assigning string to int

B

Ben

Hi all,

I would like to know if there is an easy way to assign a string to an int.

I have a struct such as:

struct Values {
int a;
int b;
}

I have 2 strings that i read from a file:
string x = "12";
string y = "10";

Now I'd like to do something like this:
Values val;
val.a = x;
val.b = y;

There is of course a mis-match.. How do I fix it? Is there an easy way?

Thanx for help!
Ben
 
M

Mike Wahler

Ben said:
Hi all,

I would like to know if there is an easy way to assign a string to an int.
Click to expand...

No, those two types are not 'compatible'. However
(as is what it appears you're really asking, yes
you can convert the textual representation of an
integer (i.e. a sequence of digit characters) to
an integer type.)
I have a struct such as:

struct Values {
int a;
int b;
}

I have 2 strings that i read from a file:
string x = "12";
string y = "10";

Now I'd like to do something like this:
Values val;
val.a = x;
val.b = y;

There is of course a mis-match.. How do I fix it? Is there an easy way?
Click to expand...

There are a few ways, which is 'easy' is a matter of opinion.
Then there are also opinions about which is 'better'.

One way is to use a stringstream
(you'll need to #include <sstream>
to use one):

std::istringstream iss(x + ' ' + y);
iss >> val.a >> val.b;

For a (imo) more flexible, 'robust' method, look up the
'strtol()' function, declared by <cstdlib>

-Mike
 
P

Petec

Ben said:
Hi all,

I would like to know if there is an easy way to assign a string to an
int.

I have a struct such as:

struct Values {
int a;
int b;
}

I have 2 strings that i read from a file:
string x = "12";
string y = "10";

Now I'd like to do something like this:
Values val;
val.a = x;
val.b = y;

There is of course a mis-match.. How do I fix it? Is there an easy
way?

Thanx for help!
Ben
Click to expand...


template<typename T, typename TCHAR>
T fromstring(const std::basic_string<TCHAR>& s)
{
std::basic_istringstream<TCHAR, std::char_traits<TCHAR>,
std::allocator<TCHAR> > iss(s);
T x;
iss >> x;
if(!iss)
throw std::invalid_argument("Bad argument!");
return x;
}

template<typename T, typename TCHAR>
std::basic_string<TCHAR> tostring(const T& x)
{
std::basic_ostringstream<TCHAR, std::char_traits<TCHAR>,
std::allocator<TCHAR> > oss;
oss << x;
if(!oss)
throw std::invalid_argument("Bad argument!");
return oss.str();
}

val.a = fromstring<int, char>(x);
val.b = fromstring<int, char>(y);

fromstring() can be used in a similar way.

- Pete
 
P

Petec

Petec wrote:
template<typename T, typename TCHAR>
std::basic_string<TCHAR> tostring(const T& x)
{
std::basic_ostringstream<TCHAR, std::char_traits<TCHAR>,
std::allocator<TCHAR> > oss;
oss << x;
if(!oss)
throw std::invalid_argument("Bad argument!");
return oss.str();
}

val.a = fromstring<int, char>(x);
val.b = fromstring<int, char>(y);

fromstring() can be used in a similar way.
Click to expand...
 
J

JKop

Ben posted:
Hi all,

I would like to know if there is an easy way to assign a string to an
int.

I have a struct such as:

struct Values {
int a;
int b;
}

I have 2 strings that i read from a file:
string x = "12";
string y = "10";

Now I'd like to do something like this:
Values val;
val.a = x;
val.b = y;

There is of course a mis-match.. How do I fix it? Is there an easy way?

Thanx for help!
Ben
Click to expand...

