R 中的 agrep max.distance 参数

Question

I need some help with the specific arguments of the agrep package in R.

In terms of cost, all, insertions, deletions and substitutions each have a "maximum number/fraction of substitutions" integer or fraction input parameter.

Ive read the documentation on it, but I still cannot figure out some specifics:

• What is the difference of a "cost=1" and "all=1"?
• How is a decimal interpreted, such as "cost=0.1", "inserts=0.9", "all=0.25", etc.?
• I understand the basics of the Levenshtein Distance, but how is it applied in terms of the cost or all arguments?

Sorry if this is fairly basic, but like I said, the documentation I have read on it is slightly confusing.

Thanks in advance

Answer 1

Not 100% certain, but here is my understanding:

• in max.distance , cost and all are interchangeable if you don't specify a costs argument (this is the next argument); if you do, then cost will limit based on the weighted (as per costs ) costs of insertion/deletion/substitutions you specified, whereas all will limit on the raw count of those operations
• The fraction represents what fraction of the number of characters in your pattern argument you want to allow as insertion/deletions/substitutions (ie 0.1 on a 10 character pattern would allow 1 change). If you specify costs , then it is the fraction of # of characters in pattern * max( costs ), though presumably fractions in max.distance{insertions/deletions/substitutions} will be # of characters * corresponding costs value.

I agree that the documentation is not as complete as it could be. I discovered the above by building simple test examples and messing around with them. You should be able to do the same an confirm for yourself, particularly the last part (ie whether costs affects the fraction measure of max.distance{insertions/deletions/substitutions} ), which I haven't tested.