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c++csyntaxpetsc

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?

Solution

  • 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.