C++ create array with dynamic size

Question

How can I create a array with dinamic size like this:

int sentLen = sentences.size();

double a[sentLen][sentLen];

for (int i = 0; i < sentLen; i++) 
{
    for (int j = 0; j < sentLen; j++)
    {
        a[i][j] = somefunction(i, j);
    }
}

My research led me to malloc which isn't recommended or other too complicated methods. After I realised that size must be constant, I tried using unordered_map, and I have tried the following:

std::unordered_map <int, int, double> a;


for (int i = 0; i < sentLen; i++) 
{
    for (int j = 0; j < sentLen; j++)
    {
        a.insert({ i, j, somefunc(i, j) });
    }
}

but still unsuccessful.

It depends on what you mean "dynamic size". Also there is nothing wrong with your first code block. — Daniel
– Daniel, Commented Apr 27, 2014 at 2:42
@Daniel I'm getting Expressions must have a constant value with the first code. — Deepsy
– Deepsy, Commented Apr 27, 2014 at 2:45
What compiler and version are you using? Older compilers do not support variable length arrays. If your compiler doesn't support it, minimal research is required to find the answer. — Daniel
– Daniel, Commented Apr 27, 2014 at 2:48

aschepler · Accepted Answer · 2014-04-27 02:40:55Z

1

You don't really want to use arrays.

std::vector<std::vector<double>> a{
   sentLen, std::vector<double>{ sentLen, 0.0 } };

for (int i = 0; i < sentLen; ++i)
{
    for (int j = 0; j < sentLen; ++j)
    {
        a[i][j] = somefunc(i, j);
    }
}

answered Apr 27, 2014 at 2:40

aschepler

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5 Comments

Deepsy Over a year ago

Thanks for the idea. Tho seems there is syntax error, because I got it running like std::vector<std::vector<double>> a(sentLen, std::vector<double>(sentLen, 0.0));

Daniel Over a year ago

Based on his first example, I don't believe the OP really needs any of the functionality of a vector, other than being able to declare it of variable length. He should use array pointers and new the memory himself. Using a vector of vectors is overkill.

David G Over a year ago

@Deepsy Put a space between the end brackets > >.

aschepler Over a year ago

Using the new operator is generally a bad idea.

aschepler Over a year ago

The {} initialization is a C++11 feature. Your post looked like you were attempting to use it. If you use g++ or clang++, try the -std=c++11 switch.

David G · Accepted Answer · 2014-04-27 02:50:54Z

1

You're getting an error because you can't use variables as static array sizes. They must be known at compile time. You have to allocate dynamically or use a vector instead.

answered Apr 27, 2014 at 2:50

David G

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