"vector<int>::iterator" problem

[arnuld]

i wrote a programme to create a vector of 5 elements (0 to 4), here is
the code & output:

#include <iostream>
#include <vector>

int main() {
std::vector<int> ivec;

// dynamically create a vector
for(std::vector<int>::size_type ix = 0; ix <= 4; ++ix)
{
ivec.push_back(ix);
}


// print-out the elements
for(std::vector<int>::iterator iter = ivec.begin();
iter != ivec.end(); ++iter)
{
std::cout << "Element: " << *iter << "..." << std::endl;
}

// create a middle iterator
std::vector<int>::iterator mid_iter = (ivec.begin() + ivec.size() /
2);
// print it out to check where it points
std::cout << "middle element: " << *mid_iter << std::endl;

return 0;
}

OUTPUT:

Element: 0...
Element: 1...
Element: 2...
Element: 3...
Element: 4...
middle element: 2 // fine


now if i change the "mid_iter" to:

/std::vector<int>::iterator mid_iter = (ivec.size() / 2);/

then i got an ERROR:
----------------------------------------------------------------
unix@debian:~/Desktop$ g++ test.cpp

test.cpp: In function `int main()':
test.cpp:22: error: conversion from `unsigned int' to non-scalar type `
__gnu_cxx::__normal_iterator<int*, std::vector<int,
std::allocator<int> > >'
requested

unix@debian:~/Desktop$
----------------------------------------------------------------


it is showing "non-scalar type". what is that? what is happening
"behind the curtains"?


"arnuld"

 

[Kai-Uwe Bux]

i wrote a programme to create a vector of 5 elements (0 to 4), here is
the code & output:

#include <iostream>
#include <vector>

int main() {
std::vector<int> ivec;

// dynamically create a vector
for(std::vector<int>::size_type ix = 0; ix <= 4; ++ix)
{
ivec.push_back(ix);
}


// print-out the elements
for(std::vector<int>::iterator iter = ivec.begin();
iter != ivec.end(); ++iter)
{
std::cout << "Element: " << *iter << "..." << std::endl;
}

// create a middle iterator
std::vector<int>::iterator mid_iter = (ivec.begin() + ivec.size() /
2);
// print it out to check where it points
std::cout << "middle element: " << *mid_iter << std::endl;

return 0;
}

OUTPUT:

Element: 0...
Element: 1...
Element: 2...
Element: 3...
Element: 4...
middle element: 2 // fine


now if i change the "mid_iter" to:

/std::vector<int>::iterator mid_iter = (ivec.size() / 2);/

then i got an ERROR:
----------------------------------------------------------------
unix@debian:~/Desktop$ g++ test.cpp

test.cpp: In function `int main()':
test.cpp:22: error: conversion from `unsigned int' to non-scalar type `
__gnu_cxx::__normal_iterator<int*, std::vector<int,
std::allocator<int> > >'
requested

unix@debian:~/Desktop$
----------------------------------------------------------------


it is showing "non-scalar type". what is that? what is happening
"behind the curtains"?

Well, look at the statement:

std::vector<int>::iterator mid_iter = (ivec.size() / 2);

The variable mid_iter has type std::vector<int>::iterator. Note that
std::vector<int>::iterator, according to the standard, is an implementation
defined type. What it is or is not 'behind the curtains' is of no concern
to the programmer.

Now, the expression (ivec.size() / 2) has type std::vector<int>::size_type.
The compiler is telling you that you cannot initialize an iterator from a
size_type. The compiler it correct: the interface of iterator does not
contain such a conversion (for good reasons!).



Best

Kai-Uwe Bux

 

[yoge]

Well, look at the statement:

std::vector<int>::iterator mid_iter = (ivec.size() / 2);

The variable mid_iter has type std::vector<int>::iterator. Note that
std::vector<int>::iterator, according to the standard, is an implementation
defined type. What it is or is not 'behind the curtains' is of no concern
to the programmer.

ok, no concern

Now, the expression (ivec.size() / 2) has type std::vector<int>::size_type.
The compiler is telling you that you cannot initialize an iterator from a
size_type. The compiler it correct: the interface of iterator does not
contain such a conversion (for good reasons!).

thanks Kai.

"arnuld"
www.arnuld.blogspot.com

 

[Ron Natalie]

arnuld wrote:
std::vector<int>::iterator mid_iter = (ivec.size() / 2);

The variable mid_iter has type std::vector<int>::iterator. Note that
std::vector<int>::iterator, according to the standard, is an implementation
defined type. What it is or is not 'behind the curtains' is of no concern
to the programmer.
=

mid_iter = ivec.begin() + (ivec.size() / 2)

would however do what the OP was trying to accomplish.

 

[Kai-Uwe Bux]

mid_iter = ivec.begin() + (ivec.size() / 2)

would however do what the OP was trying to accomplish.

True, but the OP already knew that [from the OP as quoted in my previous
posting]:

i wrote a programme to create a vector of 5 elements (0 to 4), here is
the code & output: [snip]
// create a middle iterator
std::vector<int>::iterator mid_iter = (ivec.begin() + ivec.size()/2); [snip]
now if i change the "mid_iter" to:

/std::vector<int>::iterator mid_iter = (ivec.size() / 2);/

then i got an ERROR:

Best

Kai-Uwe Bux