Problem modification and re-solving¶
KN_solve() to perform an initial solve of the model,
you may wish to make some small modifications to your model and then re-solve.
This section outlines the modifications that can be made to an existing model
after an initial solve, which allow for re-solving the existing model using the same
Knitro solver instance.
Generally, as long as no structural changes beyond linear changes are made to the model,
KN_solve() can be called in succession to re-solve a model after modifications.
For example, user options, variable bounds, and
constraint bounds can be changed between calls to
having to first call
KN_free() and reload the model from scratch.
In addition, constant and linear structures in the model may be added, deleted,
or changed without having to reconstruct the model.
More extensive additions or changes to the model (such as adding quadratic structures or
callbacks) require freeing the existing
Knitro solver object and rebuilding the model from scratch.
More specifically, the following API functions may be used to modify a problem before re-solving.
The initial point may be set via calls to
KN_set_con_dual_init_values(), or similar functions. If the initial point is not set using one of these functions, the re-solve will automatically use the solution of the previously solved problem as the initial point for the next solve.
Variable types may be changed via calls to
KN_set_var_types(), or similar API functions.
Constant structure for the objective or constraints may be added, deleted or changed using the API functions
KN_chg_con_constants()and related API functions.
Linear structures for the objective or constraints may be added, deleted or changed using the API functions
KN_chg_con_linear_struct()and related API functions.
KN_add_var()may be used to add new variables that will only appear in linear terms to the model. After adding the new variables, the API functions for adding linear structures described above may be used to add new linear terms involving these variables to the objective or constraints.
KN_add_con()may be used to add new linear constraints to the model. After adding the new constraints, the API functions for adding constant or linear structures described above may be used to add constant or linear terms to these new constraints.
There are a few important points to note regarding problem modification.
The constant and linear structural changes in the model implemented through
KN_chg_*()functions are not actually updated until
KN_solve()is called. Therefore, any API functions that retrieve these problem structures will not reflect any new changes made since the last call to
KN_solve(). In contrast, typically model changes implemented through
KN_set_*()functions (such as setting variable or constraint bounds) are updated immediately.
API functions that delete problem structure can only be used to delete structure that existed in the model prior to the last call to
KN_solve(). Any new structure added since the last call to
KN_solve()cannot be deleted. In practice, there would be no reason to delete such structure, since you can just avoid adding it in the first place if it is not wanted.
API functions for changing structures are just shorthand for deleting and then adding new structures. For example, if you want to change an existing linear constraint coefficient, calling
KN_chg_con_linear_struct()is identical to calling
KN_add_con_linear_struct()on that coefficient.
Although there is not currently an API function to directly remove linear constraints (Knitro API functions work on structures, rather than constraints), a linear constraint will effectively be removed by calling the API functions
KN_del_con_linear_struct()to remove all the constant and linear structures in the constraint, and removing any existing constraint bounds (by setting the lower constraint bound to
-KN_INFINITYand the upper constraint bound to
For more details on the API functions described above, please see the callable library API
section in the Reference Manual Callable library API reference. Below we provide a simple example, demonstrating
how to modify and then re-solve a problem using the same problem context structure. More examples
may be found in the Knitro distribution
Consider the following nonlinear optimization problem from the
Hock and Schittkowski test set provided as example
max x0*x1*x2*x3 (obj) s.t. x0^3 + x1^2 = 1 (c0) x0^2*x3 - x2 = 0 (c1) x3^2 - x1 = 0 (c2)
examples/C/exampleNLP2Resolve.c and below, we show how to modify
this problem by adding, deleting, or changing linear structures in the model,
and then re-solve using the same Knitro solver instance.
The Knitro distribution comes with several C language programs in
the directory examples/C. The instructions in
examples/C/README.txt explain how to compile and run the examples.
We will not repeat the code for solving the original model above. This is covered
in the example from section Derivatives and in file
In the code segments below, we show the modifications made after the initial
First, after the initial solve, we add a new linear term to (c2), and add a new linear inequality constraint (c3) so the model becomes:
max x0*x1*x2*x3 (obj) s.t. x0^3 + x1^2 = 1 (c0) x0^2*x3 - x2 = 0 (c1) x3^2 - x1 + 0.5x3 = 0 (c2) x1 + 2x2 + x3 <= 2.5 (c3)
Then it is re-solved using the same kc structure.
/*... Solve the initial problem. ...*/ nStatus = KN_solve (kc); /*=============== MODIFY PROBLEM AND RE-SOLVE ===========*/ /** Add 0.5x3 linear term to c2 */ nVar = 3; dCoef = 0.5; error = KN_add_con_linear_struct_one (kc, 1, 2, &nVar, &dCoef); if (error) exit(-1); /** Now add a new linear constraint * x1 + 2x2 + x3 <= 2.5 (c3) * and re-solve. */ error = KN_add_con(kc, &cIndNew); if (error) exit(-1); error = KN_set_con_upbnd(kc, cIndNew, 2.5); if (error) exit(-1); /** Coefficients for 3 linear terms */ lconNewIndexVars = 1; lconNewCoefs = 1.0; lconNewIndexVars = 2; lconNewCoefs = 2.0; lconNewIndexVars = 3; lconNewCoefs = 1.0; error = KN_add_con_linear_struct_one (kc, 3, cIndNew, lconNewIndexVars, lconNewCoefs); if (error) exit(-1); /** Tell Knitro to try a "warm-start" since it is starting from the solution * of the previous solve, which may be a good initial point for the solution * of the slightly modified problem. */ error = KN_set_int_param (kc, KN_PARAM_STRAT_WARM_START, KN_STRAT_WARM_START_YES); /** Re-solve the modified problem. */ nStatus = KN_solve (kc);
Next we remove the linear term 0.5x3 from (c2), and change the linear term x1 in c(3) to be 3.5x1 so the model becomes
max x0*x1*x2*x3 (obj) s.t. x0^3 + x1^2 = 1 (c0) x0^2*x3 - x2 = 0 (c1) x3^2 - x1 = 0 (c2) 3.5x1 + 2x2 + x3 <= 2.5 (c3)
/*=============== MODIFY PROBLEM AND RE-SOLVE AGAIN ===========*/ /** Delete 0.5x3 linear term from c2 */ nVar = 3; error = KN_del_con_linear_struct_one (kc, 1, 2, &nVar); if (error) exit(-1); /** Change x1 linear term in c3 to 3.5*x1 */ nVar = 1; dCoef = 3.5; error = KN_chg_con_linear_struct_one (kc, 1, 3, &nVar, &dCoef); if (error) exit(-1); /** Re-solve the modified problem again. */ nStatus = KN_solve (kc);