Re: playing with vectors
On Aug 3, 1:57 pm, arnuld <geek.arn...@gmail.com> wrote:
/* 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;
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.
--
James Kanze (GABI Software) email:james.kanze:gmail.com
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