Need clarification about array syntax

Question

What is the difference between declaring a 2D array in C++ like this:

int mp[3][3]={{0,2,1},
              {0,2,1},
              {1,2,0}};

And this?

int mp[3][3]={0,2,1,
              0,2,1,
              1,2,0};

Is the above an array where all 3 elements are arrays themselves while the bottom one is an array of non-array elements or are both read by the compiler as the same?

@A--C in C++. I edited to add that I need to understand it in C++ — Ishmael
– Ishmael, Commented Mar 3, 2013 at 21:45

Angew is no longer proud of SO · Accepted Answer · 2013-03-03 21:48:25Z

6

They're equivalent. The first one is a completely braced form. When the interpretation is unambiguous (such as in the second form), the standard allows eliding the braces.

answered Mar 3, 2013 at 21:48

Angew is no longer proud of SO

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5 Comments

Ishmael Over a year ago

I ask because my program was giving me different results. I'm guessing I changed something else and forgot about it.

Grijesh Chauhan Over a year ago

@Chase what is your compiler?

Ishmael Over a year ago

@Grijesh Chauhan Microsoft Visual Studio C++ 2010 Express

Angew is no longer proud of SO Over a year ago

As per [dcl.init.aggr]§10, they should be equivalent. §11 even has an example matching your case almost exactly.

Ishmael Over a year ago

@Angew Before I forget, thank you. I was able to find the real problem with my program after you clarified this for me.

Grijesh Chauhan · Accepted Answer · 2013-03-04 09:05:25Z

1

Both are same you can access elements for matrix using following loop:

for (i=0;i<3;i++)   
     for(j=0;j<3;j++)     
        printf("%d ",mp[i][j] );

One difference in when you give braces in first case then first argument can be omitted like:

int mp[][3]={{0,2,1},
              {0,2,1},
              {1,2,0}};

But C++ compiler will give you warning: missing braces around for second type of declaration.

EDIT:
As you commented: my program was giving me different results

I have written a code. working fine on C++ (gcc-4.7.2). Check here

edited Mar 4, 2013 at 9:05

answered Mar 3, 2013 at 21:53

Grijesh Chauhan

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1 Comment

Pete Becker Over a year ago

Um, some C++ compilers (and, I assume, some C compilers) will give a warning.

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