playing with vectors

[arnuld]

/* C++ Primer 4/e

* STATEMENT
* given 2 vectors of integers, write a programme to determine
whether one vector * is the prefix of the other vector e.g. if 1st vector
has elements (0,1,1,2) and 2nd * vecotr has elements (0,1,1,2,3,5,8) then
programme should say "TRUE" and if 2nd * vector is smaller then too it
should say "TRUE", else it should say "FALSE". *
*/


#include<iostream>
#include<vector>

int main()
{
std::vector<int> ivec1, ivec2;
int ival;

/* creation of 1st vector */
std::cout << "Enter elements for 1st vector" << std::endl;
while(std::cin >> ival)
{
ivec1.push_back(ival);
}

std::cout << "1st vector is created with elements: ";
for(std::vector<int>::const_iterator iter=ivec1.begin();
iter != ivec1.end(); ++iter)
{
std::cout << *iter << " ";
}

std::cout << "\n-------------------" << std::endl;

/* creation od 2nd vector */
ival = 1; /* because ival had EOF value since used last time */
std::cout << "Now enter elements for 2nd vector" << std::endl;
while(std::cin >> ival)
{
ivec2.push_back(ival);
}


std::cout << "2st vector is created with elements: ";
for(std::vector<int>::const_iterator iter=ivec2.begin();
iter != ivec2.end(); ++iter)
{
std::cout << *iter << " ";
}

std::cout << std::endl << std::endl;

unsigned sizeSmaller;
int size1 = ivec1.size();
int size2 = ivec2.size();

/* this "if-else" clause will make the next "for" loop a generalise one
and the "else clause" will work even for vectors of same length */
if(size1 < size2)
{
sizeSmaller = size1;
}
else
{
sizeSmaller = size2;
}

bool prefix_test = true;
for(std::vector<int>::size_type ix=0;
(ix != sizeSmaller) && prefix_test;
++ix)
/* notice the test-condition:
1st, "ix" is an unsigned int and that is why we made sizeSmaller an
unsigned int. 2nd condition will break the loop as soon as we will
meet with 1st false test */
{
if(ivec1[ix] != ivec2[ix])
{
prefix_test = false;
}
}


/* print the result */
if(prefix_test)
{
std::cout << "--> TRUE" << std::endl;
}
else
{
std::cout << "--> FALSE" << std::endl;
}

return 0;
}


this programme compiles and runs but it has a semantic bug. i intended
that it will ask me to input elements for both vectors but it only askes
me to input elements for the 1st vector. it seems like the 2nd while loop
never runs. here is the ouput:

[arnuld@arch cpp] $ g++ -ansi -pedantic -Wall -Wextra ex_06-15.cpp
[arnuld@arch cpp] $ ./a.out
Enter elements for 1st vector
1
2
3
1st vector is created with elements: 1 2 3 -------------------
Now enter elements for 2nd vector
2st vector is created with elements:

--> TRUE
[arnuld@arch cpp] $

 

[Victor Bazarov]

/* C++ Primer 4/e
[..]
}


this programme compiles and runs but it has a semantic bug. i intended
that it will ask me to input elements for both vectors but it only
askes me to input elements for the 1st vector. it seems like the 2nd
while loop never runs.

Well, how do you end the input of the first vector? Apparently you
tell the system your standard input has no more data, right? Did you
press Ctrl-D? If so, how do you expect to read the elements of the
second vector from the same input [that has no more data]?

Think about a different way of terminating the input of the first
vector.

here is the ouput:

The *output* is irrelevant. The *input* is what's screwing you up.

[arnuld@arch cpp] $ g++ -ansi -pedantic -Wall -Wextra ex_06-15.cpp
[arnuld@arch cpp] $ ./a.out
Enter elements for 1st vector
1
2
3
1st vector is created with elements: 1 2 3 -------------------
Now enter elements for 2nd vector
2st vector is created with elements:

--> TRUE
[arnuld@arch cpp] $

V

 

[=?ISO-8859-1?Q?Erik_Wikstr=F6m?=]

/* C++ Primer 4/e

* STATEMENT
* given 2 vectors of integers, write a programme to determine
whether one vector * is the prefix of the other vector e.g. if 1st vector
has elements (0,1,1,2) and 2nd * vecotr has elements (0,1,1,2,3,5,8) then
programme should say "TRUE" and if 2nd * vector is smaller then too it
should say "TRUE", else it should say "FALSE". *
*/

Another of the authors vague assignments, should it print TRUE if the
second vector is smaller than the first regardless of the content of the
vectors? I suspect not but its hard to tell.

