How can i automatically increase memory allocation for a struct in C?

Question

I am building a small program that takes name and age as input (stored in a struct) and spits out the output. One of the problems that I am facing is I have to enter the number of people I am going to store, something that I am sure I can solve with realloc() it's just not working. Here is what I got so far.

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

struct info
{
    int age;
    char name[30];
};

int main()
{
    struct info *Ptr;
    int i, num;

    printf("Enter number of people");
    scanf("%d", &num);

    // Allocates the memory for num structures with pointer Ptr pointing to the base address.
    Ptr = (struct info*)malloc(num * sizeof(struct info));

    for(i = 0; i < num; ++i)
    {
        printf("Enter name and age:\n");
        scanf("%s %d", &(Ptr+i)->name, &(Ptr+i)->age);
    }


    for(i = 0; i < num ; ++i)
        printf("Name = %s, Age = %d\n", (Ptr+i)->name, (Ptr+i)->age);

    return 0;
}

I have tried to realloc inside the first for loop, but it wasn't working even if it makes sense to have it there. Have also tried to convert the loop to a while loop like this:

     while(input != "stop)
    {
      allocate more memory
}

How can I use realloc to in order to skip having to enter the persons number before entering them?

Answer 1

realloc is the correct way. Just start with Ptr = NULL and num = 0 and on each input increase the number of elements by one.

Remember to limit the number of characters scanf can read, otherwise you may buffer overrun.

Also I find Ptr[i] way easier then (Ptr+i)->.

Also compare strings with strcmp not using !=. The != will compare pointers to strings, not strings themselves.

As I like reading the whole line, then scanning the line, I would do it like this:

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

struct info
{
    int age;
    char name[30];
};

int main()
{
    struct info *ptr = 0;
    size_t num = 0;

    for (;;) {
        printf("Enter name and age. If you want to stop, type only 'stop'.\n");

        char line[256];
        if (fgets(line, sizeof(line), stdin) == NULL) { 
             fprintf(stderr, "fgets error");
             exit(-1);
        }

        if (!strcmp("stop\n", line)) {
             break;
        }

        struct info tmp;
        if (sscanf(line, "%29s %d\n", tmp.name, &tmp.age) != 2) {
             fprintf(stderr, "error parsing line\n");
             exit(-1);
        }

        ptr = realloc(ptr, (num + 1) * sizeof(*ptr));
        if (ptr == NULL) { 
             fprintf(stderr, "error allocating memory!\n");
             exit(-1);
        }

        ptr[num] = tmp;
        ++num;
    }


    for (size_t i = 0; i < num ; ++i) {
        printf("Name = %s, Age = %d\n", ptr[i].name, ptr[i].age);
    }

    free(ptr);

    return 0;
}

Answer 2

If you are not sure of the no.of.elements you want to allocate and do it based on the users choice, then you can follow the below approach.

It starts with one element and the memory is reallocated as when the user wants to add new element.

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

struct info
{
  int age;
  char name[30];
};

int main()
{
    struct info *Ptr=NULL;
    int i=0, num;

    char c='Y';
    while(c=='Y'||c=='y') {

            Ptr=realloc(Ptr,(i+1)*sizeof(struct info));
            if(Ptr==NULL)
                 break;
            printf("Enter name and age:\n");
            scanf("%s %d",&Ptr[i].name,&Ptr[i].age);
            printf("Do you want to cont?\n");
            scanf(" %c",&c);
            i++;
    }
    num=i;

    for(i = 0; i < num ; ++i)
            printf("Name = %s, Age = %d\n", (Ptr+i)->name, (Ptr+i)->age);
    free(Ptr);
    return 0;
}

Answer 3

You should use a data struct like vector.

1. vector_init init vector.
2. vector_push push val to vector, if necessary, will realloc memory.
3. vector_output output the vector.

The following code could work:

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

#define INIT_SIZE 1

struct info
{
    int age;
    char name[30];
};

struct vector {
    struct info* p;
    int n;
    int index;
};

void vector_init(struct vector* ve) {
    ve->n     = INIT_SIZE;
    ve->index = 0;
    ve->p     = malloc(sizeof(struct info) * ve->n);
}

void vector_push(struct vector* ve, struct info* tmp) {
    if (ve->n == ve->index) {
        ve->n *= 2;
        ve->p = realloc(ve->p, sizeof(struct info) * ve->n);
    }
    ve->p[ve->index++] = *tmp;
}

void vector_output(const struct vector* ve) {
    for (int i = 0; i < ve->index; ++i)
        printf("Name = %s, Age = %d\n", ve->p[i].name, ve->p[i].age);
}

int main()
{
    struct vector ve;
    vector_init(&ve);

    for (;;) {
        struct info tmp;
        printf("Enter name and age:\n");
        scanf("%29s", tmp.name);
        if (strcmp(tmp.name, "stop") == 0)
            break;
        scanf("%d", &tmp.age);
        vector_push(&ve, &tmp);
    }
    vector_output(&ve);

    return 0;
}

Answer 4

To answer exactly, you can first read the input to temp variables and check if you need to stop: Break the loop if so. Or continue and reallocate the 'storage' array by increasing it's size by one and copying the values you just read to the 'storage' array.

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

struct          info
{
  int           age;
  char          name[30];
};

int             main()
{
  struct info * infos = 0;
  int           num = 0;
  char          input_name[30];
  int           input_age;

  while (1)
    {
      printf("Enter name and age:\n");
      int r = scanf("%29s", input_name);
      if (r == EOF || strcmp(input_name, "stop") == 0)
        break;
      scanf(" %d", &input_age);

      infos = realloc(infos, sizeof(struct info) * (num + 1));
      infos[num].age = input_age;
      memcpy(infos[num].name, input_name, sizeof(char) * 30);
      num++;
    }

  for(int i = 0; i < num ; ++i)
    printf("Name = %s, Age = %d\n", infos[i].name, infos[i].age);

  return 0;
}