set all elements of 2-D array to zero

Question

int a[100][100]; //globally declared array
int main()
{
    while(testcases--)
    {
        a[100][100]={0};
        //rest of code
    }
    return 0;
}

This code works only for first testcase. For next testcases, it doesn't set the elements to zero.

I can't afford to run a loop to run a loop and iterate all the elements. I want to perform this task in O(1). Is it possible?

You don't need to do anything. The array is already zero-initialized. — juanchopanza, Feb 12 '17 at 13:44
Yes, I know that. But for 2nd testcase (and so on) the array is not zero-initialised. — rishi_07, Feb 12 '17 at 13:47
You are not initializing it to zero but writing 0 in the 101X101 place (which is out of bounds) — gbehar, Feb 12 '17 at 13:50
Oh I see...for 1-D array a[100]={0} works. What is the syntax for 2-D array? — rishi_07, Feb 12 '17 at 13:52
@rishi_07 How can you "know" and then say it isn't so? Do you think your compiler has a bug? Or maybe there's a bug in your code or your reasoning. — juanchopanza, Feb 12 '17 at 13:53
It doesn't work for 1D arrays. (You can use it for all arrays when *initializing* them when they're created. What you have is an *assignment*.) — HolyBlackCat, Feb 12 '17 at 13:53
You can only "initialize" a thing once, so maybe you mean to ask how to set all the elements of a 2D array to zero. — juanchopanza, Feb 12 '17 at 13:58
@juanchopanza : But array is not already zero initialized it has to be done explicitly by something like int a[100][100] = {0} — PapaDiHatti, Feb 12 '17 at 14:10
@Kapil No. I already said it is zero initialized. Please do some research. — juanchopanza, Feb 12 '17 at 14:22

score 1 · Answer 1 · answered Feb 12 '17 at 13:57

1

You are using the same array between runs and you want to reset it to zero. You cannot avoid iterating on all the cells and setting them to zero. Even when initializing to zero on decleration, there's no such thing as promising O(1). The way the compiler initializes is up to the compiler.

answered Feb 12 '17 at 13:57

gbehar

1,229
10
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Okay, any alternative approach? – rishi_07 Feb 12 '17 at 14:00
1

I would say, give up on the O(1) requirement, and reset the cells in 2 nested loops – gbehar Feb 12 '17 at 14:08

HolyBlackCat · Answer 2 · 2017-02-12T16:05:09.450

1

First, you seem to have a slight misunderstanding of terms.

This is an initialization:

int a = b;

This is an assignment:

int a;
a = b;

You can initialize an array to zeroes using following code:

int array[10][20] = {0};

Or:

int array[10][20] = {};

Or:

int array[10][20] {};

Keep in mind that global (and static) arrays are zero-initialized by default.

You can fill array with zeroes after it was created like this:

for (int i = 0; i < size; i++)
    for (int j = 0; j < size; j++)
        array[i][j] = 0;

Or:

for (int (&a)[100] : array) // or  for (auto &a : array)
    for (int &b : a) // or  for (auto &b : a)
        b = 0;

Or:

std::memset(array, 0, sizeof array);

P.S.

You can't fill an array with zeroes at runtime for O(1).

edited Feb 12 '17 at 16:05

answered Feb 12 '17 at 14:04

HolyBlackCat

78,603
9
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1

Worth pointing out that the array is global, so it is already zero initialized. – juanchopanza Feb 12 '17 at 14:23
@juanchopanza I agree, fixed. – HolyBlackCat Feb 12 '17 at 16:05

AnT stands with Russia · Answer 3 · 2017-02-12T16:19:57.693

Your array is zero-initialized "for the first testcase" simply because it is declared with static storage duration. And such array with static storage duration begins its life zero-initialized.

Your

a[100][100]={0};

line has nothing to with it. This is actually a mere a[100][100] = 0, which is writing a single 0 into a non-existing (out of bounds) element of your array. (The behavior is undefined).

If you wan to reinitialize your array with zeros on each iteration, you have to do it either manually or using a library-level solution, since there is no core-language-level feature that would do it for you. In your case (an array of integers) you can even use the old-fashioned

std::memset(a, 0, sizeof a);

You can also use something clever-but-inefficient like

decltype(a) zeros{};
std::swap(a, zeros);

on each iteration, but it's probably not worth it.

set all elements of 2-D array to zero

3 Answers3

P.S.