
– Some programmer dude, Commented Dec 5, 2016 at 10:50
Wait, are you asking about how to make an array of the right size and add the numbers once you have them? — Iluvatar
– Iluvatar, Commented Dec 5, 2016 at 10:53
@iluvatar yes something like that, I don't know how to explain what i want properly. — juliussin
– juliussin, Commented Dec 5, 2016 at 10:55
so i should declare variable with [1000] ? That would be waste of memory right? — juliussin
– juliussin, Commented Dec 5, 2016 at 11:02

RoadRunner · Accepted Answer · 2016-12-05 13:10:26Z

1

Here is come code showing how to scan the integers into a 2D array:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define INITSIZE 5
#define BUFFSIZE 1000

void print_and_free(int **array, int rowsize, int colsize);
void check_ptr(void *ptr, const char *msg);

int
main(void) {
    int **array;
    size_t rowsize = INITSIZE, colsize = INITSIZE;
    int row = 0, col, numdigits;

    char buffer[BUFFSIZE];
    char *number;

    array = malloc(rowsize * sizeof(*array));
    check_ptr(array, "Allocation");

    printf("Enter digits(Enter blank line to end):\n");
    while (fgets(buffer, BUFFSIZE, stdin) != NULL && strlen(buffer) != 1) {
        col = 0;
        numdigits = 0;
        if (rowsize == row) {
            rowsize *= 2;
            array = realloc(array, rowsize * sizeof(*array));
            check_ptr(array, "Reallocation");
        }

        array[row] = malloc(colsize *sizeof(int));
        check_ptr(array[row], "Allocation");

        number = strtok(buffer, " ");
        while (number != NULL) {
            numdigits++;
            if (colsize == numdigits) {
                colsize *= 2;
                array[row] = realloc(array[row], colsize * sizeof(int));
                check_ptr(array[row], "Reallocation");
            }
            array[row][col] = atoi(number);
            col++;
            number = strtok(NULL, " ");
        }
        row++;
    }

    print_and_free(array, row, col);

    return 0;
}

void
print_and_free(int **array, int rowsize, int colsize) {
    int row, col;

    printf("Your numbers:\n");
    for (row = 0; row < rowsize; row++) {
        for (col = 0; col < colsize; col++) {
            printf("array[%d][%d] = %d", row, col, array[row][col]);
            if (col != colsize - 1) {
                printf(", ");
            }
        } 
        free(array[row]);
        array[row] = NULL;
        printf("\n");
    }

    free(array);
}

void
check_ptr(void *ptr, const char *msg) {
    if (!ptr) {
        printf("Unexpected null pointer: %s\n", msg);
        exit(EXIT_FAILURE);
    }
}

edited Dec 5, 2016 at 13:10

answered Dec 5, 2016 at 12:44

RoadRunner

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2 Comments

juliussin Over a year ago

Thank you, by the way what size_t rowsize = INITSIZE, colsize = INITSIZE; for?

RoadRunner Over a year ago

No worries @juliussin. Yeah they are just starting sizes to allocate space from malloc. I just assumed 5 would be a good starting size, and if your arrays end up being bigger, then just realloc more space. You can replace size_t with int if you want, that might be clearer for you.

Community · Accepted Answer · 2017-05-23 12:30:30Z

1

Here's how you can store things in a (dynamically allocated) array. This does assume that the line length is limited to 1000 chars though. (Code adapted from How do I use scanf() to take an arbitrary amount of integers?)

#include <stdio.h>
#include <stdlib.h>

int main() {
    int val_size = 2;
    int* vals = (int*)malloc(val_size * sizeof(int)); // initial array size
    char buffer[1000]; // for reading in the line
    int pos, bytes_read, num;
    int num_read = 0;

    if (fgets(buffer, sizeof(buffer), stdin) != 0) {
        for (pos = 0; sscanf(buffer+pos, "%d%n", &num, &bytes_read) != EOF; pos += bytes_read) {
            // resize the array if needed
            if (num_read >= val_size) {
                val_size *= 2;
                vals = (int*)realloc(vals, val_size * sizeof(int));
            }

            // store the value in the array
            vals[num_read] = num;
            num_read++;
        }
    }

    // print the values to prove it works
    for (int i = 0; i < num_read; i++) {
        printf("%d ", vals[i]);
    }
    printf("\n");

    free(vals); // important after you're done with it
}

You can wrap a while around the if to get multiple lines.

edited May 23, 2017 at 12:30

CommunityBot

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answered Dec 5, 2016 at 11:21

Iluvatar

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9 Comments

AndersK Over a year ago

you should not cast return value from malloc/realloc. stackoverflow.com/questions/605845/…

Iluvatar Over a year ago

This seems like a religious war, yes? Or at least in the same vein as if (a == 0) vs if (0 == a)? Perhaps strictly speaking it alleviates some user error, there are pros and cons both ways.

Iluvatar Over a year ago

stackoverflow.com/a/14879184/3761456 Granted, it's got a tenth the upvotes of the top one, yet they're there and it seems mostly stylistic to me at this point.

RoadRunner Over a year ago

Honeslty speaking int *vals = malloc(val_size * sizeof *vals); looks less cluttered than int* vals = (int*)malloc(val_size * sizeof(int));

AndersK Over a year ago

@lluvatar i dont buy that. this question is marked C, not C++. If you are using malloc in C++ you are definitely doing something wrong.

|

tdao · Accepted Answer · 2016-12-05 10:53:16Z

0

You may use a while loop and check for the return value of scanf. For example:

int num = 0;
int idx = 0;
int arr[100] = { 0 };
while( scanf( "%d", &num ) == 1 )
{
    arr[idx++] = num;
    if( idx == 99 )  // protect from array overflow
    {
        break;
    }
}
for( int i = 0; i < idx; i++ )
{
    printf( "%d\n", arr[i] );
}

answered Dec 5, 2016 at 10:53

tdao

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4 Comments

Iluvatar Over a year ago

I don't think this does what they want... it doesn't scan multiple ints on a line.

tdao Over a year ago

@Iluvatar yes it does

Weather Vane Over a year ago

No, although it scans multiple integers on one line. it does not terminate on "Enter (NOT EOF)" as requested.

Iluvatar Over a year ago

Ah, it does, but you have to enter something that causes scanf to unblock, so just hitting enter doesn't do it.

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