#include<iostream>
#include<vector>

int main()
{
std::vector<int> ivec1, ivec2;
int ival;

/* creation of 1st vector */
std::cout << "Enter elements for 1st vector" << std::endl;
while(std::cin >> ival)
{
ivec1.push_back(ival);
}

std::cout << "1st vector is created with elements: ";
for(std::vector<int>::const_iterator iter=ivec1.begin();
iter != ivec1.end(); ++iter)
{
std::cout << *iter << " ";
}

std::cout << "\n-------------------" << std::endl;

/* creation od 2nd vector */
ival = 1; /* because ival had EOF value since used last time */

No need to do this, the value of ival will be overwritten when two lines
down in the loop.

std::cout << "Now enter elements for 2nd vector" << std::endl;
while(std::cin >> ival)
{
ivec2.push_back(ival);
}


std::cout << "2st vector is created with elements: ";
for(std::vector<int>::const_iterator iter=ivec2.begin();
iter != ivec2.end(); ++iter)
{
std::cout << *iter << " ";
}

std::cout << std::endl << std::endl;

Organisational tips: Create a function taking two vectors as arguments
and returns a bool which performs the check for you.

unsigned sizeSmaller;
int size1 = ivec1.size();
int size2 = ivec2.size();

/* this "if-else" clause will make the next "for" loop a generalise one
and the "else clause" will work even for vectors of same length */
if(size1 < size2)
{
sizeSmaller = size1;
}
else
{
sizeSmaller = size2;
}

bool prefix_test = true;
for(std::vector<int>::size_type ix=0;
(ix != sizeSmaller) && prefix_test;
++ix)

You can get rid of prefix_test and sizeSmaller by rewriting the loop
like this:

for (std::vector<int>::size_type ix = 0;
(ix < ivec1.size() && ix < ivec2.size());
++ix)

/* notice the test-condition:
1st, "ix" is an unsigned int and that is why we made sizeSmaller an
unsigned int. 2nd condition will break the loop as soon as we will
meet with 1st false test */
{
if(ivec1[ix] != ivec2[ix])
{
prefix_test = false;
}
}


/* print the result */
if(prefix_test)
{
std::cout << "--> TRUE" << std::endl;
}
else
{
std::cout << "--> FALSE" << std::endl;
}

return 0;
}


this programme compiles and runs but it has a semantic bug. i intended
that it will ask me to input elements for both vectors but it only askes
me to input elements for the 1st vector. it seems like the 2nd while loop
never runs.

That's because cin has reaches EOF, you need to reset it before you can
read any more, put std::cin.clear(); before the second loop.

 

[BobR]

std::cout << std::endl << std::endl;

Just a note here, that line does a 'flush' twice. It's not a bad thing, just
not good.
Try it this way:

std::cout << '\n' << std::endl; // "\n" will also work.

 

[Jim Langston]

/* C++ Primer 4/e

* STATEMENT
* given 2 vectors of integers, write a programme to determine
whether one vector * is the prefix of the other vector e.g. if 1st vector
has elements (0,1,1,2) and 2nd * vecotr has elements (0,1,1,2,3,5,8) then
programme should say "TRUE" and if 2nd * vector is smaller then too it
should say "TRUE", else it should say "FALSE". *
*/


#include<iostream>
#include<vector>

int main()
{
std::vector<int> ivec1, ivec2;
int ival;

/* creation of 1st vector */
std::cout << "Enter elements for 1st vector" << std::endl;
while(std::cin >> ival)
{
ivec1.push_back(ival);
}

At this point std::cin is in a bad state.

std::cout << "1st vector is created with elements: ";
for(std::vector<int>::const_iterator iter=ivec1.begin();
iter != ivec1.end(); ++iter)
{
std::cout << *iter << " ";
}

std::cout << "\n-------------------" << std::endl;

/* creation od 2nd vector */
ival = 1; /* because ival had EOF value since used last time */
std::cout << "Now enter elements for 2nd vector" << std::endl;
while(std::cin >> ival)

std::cin is still in a bad state, so it won't accept any more input. You
have to reset std::cin

You should read the FAQ 15 entirely.
http://www.parashift.com/c++-faq-lite/input-output.html#faq-15 or at least
until 15.6

std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

should reset std::cin to a good state.

You'll want to

{
ivec2.push_back(ival);
}


std::cout << "2st vector is created with elements: ";
for(std::vector<int>::const_iterator iter=ivec2.begin();
iter != ivec2.end(); ++iter)
{
std::cout << *iter << " ";
}

std::cout << std::endl << std::endl;

unsigned sizeSmaller;
int size1 = ivec1.size();
int size2 = ivec2.size();

/* this "if-else" clause will make the next "for" loop a generalise one
and the "else clause" will work even for vectors of same length */
if(size1 < size2)
{
sizeSmaller = size1;
}
else
{
sizeSmaller = size2;
}

bool prefix_test = true;
for(std::vector<int>::size_type ix=0;
(ix != sizeSmaller) && prefix_test;
++ix)
/* notice the test-condition:
1st, "ix" is an unsigned int and that is why we made sizeSmaller an
unsigned int. 2nd condition will break the loop as soon as we will
meet with 1st false test */
{
if(ivec1[ix] != ivec2[ix])
{
prefix_test = false;
}
}


