# Changes from version 5.1 to version 5.5

### Main features and changes to v5.5 compared to v5.1

Why a jump from version numbers 5.1 to 5.5 ? This is done to indicate that this is more than just another update. The solver engine was revised and optimised in such a way that performance has improved considerably. Numerical stability is also better resulting in more models that can be solved. The LUSOL bfp is now also the default. In the past, the etaPFI bfp package was the default, but for larger models this leads faster to numerical instabilities and performance problems.

Overall, the v5.5 code is faster and more robust than v5.1. This robustness is for example proven by the fact that many more models can now be solved even without scaling.

The API hasn't changed very much. There are a couple of new routines and one routine has an extra argument. Some constants got new values.

- Fundamental internal change to the solver engine resulting in better performance and numerical stability. Both the LP solver and the B&B solvers are enhanced.
- Optimised MILP branch truncation, with reduced cost fixing.
- LUSOL bfp is now the default.
- Presolve is improved in functionality and performance.
- Better handling of degeneracy, with more options.
- Store and read options from a file make it easier to set options.
  See read_params and write_params
- Partial pricing for the primal simplex now works.
- Full support for xli_ZIMPL v2.0.3.
- The objective function is no longer stored as part of the constraint matrix.
- Dual-long step code is in place, but not fully activated yet.
- General code cleanup.
- Added OBJSENSE and OBJNAME headers in the free MPS format (See MPS file format).
- The MathProg xli driver has now the ability to generate a model.
- New API routines, see further.

Note the store and read options from a file. This can be a very nice new feature. lp_solve has a lot of options and with this new addition it is much easier to set these options. People can start looking for the best options that work for their models very easily and it could help to find even better default options.

#### Changed API calls

- get_basis
  - Return value is now unsigned char instead of void

- get_total_iter
  - Return value is now long long instead of long

- get_total_nodes
  - Return value is now long long instead of long

- put_abortfunc
  - Argument newctrlc is now from type lphandle_intfunc instead of ctrlcfunc

- put_logfunc
  - Argument newlog is now from type lphandlestr_func instead of logfunc

- put_msgfunc
  - Argument newmsg is now from type lphandleint_func instead of msgfunc

- set_anti_degen
  - New constants are added: ANTIDEGEN_RHSPERTURB, ANTIDEGEN_BOUNDFLIP

- set_basiscrash
  - New constant added: CRASH_LEASTDEGENERATE

- set_bb_rule
  - Constant NODE_GUBMODE was not documented.
  - New constants are added: NODE_RCOSTFIXING, NODE_STRONGINIT

- set_improve (!)
  - Constants are deleted: IMPROVE_FTRAN, IMPROVE_BTRAN, IMPROVE_SOLVE, IMPROVE_INVERSE
  - New constants are added: IMPROVE_SOLUTION, IMPROVE_DUALFEAS, IMPROVE_THETAGAP, IMPROVE_BBSIMPLEX

- set_pivoting (!)
  - Constants are deleted: PRICE_AUTOPARTIALCOLS, PRICE_AUTOPARTIALROWS, PRICE_AUTOMULTICOLS, PRICE_AUTOMULTIROWS
  - New constants added: PRICE_AUTOPARTIAL, PRICE_AUTOMULTIPLE, PRICE_HARRISTWOPASS, PRICE_TRUENORMINIT

- set_presolve (!)
  - There is a new third argument: maxloops
  - PRESOLVE_DUALS has a new value. Before it was 128, now it is 524288
  - PRESOLVE_SENSDUALS has a new value. Before it was 256, now it is 1048576
  - New constants are added: PRESOLVE_KNAPSACK, PRESOLVE_ELIMEQ2, PRESOLVE_IMPLIEDFREE, PRESOLVE_REDUCEGCD, PRESOLVE_PROBEFIX, PRESOLVE_PROBEREDUCE, PRESOLVE_ROWDOMINATE, PRESOLVE_COLDOMINATE, PRESOLVE_MERGEROWS, PRESOLVE_IMPLIEDSLK, PRESOLVE_COLFIXDUAL, PRESOLVE_BOUNDS

#### New API calls

- copy_lp
  - This routine existed before version 5, but was removed in version 5.0. Now it exists again.

- dualize_lp

- is_unbounded
  - This function is renamed. Before it was called is_free

- is_use_names

- get_columnex

- get_constr_value

- get_presolveloops

- get_rowex

- put_bb_branchfunc

- put_bb_nodefunc

- read_params

- reset_params

- set_basisvar

- set_epslevel

- set_unbounded
  - This function is renamed. Before it was called set_free

- set_use_names

- write_params

#### Removed API calls

- is_free
  - Use is_unbounded instead

- set_free
  - Use set_unbounded instead