Array with undefined length in C

Question

i was watching an exercise in my textbook that says: Create a C program that take from the keyboard an array with length "N".

The question is: In C language, how can i create an undefined length array?

Thank you all.

Answer 1

Do not create an array of undefined length.

After getting the needed length N, if C99 use a VLA (Variable Length Array)

int A[N];

... or allocate memory

int *A = malloc(sizeof *A * N);
...
// use A
...
free(A);

---

[Edit]

Good to add validation on N before proceeding. Example:

if (N <= 0 || N >= Some_Sane_Upper_Limit_Like_1000) return;

Answer 2

for more information about VLA in c99 The New C:Why Variable Length Arrays?

#include <stdlib.h>


int main(){
    int n;


    printf("size of array: ");
    scanf("%d",&n);
    // limit, and erro verification omited
    int f[n],i;

    printf("\nREADING INPUT\n");    
    for(i=0;i<n;i++){
        printf("\tValue of f[%d]: ", i);
        scanf("%d",f+i);
    }

    /*
    calc_max(f,n);
    calc_min(f,n);
    calc_avg(f,n);
    */
    printf("\n\nPRINTING VALUES:\n");
    for(i=0;i<n;i++){
        printf("\tValue of f[%d]= %d \n", i,f[i]);
    }
    return 0;
}

Answer 3

It is not easy to satisfy 100% your needs, but you can give it a try:

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

char *addMe(void);

int main(void){
    char *test = addMe();
    free(test);
    return 0;
}

char *addMe(void){
    unsigned int maxLength = 15;
    unsigned int i =0;

    char *name;
    int c;

    name = malloc(1);
    if(name == NULL){
        exit(1);
    }

    printf("Enter your name:>  ");

    while ((c = getchar()) != '\n' && c != EOF) {
        name[i++] = (char) c;

        if (i > maxLength) {
            printf("Names longer than %d not allowed!\n",maxLength);
            name[maxLength] = '\0';
            break;
        }else if (i < maxLength){
            name = realloc(name, maxLength + 2);
        }
    }

    name[i] = '\0';
    printf("Your name is:>  %s\nThe length is:>  %zu\n",name ,strlen(name));

    return name;
}

Output:

Enter your name:>  l
Your name is:>  l
The length is:>  1

As you probably noticed I allocated memory only for one letter, and if you need more it will be allocated to satisfy your needs. You can also with this approach to have control of a maximum size:

Enter your name:>  Michael jacksonnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn
Names longer than 15 not allowed!
Your name is:>  Michael jackson
The length is:>  15

Like I said, give it a try.