Assigning a default value to each element in an array of struct in C

Question

I need to initialize an array of structs with the same default values. This is a VERY large array, so setting each element by hand in an initializer is not feasable. Is the following code a correct and sane way to do this, or will I need to fall back on some initializer function and a for loop?

#define SIZE_OF_S1_ARR 10000 //just some arbitrary size for an example

typedef struct { char* id, char* description} S1;

/*
 * Array of structs, with each element having an id and a description 
 * which is an empty c-string
/*/
S1 s1_arr[SIZE_OF_S1_ARR] = {{ "", "" }};

I will add that this array already existed as a char array which only contained the ids as a single character. I am replacing it with the more useful struct.

Why don't you wrap access to this array and return a default value if NULL is found? — fge
– fge, Commented May 23, 2013 at 15:48
The "universal zero initializer" works: S1 s1_arr[SIZE_OF_S1_ARR] = {0}; — pmg
– pmg, Commented May 23, 2013 at 16:03
S1 s1_arr[SIZE_OF_S1_ARR] = {0}; stackoverflow.com/questions/1920430/c-array-initialization — kotlomoy
– kotlomoy, Commented May 23, 2013 at 19:02
That works in this specific case, but what about with non-empty strings (such as an id of "_")? — Ian T. Small
– Ian T. Small, Commented May 24, 2013 at 15:33

Matthias · Accepted Answer · 2014-06-11 05:00:27Z

1

In standard C there is not array initiaization other than 0, unless you specify each value separately.

However, if you use the GNU C compiler, you can use something like this:

char s1_arr[SIZE_OF_S1_ARR] = {[0 ... SIZE_OF_S1_ARR-1] = '_' };

Please note that this approach is not portable. In addition, please note that initalizing strings (i.e., pointer to char) to have the same value can be done in two ways: (possible automatically) allocate a number of stings and assign them or allocate one string and assign it to all elements

edited Jun 11, 2014 at 5:00

answered Jun 9, 2014 at 14:38

Matthias

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1 Comment

Ian T. Small Over a year ago

Though it's a bit late this does seem to be the best solution and possibly the only "real" answer to my question. It is unfortunate that it is not a portable solution.

Gung Foo · Accepted Answer · 2013-05-23 15:47:02Z

-1

It seems you are looking for the memset function.

answered May 23, 2013 at 15:47

Gung Foo

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3 Comments

Ian T. Small Over a year ago

That is helpful if I am forced to create an initialization function, however I'd rather have this set by the compiler if possible. I do understand this simply may not be a feature of the language, but I am hoping there is a way.

Gung Foo Over a year ago

look at this question: stackoverflow.com/questions/9669206/… your code would look very odd for 10000 items :)

Ian T. Small Over a year ago

I had seen that, however I was hoping against hope that there was another way. I will leave this question up while I work on an initialization function in case someone else can come up a static initialization solution.

zeyorama · Accepted Answer · 2013-05-23 16:00:31Z

-2

To initialize your S1 type

#define ARBITER 10000 // just an example number for elements

S1 *s1_arr = (S1*)malloc(sizeof(S1)*ARBITER);

id and description are initialized with NULL.

To set s1_arr->id and s1_arr->description you need also a *char variable

or just a literal string (char* a_string = "I am a String"; // This is a literal).

Hope I help you out.

answered May 23, 2013 at 16:00

zeyorama

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1 Comment

Gung Foo Over a year ago

malloc does NOT initialize the memory! for that u need to use cmalloc() man7.org/linux/man-pages/man3/malloc.3.html

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