Assigning elements in an array

Question

The following code allocates a 10 element array of pointers to doubles.

int i=0;

double* dp[10];


for (int i = 0; i < 10; ++i)
{
    *(dp[i]) = 0.0;
}
for (int i = 0; i < 10; ++i)
{
    cout<< *dp[i]<< endl;
}

Now how do I initialize each of these doubles to 0.0.

They aren't `double`s as you said they are pointer to `double`s and you can't initialize them to 0.0 but rather `nullptr`. — 101010, Nov 08 '14 at 21:42
You need to write a loop that allocates space for each double, and initializes it. — Barmar, Nov 08 '14 at 21:43
You'll benefit greatly from using `std::vector` and/or `std::array`. — Captain Obvlious, Nov 08 '14 at 21:47
When I modify the code as above I end up with a segmentation fault -11. Anyone can explain me why. Carry on with your down votes, but I need to understand the concept. — Umar Gul, Nov 08 '14 at 22:04

Barmar · Answer 1 · 2014-11-08T22:52:56.780

1

Write a loop that allocates space for each double and initializes it:

for (i = 0; i < 10; i++) {
    dp[i] = new double;
    *(dp[i]) = 0.0;
};

Are you sure you really need an array of pointers, not just an array of doubles? The latter would be simpler:

double dp[10] = {};

The empty initializer list defaults all the elements to 0.0.

edited Nov 08 '14 at 22:52

answered Nov 08 '14 at 21:44

Barmar

Why not just `std::vector v(10, 0.0);` or `std::array arr {0.0};`? – 101010 Nov 08 '14 at 21:52
2

Because his question was about an array of pointers to doubles, not `std::vector`. – Barmar Nov 08 '14 at 21:53
Without seeing the whole of his application, I can't comment on whether he really needs to indirect through pointers. So I just answered the question as asked. – Barmar Nov 08 '14 at 21:54
Fair enough, but IMHO all she/he wants to do is to allocate an array of 10 `double`s. – 101010 Nov 08 '14 at 21:54
[`double dp[10] = {0.0};` is misleading](http://stackoverflow.com/a/14797812/560648). Prefer `double dp[10] = {};` – Lightness Races in Orbit Nov 08 '14 at 22:51
Thanks for the correction, I had the same misunderstanding as that questioner. – Barmar Nov 08 '14 at 22:53

score -1 · Answer 2 · answered Nov 08 '14 at 21:48

-1

Example with std::vector:

std::vector<double*> myArray(10);
std::fill(myArray.begin(), myArray.end(), new double(0.0));

(prefer classes from STL instead of C-style array in C++)

answered Nov 08 '14 at 21:48

vincentp

Every element will be the same, probably not what the OP is looking for. something like `std::vector>(10, std::vector(1, 0.0))` would be more idiomatic and closer to what the OP wants. – Captain Obvlious Nov 08 '14 at 22:01
Or even `std::vector>(10, {0.0})` – Captain Obvlious Nov 08 '14 at 22:04

2 Answers2