v[long long int] in C++

Question

Why can't I declare a value like long long int v[100000000]? There's no error while compiling but when I have to input my values it simply breaks...

I have the following code:

#include <iostream>
using namespace std;
int main ()
{
    int n, i, poz, ok;
    long long int a, v[10000000], aux;
    cout << "v[0]= "; cin >> v[0];
    cout << "n= "; cin >> n;
    //cout << v[0] << " ";
    for (i=1; i<n; i++)
    {
        v[i]=((v[i-1]%31337)*(31334%31337))%31337;
        //cout << v[i] << " ";
    }
    //cout << endl;
    a=v[n-1];   
    do
    {
        ok=0;
        for (i=0; i<n-1; i++)
            if (v[i]>v[i+1])
            {
                aux=v[i];
                v[i]=v[i+1];
                v[i+1]=aux;
                ok=1;
            }
    }while (ok==1);
//  for (i=0; i<n; i++)
    //{
    //  cout << v[i] << " ";
    //}
    //cout << endl;
    for (i=0; i<n; i++)
        if (v[i]==a)
            poz=i+1;
    cout << poz;
    return 0;
}

And I have to input the values: [11863, 1661427]. Any ideas what should I do to input those values?

"it simply breaks" means what exactly? if you get an error code post it here. Also have you tried debugging your code? — UnholySheep, Oct 18 '14 at 16:00
Your program should only allocate enough memory for the amount of data requested by the user. Can you imagine if every program allocated 76mb of memory "just in case"? — Neil Kirk, Oct 18 '14 at 16:12
@NeilKirk: Modern OSes deal with allocated-but-not-committed memory. — Ben Voigt, Oct 18 '14 at 16:16
@BenVoigt Using a vector will set all integers to value 0 and could feasibly force the OS to actually commit the memory. — Neil Kirk, Oct 18 '14 at 16:17
@NeilKirk: Right, something that leads people to keep requesting a dynamically sized array that doesn't do so. — Ben Voigt, Oct 18 '14 at 16:19
@NeilKirk: Yup, if you also use `new T[n]` and not `new T[n]()` — Ben Voigt, Oct 18 '14 at 16:24

score 4 · Answer 1 · answered Oct 18 '14 at 16:00

4

The C++ Standard has an Appendix dedicated to "Implementation Quantities", explained as

Because computers are finite, C++ implementations are inevitably limited in the size of the programs they can successfully process.

You've exceeded your implementation's limit on the total size of variables of automatic storage duration. (Typically knows as "stack size").

You might try using dynamic allocation instead:

std::vector<long long int> v(10000000);

Dynamic allocations are constrained also, but the limit typically depends on the size of the largest fragment of address space, which is usually far far larger than the stack size.

Static storage duration as Vlad suggests also changes which limit applies, but there tend to be severe drawbacks to large objects of static storage duration (such as the executable file being several gigabytes on disk and taking minutes to load).

answered Oct 18 '14 at 16:00

Ben Voigt

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You don't comment on the fact 10000000 integers (76mb) shouldn't be allocated in the first place. – Neil Kirk Oct 18 '14 at 16:14
2

@NeilKirk: I guess you mean "This problem doesn't require allocation of 10000000 integers". Because some problems certainly do. – Ben Voigt Oct 18 '14 at 16:15
Number of integers is input by the user. Why not allocate the amount requested? – Neil Kirk Oct 18 '14 at 16:16
1

@NeilKirk: We can recommend a better sort routine while we're at it. This isn't code review. – Ben Voigt Oct 18 '14 at 16:18
No but we do have a professional responsibility to point out any serious flaws. – Lightness Races in Orbit Oct 18 '14 at 18:20

Vlad from Moscow · Answer 2 · 2014-10-18T16:08:13.923

0

This array is allocated in the stack the memory of which is usually very limited. Use instead

static long long v[10000000];

Or it would be better to use standard container std::vector<long long>. Moreover as it is seen from your code the actual size of the array depends on the number entered by the user. So it would be indeed better to use std::vector<long long>

For example

cout << "n= "; cin >> n;
std::vector<long long> v( n );

or

cout << "n= "; cin >> n;
std::vector<long long> v;
v.reserve( n );

edited Oct 18 '14 at 16:08

answered Oct 18 '14 at 16:02

Vlad from Moscow

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Have you ever linked a program with huge static data? – Ben Voigt Oct 18 '14 at 16:04
2

@Ben Voigt I do not write programs. I only give advices.:) – Vlad from Moscow Oct 18 '14 at 16:09
1

Good advice comes from experience. – Lightness Races in Orbit Oct 18 '14 at 18:20

v[long long int] in C++

2 Answers2