How to fill an array in C with user input values, when the array is of unknown length [duplicate]

Question

Possible Duplicate:
Dynamic array using ANSI C

I am trying to fill an array with the values that the user is typing. However, I do not know in advance how many values will my array have, or how many values the user will type in. The user types in a value at a time, the value is stored in the array, and then the user is again prompted to type in another value, and so forth, until he types in a negative number. When the user types in a negative number, the program prints out all the positive values that the user has entered so far (NOT the negative one, as it is essentially only used for termination of the program).

My problem is:

1) how to declare the array without knowing in advance how big it will be?

2) how to scan for the user input? For example, I am thinking something like this for scanning the input and assigning the values to the array (this is just a part of the code, not all of it, I just want to know if this part of the code will work when I have completed the program):

...

int working = 0;
int i = 0;
do
{          
printf("Enter a positive value \n");
scanf("%d",&x);
if (x >= 0)
{
 &array[i] = x;
 i++;
}
else
{
 printf("You have entered a negative number \n");
 working = 1;
}
} while (working = 0);

Is this code correct (of course, it is not a complete program)? Also, how do I declare the array, without knowing how big it will be (I have no way of knowing in advance how many positive values the user will type in before he types in a negative one)

I think you need to provide a pre-allocated array and then realloc in blocks (say 200 elements) when input data exceed it's size. — BigMike
– BigMike, Commented Dec 20, 2012 at 9:43
Thanks @BigMike and the rest - this link about malloc/realloc seems interesting, I didn't know about it (I am just starting to learn about C). Also, what do you think about my code? Will it work? — asterix obelisk
– asterix obelisk, Commented Dec 20, 2012 at 9:53

Sergej Christoforov · Accepted Answer · 2012-12-20 09:43:33Z

3

You can allocate an initial array for user input and save it's size into a variable so then you can reallocate the array when it's full. Or you could use linked list to save input, so later you could calculate needed element count and allocate the array.

answered Dec 20, 2012 at 9:43

Sergej Christoforov

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1 Comment

simonc Over a year ago

+1 for mallocing then reallocing the array

maverik · Accepted Answer · 2012-12-20 10:09:27Z

2

int arraySize = 256; // Or whatever
int *array = malloc(arraySize * sizeof(int));
if (!array)
{
    fprintf(stderr, "Master, please buy more RAM, I can't allocate memory\n");
    return;
}

int numberOfElements = 0;
for(;;)
{          
    printf("Enter a positive value:\n");
    scanf("%d",&x);

    if (x >= 0)
    {
        if (numberOfElements == arraySize)
        {
            arraySize *= 2; // Or whatever strategy you need
            array = realloc(array, arraySize * sizeof(int));
            if (!array)
            {
                fprintf(stderr, "Master, please buy more RAM, I can't allocate memory\n");
                break;
            }
        }
        array[numberOfElements++] = x;
    }
    else
    {
        printf("You have entered a negative number \n");
        break;
    }
}

Something like. Sorry for the possible mistakes, don't check it.

edited Dec 20, 2012 at 10:09

answered Dec 20, 2012 at 9:52

maverik

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4 Comments

asterix obelisk Over a year ago

What does the if (!array) statement do? I have never seen it before

maverik Over a year ago

You can think of it like if (array == 0) or if (array == NULL) if you prefer. It checks whether we can allocate memory, because if not then malloc/realloc returns 0 (or NULL if you want).

asterix obelisk Over a year ago

OK, thanks! What do you think of my code BTW - will it work? I used a "while-do" statement

maverik Over a year ago

Looks like working (can't check right now). for(;;) just looks prettier and clearer for me. Choose the one you like.

theharshest · Accepted Answer · 2012-12-20 09:43:33Z

1

Better use linked list, array won't help you here.

Check this out if you are not familiar with it - http://cslibrary.stanford.edu/103/LinkedListBasics.pdf

answered Dec 20, 2012 at 9:43

theharshest

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Comments

user2000 · Accepted Answer · 2012-12-20 10:19:22Z

1

   int dataarray [2];
   int no;
   int count =0;

   while(1)
   {
        printf("Enter No's = ");
        scanf("%d",&no);
        if(no<0)
             break;
       *(dataarray+count)=no;
       count++;
   }

you can use the count further to know how many elements in array.

you can get elements from this array by pointers link

no = *(dataarray+count)

edited Dec 20, 2012 at 10:19

answered Dec 20, 2012 at 10:14

user2000

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Comments

dariyoosh · Accepted Answer · 2012-12-20 10:40:30Z

You can use a linked list structure for that. There can be many possible implementations (simple linkedlist, double linkedlist ,etc) if you google you can find many pages about this. Here is an example (not necessarily the most optimized form, but just to give you an idea)

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

struct datalist
{
    int value;
    struct datalist *next;
};
typedef struct datalist *linkedList;


void addToList(linkedList *param_valueList, const int param_newValue)
{
    if (*param_valueList == NULL)
    {
        linkedList newItem = (linkedList)malloc(sizeof(struct datalist));
        newItem->value = param_newValue;
        newItem->next = NULL;
        *param_valueList = (linkedList)malloc(sizeof(linkedList));
        *param_valueList = newItem;
    }
    else
    {
        linkedList newList = (linkedList)malloc(sizeof(struct datalist));
        newList->value = param_newValue;
        newList->next = NULL;

        linkedList tmpList = *param_valueList;
        while (tmpList->next != NULL)
            tmpList = tmpList->next;
        linkedList *listPtr = &tmpList;
        (*listPtr)->next =  newList;
   }
}


void printList(const linkedList param_valueList)
{
    linkedList tmpList = param_valueList;
    while (tmpList != NULL)
    {
        printf("%d\n", tmpList->value);
        tmpList = tmpList->next;
    }
 }


int main(int argc, char *argv[])
{
    int inputNmbr = 0;
    linkedList numberList = NULL;
    while (1)
    {
        printf("print a number: ");
        scanf("%d", &inputNmbr);
        if (inputNmbr > 0)
            addToList(&numberList, inputNmbr);
        else
            break;
    }

    printf("Here are the numbers you entered:\n");
    printList(numberList);

    return 0;
}

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