/* print the result */
if(prefix_test)
{
std::cout << "--> TRUE" << std::endl;
}
else
{
std::cout << "--> FALSE" << std::endl;
}

return 0;
}


this programme compiles and runs but it has a semantic bug. i intended
that it will ask me to input elements for both vectors but it only askes
me to input elements for the 1st vector. it seems like the 2nd while loop
never runs. here is the ouput:

[arnuld@arch cpp] $ g++ -ansi -pedantic -Wall -Wextra ex_06-15.cpp
[arnuld@arch cpp] $ ./a.out
Enter elements for 1st vector
1
2
3
1st vector is created with elements: 1 2 3 -------------------
Now enter elements for 2nd vector
2st vector is created with elements:

--> TRUE
[arnuld@arch cpp] $

 

[James Kanze]

/* C++ Primer 4/e

* STATEMENT
* given 2 vectors of integers, write a programme to determine
whether one vector * is the prefix of the other vector e.g. if 1st vector
has elements (0,1,1,2) and 2nd * vecotr has elements (0,1,1,2,3,5,8) then
programme should say "TRUE" and if 2nd * vector is smaller then too it
should say "TRUE", else it should say "FALSE". *
*/

int main()
{
std::vector<int> ivec1, ivec2;

Don't ever declare two variables in the same statement. It's
very bad practice.

More to the point, don't define variables until you need them.
In this case, you shouldn't define ivec2 until much, much later.

int ival;

/* creation of 1st vector */
std::cout << "Enter elements for 1st vector" << std::endl;
while(std::cin >> ival)
{
ivec1.push_back(ival);
}

One thing: you're going to do exactly the same thing a second
time. That should have you thinking "function" immediately.

std::cout << "1st vector is created with elements: ";
for(std::vector<int>::const_iterator iter=ivec1.begin();
iter != ivec1.end(); ++iter)
{
std::cout << *iter << " ";
}

std::cout << "\n-------------------" << std::endl;

/* creation od 2nd vector */
ival = 1; /* because ival had EOF value since used last time */

No. ival has the last successfully read value. It's std::sin
which has the EOF state. And what to do about it is far from
trivial. (Under Unix, *if* the input is from a keyboard, the
just clearing the error condition would suffice to read the
second vector. Under Unix, of course, any use will also expect
to be able to redirect the input from a file, in which case,
this suddenly won't work.)

What I'd probably do (as the simplest solution) is to read the
input line by line, stopping at either end of file OR an empty
line. (In a separate function, as I said.)

std::cout << "Now enter elements for 2nd vector" << std::endl;
while(std::cin >> ival)

And this condition is guaranteed to fail immediately.

{
ivec2.push_back(ival);
}

std::cout << "2st vector is created with elements: ";
for(std::vector<int>::const_iterator iter=ivec2.begin();
iter != ivec2.end(); ++iter)
{
std::cout << *iter << " ";
}

std::cout << std::endl << std::endl;

unsigned sizeSmaller;
int size1 = ivec1.size();
int size2 = ivec2.size();

/* this "if-else" clause will make the next "for" loop a generalise one
and the "else clause" will work even for vectors of same length */
if(size1 < size2)
{
sizeSmaller = size1;
}
else
{
sizeSmaller = size2;
}

bool prefix_test = true;
for(std::vector<int>::size_type ix=0;
(ix != sizeSmaller) && prefix_test;
++ix)
/* notice the test-condition:
1st, "ix" is an unsigned int and that is why we made sizeSmaller an
unsigned int. 2nd condition will break the loop as soon as we will
meet with 1st false test */
{
if(ivec1[ix] != ivec2[ix])
{
prefix_test = false;
}
}

The above is much more complicated than necessary. Consider
using std::vector<>::swap() and std::equal().

On the other hand, the actual specification seems to say that if
the second vector is smaller, output the same thing as if the
first vector were a prefix of it. Strange, but perhaps
specifically part of the requirements so that you don't have to
worry about swap, etc. Just something along the lines of:

std::cout << ( ivec1.size() > ivec2.size()
|| std::equal(
ivec1.begin(), ivec1.end(),
ivec2.begin() )
? "TRUE"
: "FALSE" )
<< std::endl ;

/* print the result */
if(prefix_test)
{
std::cout << "--> TRUE" << std::endl;
}
else
{
std::cout << "--> FALSE" << std::endl;
}

return 0;

}

this programme compiles and runs but it has a semantic bug. i
intended that it will ask me to input elements for both
vectors but it only askes me to input elements for the 1st
vector. it seems like the 2nd while loop never runs.

End of file is an "error" condition, and error conditions are
sticky in C++. One a stream encounters an error condition, that
error condition will remain until explicitly reset.