Re: [Help-glpk] Got a wrong order of column by lpx_get_mat_row(lp, j, ndx, val)?

[Andrew Makhorin]

>Dose anyone meet this problem:
>
>for example  in jth row :  
>a*x-y+b*z=0;
>
>by len=lpx_get_mat_row(lp, j, ndx, val), I found 
>val=[-1,a,b], that mean the order of column is y,x,z.
>
>I added two new columns to this row like:
>
>ndx[len+1]=w, ndx[len+2]=v, val[len+1]=d, 
>val[len+2]=e, len+=2
>
>lpx_set_mat_row(lp,j,len,ndx,val)
>
>then I thought the jth row should be 
>a*x-y+b*z+d*w+e*v=0, but when I tried len=lpx_get_mat_row(lp, j, ndx, val) 
>again,
>
>I found it's d*w+e*v+a*x-y+b*z=0.
>
>I wonder whether the order of columns in row is 
>fixed and how the order of columns in a row is arranged there.

The order of (column) indices in sparse vectors returned by
lpx_get_mat_row (as well as other similar api routines) is *undefined*
in the sense that there is no simple way to predict that order.
However, there is the routine lpx_order_matrix, which allows ordering
the row/column lists of the constraint matrix (it is not documented, so
see the comments to this routine in glplpx1.c).