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
– SuperStormer, Commented Mar 2, 2021 at 17:31
new is an array of pointers to struct term, not an array of struct term. — jwdonahue
– jwdonahue, Commented Mar 2, 2021 at 17:33
@jwdonahue when I erase the pointer it says 'expression must have a pointer type' — beautiful world
– beautiful world, Commented Mar 2, 2021 at 17:37

anastaciu · Accepted Answer · 2021-03-05 10:13:52Z

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 5, 2021 at 10:13

answered Mar 2, 2021 at 17:34

anastaciu

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8 Comments

beautiful world Over a year ago

so you mean add something like this? 'struct term *new = malloc(12*sizeof(struct term));'

jwdonahue Over a year ago

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));

anastaciu Over a year ago

@beautifulworld, for all the pointers, I edited my answer.

jwdonahue Over a year ago

@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.

i486 Over a year ago

I can add that name new is not good choice. It is better to change it.

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