Stroustrup section 5.5 "vector of struct"

[arnuld]

this is the code:

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

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

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

std::vector<Pai rpairs;

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@localhos t 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_istr eam<_CharT, _Traits>&
std::basic_istr eam<_CharT,

[SNIP]

<_CharT, _Traits>::opera tor>>(void*&) [with _CharT = char, _Traits =
std::char_trait s<char>]
/usr/lib/gcc/i386-redhat-linux/4.1.1/../../../../include/c++/4.1.1/istream:230:
note: std::basic_istr eam<_CharT, _Traits>&
std::basic_istr eam<_CharT,
_Traits>::opera tor>>(std::basi c_streambuf<_Ch arT, _Traits>*) [with
_CharT = char, _Traits = std::char_trait s<char>]

[arnuld@localhos t cpp]$
-------------------------------------------------------------------------------

what is wrong?

[Mike Wahler]

"arnuld" <ar*****@gmail. comwrote in message
news:11******** **************@ m7g2000cwm.goog legroups.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_istr eam<_CharT, _Traits>&
std::basic_istr eam<_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]

arnuld wrote in message
<11************ **********@m7g2 000cwm.googlegr oups.com>...

>this is the code:

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

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

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

std::vector<Pa irpairs;

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@localhos t 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@localhos t 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]

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]

Mike Wahler wrote:

news:11******** **************@ m7g2000cwm.goog legroups.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@localhos t cpp]$ ./a.out
now we will check "pairs": p2
2
[arnuld@localhos t cpp]$ ./a.out
now we will check "pairs": p9
-1.1
[arnuld@localhos t 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]

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:11******** **************@ m7g2000cwm.goog legroups.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@localhos t cpp]$ ./a.out
now we will check "pairs": p2
2
[arnuld@localhos t cpp]$ ./a.out
now we will check "pairs": p9
-1.1
[arnuld@localhos t 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]

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]

Amit wrote in message
<11************ **********@h54g 2000cwb.googleg roups.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]

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]

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@localhos t ~]$ ./a.out
d1 = 8126000000.0000 000000000000000 0
d1 = 8.1e+09
d1 = 8.126e+09
[arnuld@localhos t ~]$
---------------------------------------------------------------------

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