Strange std::vector behaviour

[Hamish]

Hello, lately I've been having a lot of trouble with the std::vector. Seems
to create unpredictable behaviour within my code.

Example:

struct Switch{
int i;
int j;
double d;
};

std::vector<Switch> Switches;

Now I have a for...loop:

int h = Switches.size();
for(int i=0;i<Switches.size()-1;i++){
/*do stuff here*/
}

The problem is, when h (Switches.size()) equals 0, the for...loop is still
entered, and runs infinitely.

However, if I use the line:
for(int i=0;i<h-1;i++){
then there is not a problem.

Anyone got any ideas as to what is going on, cos this is ruining my New
Year!

 

[Cy Edmunds]

Hello, lately I've been having a lot of trouble with the std::vector.
Seems
to create unpredictable behaviour within my code.

Example:

struct Switch{
int i;
int j;
double d;
};

std::vector<Switch> Switches;

Now I have a for...loop:

int h = Switches.size();
for(int i=0;i<Switches.size()-1;i++){
/*do stuff here*/
}

The problem is, when h (Switches.size()) equals 0, the for...loop is still
entered, and runs infinitely.

However, if I use the line:
for(int i=0;i<h-1;i++){
then there is not a problem.

Anyone got any ideas as to what is going on, cos this is ruining my New
Year!

std::vector<T>::size() returns an unsigned type. Hence 0 - 1 evaluates to
something like 0xffffffff

 

[Chris]

std::vector<T>::size() returns an unsigned type. Hence 0 - 1 evaluates to
something like 0xffffffff

or, you could use iterators:
typedef std::vector< Switch > tSwitchContainer
typedef tSwitchContainer::iterator tSwitchItr;

tSwitchContainer Switches;

for ( tSwitchItr cur = Switches.begin(); cur != Switches.end(); cur++)
{
// do stuff.....
}

nice thing about this is you could use any container and the code would
still work

-c

 

[Ioannis Vranos]

Hello, lately I've been having a lot of trouble with the std::vector. Seems
to create unpredictable behaviour within my code.

Example:

struct Switch{
int i;
int j;
double d;
};

std::vector<Switch> Switches;

Now I have a for...loop:

int h = Switches.size();
for(int i=0;i<Switches.size()-1;i++){
/*do stuff here*/
}

The problem is, when h (Switches.size()) equals 0, the for...loop is still
entered, and runs infinitely.

However, if I use the line:
for(int i=0;i<h-1;i++){
then there is not a problem.

Anyone got any ideas as to what is going on, cos this is ruining my New
Year!

Switches.size() returns a vector<Switch>::size_type which is an unsigned
integer type.


Thus vector<Switch>::size_type(-1) is the maximum value of that type.


Your code can be fixed like this:


for(int i=0; i<Switches.size(); ++i)
/*do stuff here*/


or better:


for(vector<Switch>::size_type i=0; i<Switches.size(); ++i)
/*do stuff here*/

 

[Mike Wahler]

Hello, lately I've been having a lot of trouble with the std::vector. Seems
to create unpredictable behaviour within my code.

Example:

struct Switch{
int i;
int j;
double d;
};

std::vector<Switch> Switches;

Now I have a for...loop:

int h = Switches.size();
for(int i=0;i<Switches.size()-1;i++){
/*do stuff here*/
}

The problem is, when h (Switches.size()) equals 0, the for...loop is still
entered, and runs infinitely.

for(std::vector<Switch>::size_type i = 0; i < Switches.size(); ++i)
/* etc */

If for some reason you really want to ignore the last
element (if one exists):

if(!Switches.empty())

However, if I use the line:
for(int i=0;i<h-1;i++){
then there is not a problem.

If 'h' starts at zero, the behavior is undefined.

Anyone got any ideas as to what is going on, cos this is ruining my New
Year!

Vector indices begin with zero and run through 'size()' - 1

-Mike

 

[Ioannis Vranos]

Hello, lately I've been having a lot of trouble with the std::vector. Seems
to create unpredictable behaviour within my code.

Example:

struct Switch{
int i;
int j;
double d;
};

std::vector<Switch> Switches;

Now I have a for...loop:

int h = Switches.size();
for(int i=0;i<Switches.size()-1;i++){
/*do stuff here*/
}

The problem is, when h (Switches.size()) equals 0, the for...loop is still
entered, and runs infinitely.

However, if I use the line:
for(int i=0;i<h-1;i++){
then there is not a problem.

Anyone got any ideas as to what is going on, cos this is ruining my New
Year!

Switches.size() returns a vector<Switch>::size_type which is an unsigned
integer type.


Thus vector<Switch>::size_type(-1) is the maximum value of that type.


Your code can be fixed like this:


for(int i=0; i<Switches.size(); ++i)
/*do stuff here*/


or better:


for(vector<Switch>::size_type i=0; i<Switches.size(); ++i)
/*do stuff here*/

 

[E. Mark Ping]

If for some reason you really want to ignore the last
element (if one exists):

if(!Switches.empty())
for(std::vector<Switch>::size_type i = 0; i < Switches.size() - 1; ++i)
/* etc */

Which of course is a big problem if the size is 0. When dealing with
unsigned types, I find avoiding subtraction to be much safer.

for(int i = 0; i+1 < Switches.size(); ++i)

 

[John Carson]

Which of course is a big problem if the size is 0.

You seem not to have noticed the test:

if(!Switches.empty())