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-   -   Stroustrup section 5.5 "vector of struct" (http://www.velocityreviews.com/forums/t458242-stroustrup-section-5-5-vector-of-struct.html)

arnuld 11-08-2006 03:21 AM

Stroustrup section 5.5 "vector of struct"
 
this is the code:

-------------------------------------------------------------------------

#include <iostream>
#include <string>
#include <vector>

struct Pair {
std::string name;
double val;
};

std::vector<Pair> pairs;

double& value(const std::string s) {
for(int i=0; i < pairs.size(); i++)
if(s == pairs[i].name) return pairs[i].val;

Pair p = {s, -1.1};
pairs.push_back(p); // pair added at the end

return pairs[pairs.size() - 1].val;
}


int main() {

Pair p0 = {"p0", 0.0};
Pair p1 = {"p1", 1.0};
Pair p2 = {"p2", 2.0};

// add to "pairs"
pairs.push_back(p0);
pairs.push_back(p1);
pairs.push_back(p2);

const std::string s1;
std::cout << "now we will check \"pairs\": ";
std::cin >> s1;

value(s1);
}
--------------------------------------------------------------------------

basically it is a "vector" of "Pair" to which i added 3 "Pair values".
then i called "value" function which returns a reference to "val"
related to "string" & if "string" is not found in "pairs" then it
simply creates a new "Pair" & adds it to the end of "pairs" .
compilation gave me this error (using "g++ 4.1.1" on BLAG Linux. ):

-------------------------------------------------------------------------
[arnuld@localhost cpp]$ g++ 05_55-references.cpp

05_55-references.cpp: In function 'int main()':
05_55-references.cpp:41: error: no match for 'operator>>' in
'std::cin >> s1'
/usr/lib/gcc/i386-redhat-linux/4.1.1/../../../../include/c++/4.1.1/istream:131:
note: candidates are: std::basic_istream<_CharT, _Traits>&
std::basic_istream<_CharT,

[SNIP]

<_CharT, _Traits>::operator>>(void*&) [with _CharT = char, _Traits =
std::char_traits<char>]
/usr/lib/gcc/i386-redhat-linux/4.1.1/../../../../include/c++/4.1.1/istream:230:
note: std::basic_istream<_CharT, _Traits>&
std::basic_istream<_CharT,
_Traits>::operator>>(std::basic_streambuf<_CharT, _Traits>*) [with
_CharT = char, _Traits = std::char_traits<char>]

[arnuld@localhost cpp]$
-------------------------------------------------------------------------------

what is wrong?


Mike Wahler 11-08-2006 03:31 AM

Re: Stroustrup section 5.5 "vector of struct"
 

"arnuld" <arnuld3@gmail.com> wrote in message
news:1162956084.425332.170130@m7g2000cwm.googlegro ups.com...
> this is the code:


Remarks in-line e
>
> -------------------------------------------------------------------------
>
> #include <iostream>
> #include <string>
> #include <vector>


[...]

> const std::string s1;


Since you used the 'const' qualifer here, that's
your promise that your code will *not* modify
the object 's1'.

> std::cout << "now we will check \"pairs\": ";
> std::cin >> s1;


.... but here you go trying to modify 's1'. :-)

>
> value(s1);
> }


[...]
> 05_55-references.cpp: In function 'int main()':
> 05_55-references.cpp:41: error: no match for 'operator>>' in
> 'std::cin >> s1'
> /usr/lib/gcc/i386-redhat-linux/4.1.1/../../../../include/c++/4.1.1/istream:131:
> note: candidates are: std::basic_istream<_CharT, _Traits>&
> std::basic_istream<_CharT,


[...]

> what is wrong?


You're trying to modify a const object. Not allowed.
Remove 'const' from your definition of 's1'.

I suspect this was just an 'absent-minded' thing you did, and you
do understand what 'const' means. If not, say so, and we'll explain.

-Mike



BobR 11-08-2006 03:40 AM

Re: Stroustrup section 5.5 "vector of struct"
 

