CODEWITHSUNDEEP
c++cmemorystructmemcpy
Greg Witczak
Greg Witczak

Reputation: 1684

C/C++ fill struct array with one value

I'd like to fill array of MyStruct with same value. How can it be done in fastest and simplest way? I'm operating on rather low-level methods, like memset or memcpy.

edit: std::fill_n indeed complies and works fine. But it's C++ way. How can it be done in pure C?

struct MyStruct
{
    int a;
    int b;
};

void foo()
{
    MyStruct abc;
    abc.a = 123;
    abc.b = 321;

    MyStruct arr[100];
    // fill 100 MyStruct's with copy of abc
    std::fill_n(arr, 100, abc); // working C++ way

    // or maybe loop of memcpy? But is it efficient?
    for (int i = 0; i < 100; i++)
        memcpy(arr[i],abc,sizeof(MyStruct));
}

Upvotes: 1

Views: 14267

Answers (6)

silver takana
silver takana

Reputation: 186

here is how i did it in c:

#include <stdio.h>

struct MyStruct{
    int a;
    int b;
};

/*fill the array with a specified element.*/
void fillArray(void* array, int arraySize, void* elem, int elemSize){
    for(int i = 0; i < arraySize; i++){
        memcpy((char*)array + i * elemSize, elem, elemSize);
    }
}

int main(){
    struct MyStruct array[10];
    struct MyStruct elem;
    elem.a = 123;
    elem.b = 321;
    fillArray(array, 10, &elem, sizeof(struct MyStruct));
    for(int i = 0; i < 10; i++){
        printf("%i, %i\n", array[i].a, array[i].b);
    }
    return 0;
}

I create a function called fillArray which uses memcpy to copy a specified element into each slot of the array. I now just put the corresponding element into the function parameter and print it out.

Upvotes: 0

Lazureus
Lazureus

Reputation: 461

My guess in the C99 would be below code:

#include <stdio.h>

typedef struct MyStruct
{
    int a ;
    int b ;
} MyStruct_t;

const MyStruct_t abc = 
{
    .a = 0,
    .b = 321, 
};

void main(void)
{
    int i = 0 ;

    MyStruct_t arr[100] = {0} ;

    for(i=0 ;i <sizeof(arr)/sizeof(arr[0]);i++)
    {
        arr[i] = abc ;
    }
}

In my opinion this is cleanest, safest solution.

Upvotes: 0

ugoren
ugoren

Reputation: 16441

Not pure C, but GCC allows you to do it quite nicely:

MyStruct arr[100] = { [ 0 ... 99 ] = { .a = 123, .b = 321 } };

If you need pure C, I'd follow Tri-Edge Al's answer.

Upvotes: 0

Loki Astari
Loki Astari

Reputation: 264391

The following should work in C

MyStruct arr[100] = {
   {123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},
   {123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},
   {123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},
   {123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},
   {123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},
   {123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},
   {123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},
   {123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},
   {123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},
   {123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321},{123,321}};

Upvotes: 2

Tri-Edge AI
Tri-Edge AI

Reputation: 340

for(int i = 0; i < 100; i++)
{
    arr[i] = abc;
}

Fastest and cleanest. The optimizer will most likely work it's magic too.

Upvotes: 2

LihO
LihO

Reputation: 42083

Be careful to type names of your types correctly (it's case sensitive) and don't forget the semicolon after the definition of your struct, apart from these, your program should compile with no problems:

#include <iostream>
#include <algorithm>

struct MyStruct
{
    int a;
    int b;
}; // <------------- HERE

int main() {
    MyStruct abc;
    abc.a = 123;
    abc.b = 321;

    MyStruct arr[100];
    std::fill_n(arr, 100, abc);

    std::cout << arr[99].b;
}

outputs 321.

"How can it be done in fastest and simplest way?"

The simplest way would probably be using std::vector and its appropriate constructor instead:

#include <vector>

void foo()
{
    MyStruct abc;
    abc.a = 123;
    abc.b = 321;

    std::vector<MyStruct> vec(100, abc);
    ...
}

Upvotes: 8

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