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return value of function

Bob
Hi,

I have a std::vector, say myVec, of some user defined object, say
myOb. In my code, I have a function that searches myVec for a
particular myOb.

The way I was doing this was searching myVec for the element that has
a member equal to a value that was passed into the function. For
example:

bool myFunc(const std::string& s)
{
std::vector<myOb>::iterator i = myVec.begin();

while(i != myVec.end()) {
if((*i).name == s) return true;
}
return false;
}

This hopefully returns true if the element is found, or false if not.
However, I don't really want to return true or false, I was wanting to
return either the offset from myVec.begin() (and maybe -1 if not
found), or a reference to the element itself.

How should I do this? Basically, I'm not sure how to return an offset
from begin(), and I didn't know what to return for the reference in
the case of 'element not found'. Or, should I really be returning the
iterator, and de-referencing that in my calling program?

Many thanks for your valued advice,
Bob.
Jul 22 '05 #1
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5 Replies
Bob wrote:

Hi,

I have a std::vector, say myVec, of some user defined object, say
myOb. In my code, I have a function that searches myVec for a
particular myOb.

The way I was doing this was searching myVec for the element that has
a member equal to a value that was passed into the function. For
example:

bool myFunc(const std::string& s)
{
std::vector<myOb>::iterator i = myVec.begin();

while(i != myVec.end()) {
if((*i).name == s) return true;
}
return false;
}

This hopefully returns true if the element is found, or false if not.
However, I don't really want to return true or false, I was wanting to
return either the offset from myVec.begin() (and maybe -1 if not
found), or a reference to the element itself.

How should I do this? Basically, I'm not sure how to return an offset
from begin(), and I didn't know what to return for the reference in
the case of 'element not found'. Or, should I really be returning the
iterator, and de-referencing that in my calling program?

Many thanks for your valued advice,
Bob.


Without knowing the specifics of your problem, the first idea is to
return an iterator to the element if found, myVec.end() otherwise.

Don't write it as above. Look up the standard function template
std::find_if. All you need to do is to define a predicate function object
which stores a const std::string& s, accepts myOb and returns true
if s == myOb.name, something like

class MyPred {
const std::string& s_;
public:
MyPred(const std::string& s) : s_(s) {}
bool operator()(const myOb& a) const {
return s_ == a.name;
}
};

Denis
Jul 22 '05 #2
"Bob" <bo******@hotmail.com> wrote in message
news:77**************************@posting.google.c om...
However, I don't really want to return true or false, I was wanting to
return either the offset from myVec.begin() (and maybe -1 if not
found), or a reference to the element itself. [..] Or, should I really be returning the
iterator, and de-referencing that in my calling program?


Yes, seems like the best solution. And if no match was found, you return
myVec.end (). Tho, then you have to check if the returned iterator is valid
before dereferncing it.

hth
--
jb

(replace y with x if you want to reply by e-mail)
Jul 22 '05 #3

"Bob" <bo******@hotmail.com> wrote in message
news:77**************************@posting.google.c om...
Hi,

I have a std::vector, say myVec, of some user defined object, say
myOb. In my code, I have a function that searches myVec for a
particular myOb.

The way I was doing this was searching myVec for the element that has
a member equal to a value that was passed into the function. For
example:

bool myFunc(const std::string& s)
{
std::vector<myOb>::iterator i = myVec.begin();

while(i != myVec.end()) {
if((*i).name == s) return true;
}
return false;
}

This hopefully returns true if the element is found, or false if not.
However, I don't really want to return true or false, I was wanting to
return either the offset from myVec.begin() (and maybe -1 if not
found), or a reference to the element itself.
You can't return a reference to the element in the case where the element
isn't found. You could throw an exception in this case however.

You could also return a pointer to the element, and a null pointer in the
not found case.

How should I do this? Basically, I'm not sure how to return an offset
from begin(), and I didn't know what to return for the reference in
the case of 'element not found'.
return i - myVec.begin();

Or, should I really be returning the iterator, and de-referencing that in my calling program?


Yes probably, although checking the return value against myVec.end() in the
calling program is tedious.

john
Jul 22 '05 #4
On Wed, 12 May 2004 08:18:00 +0200, Bob wrote:
[...]
However, I don't really want to return true or false, I was wanting to
return either the offset from myVec.begin() (and maybe -1 if not found),


Have a look at distance()

HTH, Darius.
Jul 22 '05 #5
Bob,

use the stl find() algorithm - this will return the iterator to the
element you want or end() to indicate t he object was not found,
why reinvent the wheel when you can rip one off somebody else car :).
std::vector<myOb>::iterator i find( myVec.begin(), myVec.end(), foo)

if ( i!= myVec.end())
{
// found.
myObj bar = *i;
EurekaImFound(bar);
}
else
{
// not found.
}
Do you really need the offset, the iterator would give you direct
access to the object anyway.

dave
"Bob" <bo******@hotmail.com> wrote in message
news:77**************************@posting.google.c om...
Hi,

I have a std::vector, say myVec, of some user defined object, say
myOb. In my code, I have a function that searches myVec for a
particular myOb.

The way I was doing this was searching myVec for the element that has
a member equal to a value that was passed into the function. For
example:

bool myFunc(const std::string& s)
{
std::vector<myOb>::iterator i = myVec.begin();

while(i != myVec.end()) {
if((*i).name == s) return true;
}
return false;
}

This hopefully returns true if the element is found, or false if not.
However, I don't really want to return true or false, I was wanting to
return either the offset from myVec.begin() (and maybe -1 if not
found), or a reference to the element itself.

How should I do this? Basically, I'm not sure how to return an offset
from begin(), and I didn't know what to return for the reference in
the case of 'element not found'. Or, should I really be returning the
iterator, and de-referencing that in my calling program?

Many thanks for your valued advice,
Bob.

Jul 22 '05 #6

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