arnuld wrote in message
<1162956084.425332.170130@m7g2000cwm.googlegroups. com>...
>this is the code:
>
>-------------------------------------------------------------------------
>
>#include <iostream>
>#include <string>
>#include <vector>
>
>struct Pair {
> std::string name;
> double val;
>};
>
>std::vector<Pair> pairs;
>
>double& value(const std::string s) {
> for(int i=0; i < pairs.size(); i++)
> if(s == pairs[i].name) return pairs[i].val;
>
> Pair p = {s, -1.1};
> pairs.push_back(p); // pair added at the end
>
> return pairs[pairs.size() - 1].val;
>}
>
>
>int main() {
>
> Pair p0 = {"p0", 0.0};
> Pair p1 = {"p1", 1.0};
> Pair p2 = {"p2", 2.0};
>
> // add to "pairs"
> pairs.push_back(p0);
> pairs.push_back(p1);
> pairs.push_back(p2);
>
> const std::string s1;
> std::cout << "now we will check \"pairs\": ";
> std::cin >> s1;
>
> value(s1);
>}
>--------------------------------------------------------------------------
>
>basically it is a "vector" of "Pair" to which i added 3 "Pair values".
>then i called "value" function which returns a reference to "val"
>related to "string" & if "string" is not found in "pairs" then it
>simply creates a new "Pair" & adds it to the end of "pairs" .
>compilation gave me this error (using "g++ 4.1.1" on BLAG Linux. ):
>
>-------------------------------------------------------------------------
>[arnuld@localhost cpp]$ g++ 05_55-references.cpp
>
>05_55-references.cpp: In function 'int main()':
>05_55-references.cpp:41: error: no match for 'operator>>' in
>'std::cin >> s1'
>
>[SNIP]
>[arnuld@localhost cpp]$
>----------------------------------------------------------------------------

---
>
>what is wrong?


Look at std::string 's1'.

> const std::string s1;


The 'const' means you (or anything else in your program) is allowed to change
it. Remove the 'const', and see if it will work.

Usually you would use a const string like:

const std::string name( "This will stay the same for the whole program" );

--
Bob R
POVrookie



BobR 11-08-2006 04:00 AM

Re: Stroustrup section 5.5 "vector of struct" [correction]
 

BobR wrote in message ...
>
>>what is wrong?

>
>Look at std::string 's1'.
>> const std::string s1;

>
>The 'const' means you (or anything else in your program) is allowed to

change
>it. Remove the 'const', and see if it will work.


The 'const' means you (or anything else in your program) is NOT allowed to
change it. Remove the 'const', and see if it will work.

--
Bob R
POVrookie



arnuld 11-08-2006 04:22 AM

Re: Stroustrup section 5.5 "vector of struct"
 
> Mike Wahler wrote:

> news:1162956084.425332.170130@m7g2000cwm.googlegro ups.com...
> > this is the code:


> Remarks in-line e


what does that mean?

> You're trying to modify a const object. Not allowed.
> Remove 'const' from your definition of 's1'.


> I suspect this was just an 'absent-minded' thing you did, and you
> do understand what 'const' means. If not, say so, and we'll explain.


i do understand what "const" means but that was really absent-minded
thing i did. anyway i removed it & programme compiled & ran but it does
not do what i intended. i also made some minor changes to function
"value" to get output on my terminal:

-----------------------------------------------------------
double& value(const std::string s) {
for(int i=0; i < pairs.size(); i++)
if(s == pairs[i].name)
{
std::cout << pairs[i].val << "\n";
return pairs[i].val;
}

Pair p = {s, -1.1};
pairs.push_back(p); // pair added at the end
std::cout << pairs[pairs.size() - 1].val << "\n";
}
-------------------------------------------------------------


------------------- OUTPUT --------------------------------
[arnuld@localhost cpp]$ ./a.out
now we will check "pairs": p2
2
[arnuld@localhost cpp]$ ./a.out
now we will check "pairs": p9
-1.1
[arnuld@localhost cpp]$
---------------------------------------------------------------

function "value" returns a "double&" not "int". then why did i get /2/
which is of type /int/ instead of double /2.0/?


Amit 11-08-2006 06:14 AM

Re: Stroustrup section 5.5 "vector of struct"
 
Hi arnuld,

If u want to view decimal precision values, u need to set some
properties in cout

try using these statments

cout << showpoint;
cout.precision(2);

Regards.
-Amit Gupta

arnuld wrote:
> > Mike Wahler wrote:

>
> > news:1162956084.425332.170130@m7g2000cwm.googlegro ups.com...
> > > this is the code:

>
> > Remarks in-line e

>
> what does that mean?
>
> > You're trying to modify a const object. Not allowed.
> > Remove 'const' from your definition of 's1'.

>
> > I suspect this was just an 'absent-minded' thing you did, and you
> > do understand what 'const' means. If not, say so, and we'll explain.

