# slack variables in optimization: a different usage

17 views (last 30 days)
RPatel on 11 Oct 2017
Commented: RPatel on 27 Oct 2017
Hello, I have an optimisation problem with equalities and inequalities.
A x < b is an inequality, with x,b being a vector.
If the problem is infeasible with b, I want to allow b to change to b*, only enough for the problem to be solved and (b*-b) should be heavily punished. This corresponding cost would be k*ϵ, where k = 10^6 for example. Thus, my slack variable is ϵ = b* - b > 0.
How could I formulate this in matlab? I am using quadprog for optimization.
Thanks,
##### 0 CommentsShow -2 older commentsHide -2 older comments

Sign in to comment.

### Answers (1)

Carl on 13 Oct 2017
I don't believe this functionality is built into the quadprog function, or for the Optimization Toolbox in general. The closest thing I was able to find was the ConstraintTolerance option:
The documentation page here gives a good explanation of that tolerance value. If your problem cannot be solved with the current value of b, you can try increasing the value of ConstraintTolerance.
##### 1 CommentShow -1 older commentsHide -1 older comments
RPatel on 27 Oct 2017
Thanks Carl for your response. Please refer to my follow up... 1. An issue with this approach is that if there are multiple agents, corresponding to which I have different values of b, I would ideally like to change the value of the constraint tolerance corresponsing to the agent which causes failure of quadprog. This agent which is the reason behind quadprog returning infeasible is difficult to be found (or is it otherwise?). Thus, If I apply your approach, I will be increasing constraint tolerance for all agents, which is not really what I would want (as this would result into a huge increase in the cost). 2. Do you see this feature being updated/available to the Matlab users in near future?
Thanks :)

Sign in to comment.

### Categories

Find more on Quadratic Programming and Cone Programming in Help Center and File Exchange

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!