How to store string in a struct array?

Question

How can you store a string in a struct array? My code gave me a segmentation fault when I try this. This also happens with the integer.

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <string.h>
/* get the weight and the object */

struct term{
    char term[200]; // assume terms are not longer than 200
    double number;
};

int main(void)
{   
    struct term *new[12];
    
    char word[13]= "hello world";
    int num =123;
    strcpy(new[0]->term,word);
    new[0]->number = num;


    char word2[20]= "hello new world";
    int num2 =123;
    strcpy(new[1]->term,word2);
    new[0]->number = num2;
}

new[0] is unintialized, so of course assigning to its properties is going to error. Please turn on compiler warnings, which would have caught this easily. — SuperStormer, Mar 02 '21 at 17:31
`new` is an array of pointers to `struct term`, not an array of `struct term`. — jwdonahue, Mar 02 '21 at 17:33
@jwdonahue when I erase the pointer it says 'expression must have a pointer type' — beautiful world, Mar 02 '21 at 17:37
@beautifulworld That's an array of pointers. You never allocated memory for them to point to. — Barmar, Mar 02 '21 at 17:38
Use `new[0].term` to access struct members. The `->` is for dereferencing struct members through a pointer. — jwdonahue, Mar 02 '21 at 17:38

anastaciu · Answer 1 · 2021-03-05T10:13:52.417

2

struct term *new[12] is an array of pointers, they are uninitialized, in order to store anything there you must make your pointers point to some valid memory location, either by allocating memory for them:

struct term *new[12];

for (int i = 0; i < 12; i++)
{
    new[i] = malloc(sizeof **new);
}

Or by othewise assigning them valid struct term variables, i.e.:

struct term st;
new[0] = &st;

Anyway, for such a small array, you could just use a fixed size array, i.e. struct term new[12].

malloc is a heavy function that involves multiple system calls, if you can, you should avoid it.

For better understanding when and where you should use dynamic memory allocation take a look at these:

When and why to use malloc?

When do I need dynamic memory?

There is another problem you should address in your code, char word2[13] = "hello new world" is ill formed, the string is too large for its container. It'll need space for at least 16 chars.

Use empty bounds when assigning strings, i.e. char word2[] = "hello new world", the complier will deduce the needed size for the string, avoiding mistakes like these.

Footnote

new is a reserved keyword in C++, it's used to allocate memory and is the functional equivalent of C's malloc, though it is allowed in C, I would avoid using it to name a variable.

edited Mar 05 '21 at 10:13

answered Mar 02 '21 at 17:34

anastaciu

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so you mean add something like this? 'struct term *new = malloc(12*sizeof(struct term));' – beautiful world Mar 02 '21 at 17:41
No, you've allocated the array of pointers on the stack already. It's the pointers you need to allocate with `new[0] = malloc(sizeof(struct term));` – jwdonahue Mar 02 '21 at 17:42
@beautifulworld, for all the pointers, I edited my answer. – anastaciu Mar 02 '21 at 17:44
@anastaciu, or the OP can avoid all that hassle by simply declaring `struct term new[MAX_TERMS];` and then use dot to dereference them. I think the OP doesn't yet understand the difference between local and heap storage. – jwdonahue Mar 02 '21 at 17:46
@jwdonahue, definitely, for such a small array it's much better. – anastaciu Mar 02 '21 at 17:50
@jwdonahue, *I think the OP doesn't yet understand the difference between local and heap storage*, let's fix that, at least, begin to. – anastaciu Mar 02 '21 at 18:01
1

I can add that name `new` is not good choice. It is better to change it. – i486 Mar 05 '21 at 10:00
@i486 for sure. – anastaciu Mar 06 '21 at 22:23

How to store string in a struct array?

1 Answers1

Footnote