Stange Occurance After Using std::Merge On std::vector's

[Adam Teasdale Hartshorne]

Using the code below I get a very stange result. I assume that I must be
doing something wrong, but I can't figure out what it is.
getTriangleEdges(t[0],t1e) ;

for (int i = 0 ; i < (int)t1e.size() ; i++) {
printf("T1 Edges Index:- %i \n", t1e[i]) ;

}

getTriangleEdges(t[1],t2e) ;

for (int i = 0 ; i < (int)t1e.size() ; i++) {
printf("T2 Edges Index:- %i \n", t2e[i]) ;
}

std::vector<int> edges ;
edges.reserve(6);

//printf("Merging lists \n") ;
std::merge(t1e.begin(), t1e.end(), t2e.begin(), t2e.end(), edges.begin());

printf("Edges Size:- %i \n", (int) edges.size()) ;

for (int i = 0 ; i < 6 ; i++) {
printf("Edges Index:- %i \n", edges[i]) ;
}
The following output is seen in the terminal

T1 Edges Index:- 7
T1 Edges Index:- 8
T1 Edges Index:- 6
T2 Edges Index:- 10
T2 Edges Index:- 8
T2 Edges Index:- 9
Edges Size:- 0
Edges Index:- 7
Edges Index:- 8
Edges Index:- 6
Edges Index:- 10
Edges Index:- 8
Edges Index:- 9

It seems to think the size of edges is 0!!!!!

Any thoughts would be much appreciated,

Adam

[Thomas Tutone]

Adam Teasdale Hartshorne wrote:

Using the code below I get a very stange result. I assume that I must be
doing something wrong, but I can't figure out what it is.
No offense, but there are so many problems with your code, it's hard to
know where to begin.
getTriangleEdges(t[0],t1e) ;

for (int i = 0 ; i < (int)t1e.size() ; i++) {
C-style casts like this are a bad habit, particularly in a situation
like this where there's no reason to include a cast at all.
printf("T1 Edges Index:- %i \n", t1e[i]) ;

}

getTriangleEdges(t[1],t2e) ;

for (int i = 0 ; i < (int)t1e.size() ; i++) {
C-style cast again.
printf("T2 Edges Index:- %i \n", t2e[i]) ;
}

std::vector<int> edges ;
edges.reserve(6);

//printf("Merging lists \n") ;
std::merge(t1e.begin(), t1e.end(), t2e.begin(), t2e.end(), edges.begin());
That last parameter needs to be std::back_inserter(edges), not
edges.begin(). That's the error that resulted in edges.size()
returning the wrong result.

But there's a more fundamental problem. Merge requires that the two
constituent ranges both already be sorted (and in ascending order
unless you tell it otherwise). Your two ranges are not sorted at all.
Accordingly, the result of calling merge as you do here is undefined.
printf("Edges Size:- %i \n", (int) edges.size()) ;
Another C-style cast. edges.size() returns a std::size_t, which
generally means an unsigned int. Just change your printf accordingly -
or better yet, use std::cout - and get rid of the cast.
for (int i = 0 ; i < 6 ; i++) {
printf("Edges Index:- %i \n", edges[i]) ;
}
The following output is seen in the terminal

T1 Edges Index:- 7
T1 Edges Index:- 8
T1 Edges Index:- 6
T2 Edges Index:- 10
T2 Edges Index:- 8
T2 Edges Index:- 9
Like I said, two unsorted ranges, so undefined behavior when you call
std::merge
Edges Size:- 0
Edges Index:- 7
Edges Index:- 8
Edges Index:- 6
Edges Index:- 10
Edges Index:- 8
Edges Index:- 9

It seems to think the size of edges is 0!!!!!

Because you mistakenly told it to send the result to edges.begin()
instead of std::back_inserter(edges).

Hope that helps.

Best regards,

Tom