For large models, it can sometimes be helpful to see summaries of nonzero counts of the columns or rows of the constraint matrix. This kind of display is known as a histogram. There are two commands for displaying histograms: one for columns, one for rows.

For the small example in this tutorial, the column histogram looks like this:

Column counts (excluding fixed variables):

    Nonzero Count:   2
Number of Columns:   3

It tells you that there are three columns each having two nonzeroes, and no other columns. Similarly, the row histogram of the same small problem looks like this:

Row counts (excluding fixed variables):

 Nonzero Count:   3
Number of Rows:   2

It tells you that there are two rows with three nonzeroes in each of them.

Of course, in a more complex model, there would usually be a wider variety of nonzero counts than those histograms show. Here is an example in which there are sixteen columns where only one row is nonzero, 756 columns where two rows are nonzero, and so forth.

Column counts (excluding fixed variables):
Nonzero Count:      1   2     3    4     5    6  15  16
Number of Columns: 16 756  1054  547   267  113   2   1

If there has been an error during entry of the problem, perhaps a constraint coefficient having been omitted by mistake, for example, summaries like these, of a model where the structure of the constraint matrix is known, may help you find the source of the error.