Reference to a static 2D array?

Question

I have some 2D arrays of a static size:

double quickStats[NUM_REPETITIONS][NUM_TRIALS];
double mergeStats[NUM_REPETITIONS][NUM_TRIALS];
double bstStats[NUM_REPETITIONS][NUM_TRIALS];

Later, I want a reference to one of them:

double stats[NUM_REPETITIONS][NUM_TRIALS];

    if(i == 0) {
        stats = quickStats;
    }
    else if(i == 1) {
        stats = mergeStats;
    }
    else if(i == 2) {
        stats = bstStats;
    }

However, this does not work.

What can I do to do what I am trying to do without nested for loops to manually copy each element into the reference array?

I just need to read from stats, it is to avoid redundant code.

Thanks

I am assuming you want a pointer not reference? References have to be initialized upon declaration so you cannot assign it later on. Your best bet is using a pointer or declare the reference inside the if branches. — SwiftMango
– SwiftMango, Commented Apr 9, 2014 at 23:46

M.M · Accepted Answer · 2014-04-09 23:21:54Z

2

Try this:

#define NUM_REPETITIONS 10
#define NUM_TRIALS 10
double quickStats[NUM_REPETITIONS][NUM_TRIALS];
double mergeStats[NUM_REPETITIONS][NUM_TRIALS];
double bstStats[NUM_REPETITIONS][NUM_TRIALS];

int main(){
    double (* ptr)[NUM_TRIALS] =quickStats;
}

edited Apr 9, 2014 at 23:21

M.M

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answered Apr 9, 2014 at 23:18

merlin2011

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2 Comments

M.M Over a year ago

Explaining a bit more for OP; with this syntax you would then access elements via ptr[3][4]. You can also point to lists of different numbers of repetitions, although the number of trials must remain fixed.

merlin2011 Over a year ago

@MattMcNabb, Thanks for the edit and the comment. I was using Vim's autocomplete and did not pay enough attention. :)

David G · Accepted Answer · 2014-04-09 23:31:44Z

0

Create a pointer to it like this:

double (*stats)[NUM_REPETITIONS][NUM_TRIALS];

Since your dimensions are known at compile time you can use std::array<> which is more versatile and readable:

std::array<std::array<double, NUM_TRIALS>, NUM_REPETITIONS> stat;

edited Apr 9, 2014 at 23:31

answered Apr 9, 2014 at 23:16

David G

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1 Comment

WhozCraig Over a year ago

That reference should be assigned to something, right ? Given the OP's question an only-slightly-wicked stack of ternary expressions would work.

SwiftMango · Accepted Answer · 2014-04-09 23:50:15Z

0

References have to be initialized upon declaration so you cannot assign it later on. Your best bet is using a pointer or declare the reference inside the if branches.

constexpr unsigned int N = 100;

double a[N][N] = {0};

/* Pointer. In your case, N should be NUM_TRAILS here */
double (*a_ptr)[N] = a;


/* Specified Reference Type */
double (&a_ref)[N][N] = a;

/* C++11 Auto Reference */
auto &a_autoref = a;

Test here: Coliru

answered Apr 9, 2014 at 23:50

SwiftMango

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