Re: I need help! Substrings

No offense, but does your course have an instructor?  TAs?  A textbook?
Which college is that?  Who is the instructor?  Is there a course
web page we can look at, so we can get a feel of the bigger picture of
what kind of course this is and what is being asked of you?

I hope you realize that you have already received more help than you
could have expected.  David MacQueen has kindly taken the time to
provide you with a fairly detailed reply that you should be able to work
with.  If not, then this may be a sign that, indeed, you have bitten off
more than you can chew at this point.

The problem you are supposed to program is quite basic and does not
require more than beginner's programming skills.
If you can do it in Java but are stuck with SML syntax issues, perhaps
you can show us what you are trying to do so far?  In fact, how
about you show us your Java code, since this seems to be what you
know.  Then we can talk about how to derive an SML solution from that.

In any case, I'm sure your instructor, your TAs, and your textbook have
answers, too.  Have you asked them?  How about your classmates?

By the way, your original problem description looks inconsistent to me.
Are you sure your first example returns 1?  I would think it should
return 0.

Kind regards,
Matthias


On May 29, 2007, at 11:23 PM, turbosol wrote:

>
> i appreciate the help, but i must admit, I am a definite beginner  
> in this
> class, and i am completely lost.  also, we are not allowed to use  
> any built
> in librbaries like string.sub, or anything like that.  the -1 is only
> returned because that is what is asked of me to do it.  I have the  
> basic
> idea as to how to do the program, its just the syntax that is  
> screwing me
> up.  If this were java, i would have no problem. thanks for the  
> pointers,
> but any more help would be much appreciated.
>
>
>
> David MacQueen-2 wrote:
>>
>> This is almost the same as the library function String.isSubstring,
>> except for
>> the type.
>>
>> Here is a hint (for a fairly simple, but not optimal solution).
>> Define two auxiliary functions
>>
>> val firstMatch : char * string * int -> int option
>> fun firstMatch(c: char, s: string, i: int) : int option =
>>         find the index of the first occurrence of character c in
>> string s starting at index i.
>> 	if c is found at index j in s, return SOME j, otherwise return NONE
>>
>> val checkRest : string * int * string * int -> bool
>> fun checkRest(s1, s2, i) : bool =
>>        check that all the characters in string s1, starting at index
>> 1 (i.e. the second
>>        character) are matched by the corresponding characters in
>> string s2, starting
>>        at index i.  return true if so, or false if there is a
>> mismatch or s2 is exhausted
>>        before s1
>>
>> Once you have these, it should be straightforward to put them
>> together to define
>> the function subString function you want.
>>
>> By the way, returning -1 for the failure case is bad style.  Much
>> better to return
>> an int option (NONE or SOME) instead, as I suggested for firstMatch
>> above.
>>
>> Good luck.
>>
>> Dave MacQueen
>>
>> On May 29, 2007, at 3:41 PM, turbosol wrote:
>>
>>>
>>> So i signed up for my last compscience class so i can graduate...
>>> the class i
>>> needed was canceled so i was put into a more difficult class for
>>> majors.
>>> Comp science is my minor... anyways, I am currently doing twice the
>>> work i
>>> would normally do in half the time.  I am lost.  if anyone can help
>>> me, i
>>> would appreciate it SOOO much.
>>>
>>> I need to write a function SubString : string * string -> int that
>>> checks
>>> whether the first string is a substring of the second (case
>>> sensitive).
>>> return the index position (starting from 0) if the string is a
>>> substring or
>>> -1 otherwise.  for multiple occurances, just return the index of
>>> the first
>>> appearance.  you may assume both input parameters are strings.
>>>
>>> subString("ab","abcdefg")=> 1
>>> subString("aaa","aaaa")=>0
>>> subString("bc","absge") =>-1
>>> subString("ab","cdabd") => 2
>>> -- 
>>> View this message in context: http://www.nabble.com/I-need-help%21--
>>> Substrings-tf3836438.html#a10862221
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>>>
>>>
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>
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> Substrings-tf3836438.html#a10867231
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>
>
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