C: Problem with initializing array values to zero

Question

I'm trying to construct the following 14*14 array i C: [I 0; 0 -I], that is a 7*7 identity matrix upper left, minus the identity lower right and zeros otherwise.

This is the method:

#define DIM 7

 double S[2*DIM][2*DIM];

 for(i = 0; i < DIM; i++){
     for(j = 0; j < DIM; j++){
         if(i == j){
             S[i][j] = 1.0;
             S[i+7][j+7] = -1.0;
         }
         else{
             S[i][j] = 0.0;
         }
     } 
 }

This works fine for all the diagonal elements; however, some elements of the array get initialized to crazy values; for example, 13,6 gets initialized to

68111186113812079535019899599437200576833320031036694798491976301968333351950125611739840800974137748034248687763243996679617222196278187875968953700681881752083957666277350377710107236511681624408064.000000

This seems to be happening consistently (at least thrice) to entries 11,13, 12,9, 12,10, 13,12 and 13,6.

Can anybody tell me what's at play here or provide an alternative solution?

Cheers!

EDIT: The weird entries aren't consistent.

EDIT2: Typo: 13,12, not 13,15

How are you getting 13,15 with that?

cwallenpoole
– cwallenpoole

2011-09-15 13:44:04 +00:00
Commented Sep 15, 2011 at 13:44 — cwallenpoole
– cwallenpoole, Commented Sep 15, 2011 at 13:44

Ernest Friedman-Hill · Accepted Answer · 2011-09-15 13:44:50Z

4

Your loop only covers the upper left quadrant, so the off-diagonal elements in the other quadrants are not initialized and contain garbage. Your loop should go up to 2*DIM for each dimension, so that the off-diagonal elements are zeroed, and then your conditional for the diagonal elements should just be a little more complex to decide which value to set the diagonal element to.

Note that [13, 15] is entirely outside of this array!

answered Sep 15, 2011 at 13:44

Ernest Friedman-Hill

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1 Comment

trolle3000 Over a year ago

Of course. I'm not feeling too smart now, let me tell you that ;-)

pmg · Accepted Answer · 2011-09-15 13:49:06Z

4

You can initialize the whole array with zeros, then set only the diagonal

double S[2*DIM][2*DIM] = {0};
for (i = 0; i < DIM; i++) {
    s[i][i] = 1;
    s[i + DIM][i + DIM] = -1;
}

answered Sep 15, 2011 at 13:49

pmg

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6 Comments

Paul R Over a year ago

ITYM double S[2*DIM][2*DIM] = { { 0 } };

pmg Over a year ago

@Paul: your version may be more explicit, but = {0} "works" for initializing anything -- eg int k = {0};, struct whatever obj[2][15] = {0};, .... I like to call it the universal zero initializer :-)

Paul R Over a year ago

It may work, but it generates warnings from gcc with -Wall: warning: missing braces around initializer warning: (near initialization for ‘S[0]’)

pmg Over a year ago

Yep ... gcc can be too helpful sometimes. I have -Wno-missing-braces added to my gcc alias because of that.

pmg Over a year ago

6.7.8/19 in n1256.pdf: "... all subobjects that are not initialized explicitly shall be initialized implicitly ..."

|

michael_wu · Accepted Answer · 2011-09-15 13:44:21Z

2

You are never writing to s[i][j] for i != j and i >= DIM or j >= DIM. If your array has automatic storage (is "local") it contains arbitrary init values.

answered Sep 15, 2011 at 13:44

michael_wu

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4 Comments

Ernest Friedman-Hill Over a year ago

It would contain garbage if it were malloced as well.

michael_wu Over a year ago

Yes, but it isn't malloced: double S[2*DIM][2*DIM];

Ernest Friedman-Hill Over a year ago

My point is that however it were to be allocated -- on the stack, or on the heap -- it contains garbage, so that most of your second sentence is redundant. The fact that it's automatic is immaterial.

michael_wu Over a year ago

Sorry, I was taught variables (and arrays) with static storage are initialized to 0 automatically. Is this not true?

brain · Accepted Answer · 2011-09-15 13:44:02Z

1

I would say most of the elements that are outside 7x7 will not be initialized at all unless i == j (diagonal elements).

What do you want to initialize them to?

answered Sep 15, 2011 at 13:44

brain

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Comments

moteutsch · Accepted Answer · 2011-09-15 13:46:40Z

1

That's because your not initializing those elements. Here is some better code:

#define DIM 14

double S[DIM][DIM];

for (i = 0; i < DIM; i++) {
    for (j = 0; j < DIM; j++) {
        if (i == j) {
            if (i < 7) {
                S[i][j] = 1.0;
            } else {
                S[i][j] = -1.0;
            }
        } else {
            S[i][j] = 0.0;
        }
    } 
}

answered Sep 15, 2011 at 13:46

moteutsch

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Comments

Jodaka · Accepted Answer · 2011-09-15 13:44:25Z

0

You never initialize values with an i or j between DIM + 1 and 2*DIM. So when you look at a value stored in one of those positions, you see whatever was there before that space was accessed by your matrix.

answered Sep 15, 2011 at 13:44

Jodaka

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