>
> i do understand what "const" means but that was really absent-minded
> thing i did. anyway i removed it & programme compiled & ran but it does
> not do what i intended. i also made some minor changes to function
> "value" to get output on my terminal:
>
> -----------------------------------------------------------
> double& value(const std::string s) {
> for(int i=0; i < pairs.size(); i++)
> if(s == pairs[i].name)
> {
> std::cout << pairs[i].val << "\n";
> return pairs[i].val;
> }
>
> Pair p = {s, -1.1};
> pairs.push_back(p); // pair added at the end
> std::cout << pairs[pairs.size() - 1].val << "\n";
> }
> -------------------------------------------------------------
>
>
> ------------------- OUTPUT --------------------------------
> [arnuld@localhost cpp]$ ./a.out
> now we will check "pairs": p2
> 2
> [arnuld@localhost cpp]$ ./a.out
> now we will check "pairs": p9
> -1.1
> [arnuld@localhost cpp]$
> ---------------------------------------------------------------
>
> function "value" returns a "double&" not "int". then why did i get /2/
> which is of type /int/ instead of double /2.0/?



arnuld 11-08-2006 06:24 AM

Re: Stroustrup section 5.5 "vector of struct"
 
Amit wrote:
> Hi arnuld,
>
> If u want to view decimal precision values, u need to set some
> properties in cout
>
> try using these statments
>
> cout << showpoint;
> cout.precision(2);


1st, do not top-post.

2nd, you did not get my point. my input was "2.0" ( Pair p2 = { "p2",
2.0} )

i wanted to know why compiler changed it to /int 2/. on the contrary,
if i input / Pair p3 = { "p3", 3.3} / i get 3.3 as output. so this time
compiler did not change it to /int 3/. why so?


BobR 11-08-2006 08:04 AM

Re: Stroustrup section 5.5 "vector of struct"
 

Amit wrote in message
<1162966453.352575.211360@h54g2000cwb.googlegroups .com>...
>Hi arnuld,
>
>If u want to view decimal precision values, u need to set some
>properties in cout
>
>try using these statments
>
> cout << showpoint;
> cout.precision(2);
>
>Regards.
>-Amit Gupta


Nice top-post. Been takeing lessons long?


(http://www.parashift.com/c++-faq-lit....html#faq-5.4).

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?



BobR 11-08-2006 08:04 AM

Re: Stroustrup section 5.5 "vector of struct"
 

arnuld wrote in message ...
>Amit wrote:
>> Hi arnuld,
>> If u want to view decimal precision values, u need to set some
>> properties in cout
>>
>> try using these statments
>>
>> cout << showpoint;
>> cout.precision(2);

>
>1st, do not top-post.
>
>2nd, you did not get my point. my input was "2.0" ( Pair p2 = { "p2",
>2.0} )
>
>i wanted to know why compiler changed it to /int 2/. on the contrary,
>if i input / Pair p3 = { "p3", 3.3} / i get 3.3 as output. so this time
>compiler did not change it to /int 3/. why so?


What evidence do you have that it is an 'int'? What you see in the output may
be very different than what the variable holds.

Put this in main() and try it:

{
using std::cout;
cout.setf( std::ios_base::fixed );
cout.precision( 20 );
double d1 = 8.126e9;
cout <<"d1 = "<< d1 <<std::endl;

cout.setf( std::ios_base::scientific );
cout.precision(2);
cout <<"d1 = "<< d1 <<std::endl;
cout.precision(6);
cout <<"d1 = "<< d1 <<std::endl;
cout.setf( std::ios_base::fixed );
}

--
Bob R
POVrookie



arnuld 11-08-2006 08:54 AM

Re: Stroustrup section 5.5 "vector of struct"
 
> BobR wrote:

> What evidence do you have that it is an 'int'?


becuase i get plain 2 ( not 2.0 )

> What you see in the output may
> be very different than what the variable holds.


then how will i get the desired output? or how will i know what the
variable holds?
2nd, is it really necessary, practically, to know answers to these 2
questions i have asked?

BTW, what is the "type" of "2" (the output i got)?

> Put this in main() and try it:
>
> {
> using std::cout;
> cout.setf( std::ios_base::fixed );
> cout.precision( 20 );
> double d1 = 8.126e9;
> cout <<"d1 = "<< d1 <<std::endl;
>
> cout.setf( std::ios_base::scientific );
> cout.precision(2);
> cout <<"d1 = "<< d1 <<std::endl;
> cout.precision(6);
> cout <<"d1 = "<< d1 <<std::endl;
> cout.setf( std::ios_base::fixed );
> }


this is the output from "g++ 4.1.1 on BLAG Linux":

-------------------------------------------------------------------
[arnuld@localhost ~]$ ./a.out
d1 = 8126000000.00000000000000000000
d1 = 8.1e+09
d1 = 8.126e+09
[arnuld@localhost ~]$
---------------------------------------------------------------------

you have used what "Amit" told to me: / cout.precision(n) / with your
mysterious / cout.setf( std::ios_base::fixed ); /



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