C++ Vector<int> array initialization

Question

Given the following code:

vector<int> A[1000000];
for( int i = 0; i < 1000000; i++ ){
    A[i].clear();
}

I am running it on an automated terminal that has test cases run with my code, so I can't get full debug messages. I don't get any errors, and the program seems to run fine with single test cases. However when I run against the full set I pass/fail test cases randomly (one time I may pass the first 3, another I pass everything but them, another I only pass the second, etc). I want to make sure this kind of initialization would get rid of "undefined" values on the array (eg char *buffer = new char[100]; buffer[0] = 0;) so that it is not a cause of random crashes.

sizeof(A) == 24000000 (~23MB), sizeof(int) == 4. I only add up to 1 million integers to these vectors (in total), so in the worst tests, each vector could be a one element vector, or a single vector could have 1 million integers, while the rest remains empty.

Do I have to call a destructor? I assumed not since I never called new, but I'm new to STL. Finally it could be not an issue with my code but the tester, but still, want to make sure this is fine on my side.

This code is OK, there could be a bug elsewhere. Or a stack overflow — M.M
– M.M, Commented Jan 10, 2017 at 5:10
Have you tried, vector<int> A(1000000,0);? This will fill the vector with zeros. — tinstaafl
– tinstaafl, Commented Jan 10, 2017 at 5:13
@tinstaafl that is a single vector, OP code makes a million vectors (which I assume is intentional) — M.M
– M.M, Commented Jan 10, 2017 at 5:17
check for sizeof(int) in your compiler, if it's 2 byte then the for loop is culprit. — VivekD
– VivekD, Commented Jan 10, 2017 at 5:19
@M.M Yes. it is 1 million vectors (that will hold 1 million ints between all of them, so that shouldnt be the cause of an out of memory), not a single 1 million element vector sadly. I'll guess its the tester code then... — gia
– gia, Commented Jan 10, 2017 at 5:20

cthl · Accepted Answer · 2017-01-10 05:27:11Z

6

I think this is a stack overflow, as M.M suggested. The line

std::vector<int> A[1000000];

creates a million vector objects on the stack, which is too much. On my machine, the example failed for one million vectors, but it worked fine for one thousand.

answered Jan 10, 2017 at 5:27

cthl

4092 silver badges9 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

gia Over a year ago

Are you sure? sizeof(A) returns 24000000 to me, thats only 23MB I will only add 1 million integers to these vectors (in total), so each vector could be a one element vector, or a single vector could have 1 million, while the rest remains empty.

Ari0nhh Over a year ago

Default stack size in the Visual Studio for example is 1 MB. Create your array of vectors on the heap using new operator instead.

M.M Over a year ago

don't use new ... vector of vectors, or unique_ptr of vector array would both be preferable

cthl Over a year ago

23MB is certainly too big for the stack, so you have to allocate on the heap. I would try to avoid using new in this case. Instead, you could use a vector of vectors: std::vector<std::vector<int> > A(1000000);.

M.M Over a year ago

@cthl well it is possible to modify stack size to be in the gigabytes. Seems likely to exceed common compilers' default stack size though.

|

VivekD · Accepted Answer · 2017-01-10 10:22:34Z

0

In place of for loop use below code

std::fill(A.begin(), A.end(), 0);

answered Jan 10, 2017 at 10:22

VivekD

3281 silver badge11 bronze badges

Collectives™ on Stack Overflow

C++ Vector<int> array initialization

2 Answers 2

7 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

7 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related