Unclear sentence in examples

[infinity77]

Dear HiGHS community,

mi was thinking of using HiGHS via the C interface and I was browning through the examples folder. I got to this sentence:

// Note that a_start[0] must be zero

You can find it here:

HiGHS/examples/call_highs_from_c.c

Line 41 in 8c76346

// Note that a_start[0] must be zero

Why?

The documentation states:

“a_start is an array of length [num_col] containing the starting index of each column in a_index.”

What if I do not have any non-nonzero elements in column 0? I’m confused.

My apologies if it’s a stupid question, thank you for any clarification.

[jajhall]

If you have no nonzeros in column 0, then you set start_[1] to zero. Then a loop

for (int i=start_[col], i<start_[col+1], i++)

is vacuous for col=0

[infinity77]

My apologies, I had misunderstood the matrix format that HiGHS is expecting, which is kind-of CSC/CSR. "Kind of" because my understanding of the CSC format says:

a_start: The pointers to the column offsets array of length number of columns + 1 that represents the starting position of each column in the columns and values arrays.

While in HiGHS it seems to me that a_start has one element less:

a_start is an array of length [num_col] containing the starting index of each column in a_index

Unless of course I am misunderstanding again, which is quite possible. Will HiGHS complain if a_start has num_col+1 elements or will it cut away the last one? Hopefully it won't complain, I am ok if it silently removes the last element.

[odow]

The format is CSC except that HiGHS does not check a_start[num_col], so the length need only be num_col. If you pass an array with num_col+1 elements HiGHS will ignore the last entry.

[jajhall]

Internally (in C++) HiGHS uses std::vectors to store the matrix as pure CSC and CSR with num_col+1 start_ values (and the number of nonzeros not held explicitly). However, since C only passes pointers, we felt that it was clearer to pass the number of nonzeros explicitly, and not require a num_col+1st value of start_.