[Previous] [Next chapter]

Callbacks

Purpose

A callback is a user-specified function called at a specific point or points in the SLP optimization procedure, and which allows the user to intervene in the optimization process.

Synopsis

(1) procedure CbMessage(Message:string, MsgLen:integer, MsgType:integer)

(2) function CbType1:integer

(3) function CbType2(VarNum:integer):integer

(4) function CbType3(Flag:integer):integer

Arguments

Message  

Message text to be printed

MsgLen  

Length of message text

MsgType  

Type of message

4	Error
3	Warning
0,1,2	Information
negative	Program is finishing

VarNum  

Number of the variable being processed

Flag  

Feasibility flag

Return value

The procedure form of the callback is only used for xslp_slpmessage. All other forms return a value to the calling program.

0

Normal

1

Mark node as infeasible (MISLP callbacks) or terminate current SLP procedure

Example

The following example defines a callback for xslp_optnode which marks the node as infeasible (so that the branch is not worth pursuing further) if the error costs are too high.

SLPsetcallback(xslp_optnode,"MyOptNode")
  ...
function MyOptNode(Flag:integer):integer
  if getparam("xslp_errorcosts") > 1000 then
    returned := 1
  else
    returned := 0
  end-if
end-function

Further information

All callbacks are of type CbType1 except the following:

xslp_cascadevar: CbType2
xslp_itervar: CbType2
xslp_optnode: CbType3
xslp_prenode: CbType3
xslp_slpmessage: CbMessage
xslp_slpnode: CbType3

If a function of type CbType1 returns a nonzero value then generally the current part of the SLP optimization procedure is terminated as follows:

xslp_cascadeend: No effect
xslp_cascadestart: Cascading for the current SLP iteration is omitted
xslp_slpdestroy: No effect
xslp_intsol: No effect
xslp_iterend: The SLP optimization is terminated
xslp_iterstart: The SLP optimization is terminated
xslp_slpend: The SLP optimizer returns an error code
xslp_slpstart: The SLP optimization is omitted

Functions of type CbType2 receive the number of the variable which is currently being processed.
xslp_itervar returns the convergence status of the variable:

0: No change to the convergence status
<0: Variable has not converged
n >0: Variable has converged with status n

xslp_cascadevar can change the cascaded value for the variable. Its return value is ignored.

Functions of type CbType3 receive the feasibility status flag. This is normally zero (node is feasible) on entry. The functions cannot change the value of the flag, but the flag is set to a nonzero value if the function returns a nonzero value. A nonzero value for the flag means that the node has been declared infeasible and will not be examined further during the MISLP search.