CODEWITHSUNDEEP
c++csyntaxpetsc
Michael
Michael

Reputation: 7809

what is the role of the comma in below statements?

Currently reading into PETSc when I came up to this syntax in C/C++:

PetscInt i, n = 10, col[3], its;
PetscScalar neg_one = -1.0, one = 1.0, value[3];

I do not understand the meaning of the commas here. Has it to do with tuples? Or is there something overloaded?

Upvotes: 2

Views: 121

Answers (2)

unwind
unwind

Reputation: 399881

That's just declaring multiple variables of the same types.

It's like

int a, b;

The first line declares four variables of the type PetscInt, called i, n (which is initialized to 10), the array col[3] and finally its. The second line declares three variables of the type PetscScalar.

So this:

PetscInt i,n = 10,col[3],its;

is the same as:

PetscInt i;
PetscInt n = 10;
PetscInt col[3];
PetscInt its;

Some find the original way shorter, easier to type, and also nice since it shows that the variables share (part of) the same type. Some find it confusing and/or error-prone, this is subjective of course but I felt I should mention it to kind of motivate why you often find code like this.

Upvotes: 12

Sourav Ghosh
Sourav Ghosh

Reputation: 134346

The commas here are just to declare multiple variables of the same type in a single line statement. You may very well break them into one individual line, each, like

PetscInt i;
PetscInt n = 10;
PetscInt col[3];
PetscInt its;

While both are valid and correct syntax, some of us prefer to use the broad version anyways, as it gives a better readability (IMHO) and avoid any possible confusion while having a pointer type or storage-class specifier associated with the type.

For example, for the shorthand

int * p, q;

is the same as 

int *p;
int q;   //not int *q

q is not a pointer type there.

Upvotes: 4

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