The Matrix Alteration (.alt) File
The Alter File is an ASCII file containing matrix revision statements, read in by use of the XPRSalter (ALTER) command, and should be named problem_name.alt by default. Each statement occupies a separate line of the file and the final line is always empty. The statements consist of identifiers specifying the object to be altered and actions to be applied to the specified object. Typically the identifier may specify just a row, for example R2, specifying the second row if that name has been assigned to row 2. If a coefficient is to be altered, the associated variable must also be specified. For example:
RRRRRRRR CCRider 2.087
changes the coefficient of CCRider in row RRRRRRRR to 2.087. The action may be one of the following possibilities.
Changing Upper or Lower Bounds
An upper or lower bound of a column may be altered by specifying the special 'rows' **LO and **UP for lower and upper bounds respectively.To change the objective coefficient of a column use the string **OBJ. For example, to change the lower bound (to 1.234), upper bound (to 5.678) and the objective (to 1234.0) of column x___0305 would look like:-
**LO x___0305 1.234 **UP x___0305 5.678 **OBJ x___0305 1234.0
Changing Right Hand Side Coefficients
Right hand side coefficients of a row may be altered by changing values in the 'column' with the name of the right hand side.
Changing Constraint Types
The direction of a constraint may be altered. The row name is given first, followed by an action of **NTx, where x is one of:
| N | for the new row type to be constrained; |
| L | for the new row type to be 'less than or equal to'; |
| G | for the new row type to be 'greater than or equal to'; |
| E | for the new row type to be an equality. |
| R | for the new row type to be a range row. |
Note that N type rows will not be present in the matrix in memory if the control KEEPNROWS has been set to zero before XPRSreadprob (READPROB).
When turning a row to ranged row, a third entry, the range, is expected to be defined following **NTR. The rules for changing a row to a ranged row follow that of XPRSchgrhsrange.
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