Loop through a multidimensional array?

Question

How would I loop through a multidimensional array? Say we had something like this:

class blah
{
    public:
    blah();
    bool foo;
};

blah::blah()
{
    foo = true;
}

blah testArray[1][2];
testArray[1][0].foo = false;

How would I go about looping through testArray to find which one of foo is false?

Same way you loop through any other array, except the body of the loop will be another loop — Robert
– Robert, Commented Nov 13, 2011 at 6:08
It's pseudocode. Doesn't matter. You can even fly in pseudocode. — Lemmons
– Lemmons, Commented Nov 13, 2011 at 6:16
@Lemmons: I was only pointing it out because if you were testing with that and couldn't find the false value, that may have been your problem. — Tyler Hyndman
– Tyler Hyndman, Commented Nov 13, 2011 at 6:22

Pubby · Accepted Answer · 2011-11-14 22:31:57Z

8

This one isn't dependent on magic numbers:

#include <cstddef>
for (size_t x = 0; x < sizeof(*testArray) / sizeof(**testArray); ++x)
for (size_t y = 0; y < sizeof(testArray)  / sizeof(*testArray);  ++y) {
  if (testArray[x][y].foo == false) {

  }
}

Having x in the outer loop leads to better caching.

edited Nov 14, 2011 at 22:31

answered Nov 13, 2011 at 6:17

Pubby

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5 Comments

Keith Layne Over a year ago

+1...but maybe add #include <cstddef> and using std::size_t? It's a pet peeve of mine. What if (not likely, and not in the question) the rows are not all the same length?

Pubby Over a year ago

@keith.layne I'll add the header, but the std isn't needed. Can array types even be defined with differing rows?

Keith Layne Over a year ago

I feel dumb now, didn't know it was declared in the global namespace. I'm checking on your question.

Keith Layne Over a year ago

I guess not, but you could make an array of pointers that are arrays of different lengths, like so: int* a[2]; a[0] = new int[7]; a[1] = new int[4];. I don't think my point really pertains to anything, I'm just your local devil's advocate. I think your technique would work anyway if you adjusted a little and then ordered the loops oppositely...you'd lose your optimization though. I think that's another good question...won't most compilers reorder that loop anyway for the exact same reason?

gsamaras Over a year ago

Shouldn't the first loop use the stop condition of the second loop and vice versa?

DanZimm · Accepted Answer · 2011-11-13 06:11:12Z

6

class blah
{
    public:
    blah();
    bool foo;
};

blah::blah()
{
    foo = true;
}

int testArrayFirstLength = 1;
int testArraySecondLength = 2;

blah testArray[testArrayFirstLength][testArraySecondLength];
testArray[1][0].foo = false;


for (int i = 0; i < testArrayFirstLength; i++) {
    for (int j = 0; j < testArraySecondLength; j++) {
        if (!testArray[i][j]) {
            blah thing = testArray[i][j]
        }
    }
}

That good? Or were you looking for something else?

answered Nov 13, 2011 at 6:11

DanZimm

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Comments

Homer6 · Accepted Answer · 2011-11-13 06:12:21Z

0

int x = 0;
int y = 0;

for( x = 0; x < 1; x++ ){
    for( y = 0; y < 2; y++ ){
       if( testArray[x][y].foo == false ){
           //yeah!
       }
    }
}

answered Nov 13, 2011 at 6:12

Homer6

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Comments

Mankarse · Accepted Answer · 2011-11-13 06:13:07Z

0

for (std::size_t i(0); i != 1; ++i){
    for (std::size_t j(0); j != 2; ++j) {
        if (!testArray[i][j].foo) {
            //testArray[i][j].foo is false
            //Perform the required operation
        }
    }
}

answered Nov 13, 2011 at 6:13

Mankarse

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Comments

Thinkal VB · Accepted Answer · 2017-06-25 18:38:52Z

0

Tested in c++ +14 (using pointers, so be safe)

#include <iostream>
int main()
{
int array[2][2][2]; 
int* pointer=&array[0][0][0]; 
for(int i=0; i<2*2*2; i++)
    {
        std::cout<<*pointer<<std::endl;     
        pointer++; 
    }
}

answered Jun 25, 2017 at 18:38

Thinkal VB

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1 Comment

Steven Lu Over a year ago

You didn't use the variable i here.

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