First of all, I'd like to explain what's happening. Take the following:

"12"

What this is, is three char's side by side in memory:

char twelve[3];

twelve[0] = 31; //'1' = 31 The ASCII keycode
twelve[1] = 32; //'2' = 32
twelve[2] = 0; //'\0' = 0


What you're trying to do is allot more complicated than you think! You need
a function that wil see that 32 == 2 and then see that 31 == 1 * 10, see
what I mean. If I were to write such a function, I'd do it something like
so:

UNTESTED CODE

unsigned long int GenerateNumber(const char* pString)
{
unsigned long int Number;

while (pString += 1)
{
;
}

//We've found the terminating null character

pString -= 1;
//Now we're at the last digit


Number = *pString - '0';
//This gets the actual number from the ASCII keycode


//Now go to the second last digit
pString -= 1;

//This time it's to be multiplied by 10
Number += ( ( *pString - '0' ) * 10 );


//You'd use a loop to go through all the digits,
//then you'd:

return Number;
}


But ofcourse, there's reusable code already there to do this for you. I
myself am not familiar with the "string" class, so some-one else will have
to give you a hand with that.


Hope the explanation gives you some in-sight in anyway.

-JKop
 
D

DigitaluX

Op Sat, 29 May 2004 09:48:55 -0700, schreef Ben:
Hi all,

I would like to know if there is an easy way to assign a string to an int.

I have a struct such as:

struct Values {
int a;
int b;
}

I have 2 strings that i read from a file:
string x = "12";
string y = "10";

Now I'd like to do something like this:
Values val;
val.a = x;
val.b = y;

There is of course a mis-match.. How do I fix it? Is there an easy way?

Thanx for help!
Ben
Click to expand...

You can use the function itoa ;-)

val.a = itoa(x);
 
D

DigitaluX

Op Sat, 29 May 2004 09:48:55 -0700, schreef Ben:
Hi all,

I would like to know if there is an easy way to assign a string to an int.

I have a struct such as:

struct Values {
int a;
int b;
}

I have 2 strings that i read from a file:
string x = "12";
string y = "10";

Now I'd like to do something like this:
Values val;
val.a = x;
val.b = y;

There is of course a mis-match.. How do I fix it? Is there an easy way?

Thanx for help!
Ben
Click to expand...

EUH, it was atoi not itoa :-(

val.a = atoi(x);
 
P

Petec

DigitaluX wrote:
EUH, it was atoi not itoa :-(

val.a = atoi(x);
Click to expand...

Aside from atoi() having no error detection, you example will not compile.
If you must use atoi(), this will work:
val.a = atoi(x.c_str());

I highly reccomend staying away from it, though.

- Pete
 
D

DigitaluX

Op Sat, 29 May 2004 21:48:51 +0000, schreef Petec:
DigitaluX wrote:



Aside from atoi() having no error detection, you example will not compile.
If you must use atoi(), this will work:
val.a = atoi(x.c_str());

I highly reccomend staying away from it, though.

- Pete
Click to expand...

You are totaly right, forgot the c_str() :-$.

Why should a programmer stay away from this?
 
P

Petec

DigitaluX said:
Op Sat, 29 May 2004 21:48:51 +0000, schreef Petec:


You are totaly right, forgot the c_str() :-$.

Why should a programmer stay away from this?
Click to expand...

No error handling is possible. If you pass in, for example, "one two three",
the results of atoi will be zero, which is impossible to tell from a
legitimate zero.

- Pete
 
B

Ben

Mike Wahler said:
There are a few ways, which is 'easy' is a matter of opinion.
Then there are also opinions about which is 'better'.

One way is to use a stringstream
(you'll need to #include <sstream>
to use one):

std::istringstream iss(x + ' ' + y);
iss >> val.a >> val.b;

For a (imo) more flexible, 'robust' method, look up the
'strtol()' function, declared by <cstdlib>

-Mike
Click to expand...

using istringstream gave me following error:
variable `std::istringstream iss' has initializer but incomplete
type

i'm not sure what it means. i'm not very familiar with
istringstream... i'm using gcc compiler version 3.2 in Linux.

Thanx!
Ben
 
R

Robert Bauck Hamar

unsigned long int GenerateNumber(const char* pString)
{
unsigned long int Number;

while (pString += 1)
{
;
}

//We've found the terminating null character

pString -= 1;
//Now we're at the last digit


Number = *pString - '0';
//This gets the actual number from the ASCII keycode


//Now go to the second last digit
pString -= 1;

//This time it's to be multiplied by 10
Number += ( ( *pString - '0' ) * 10 );


//You'd use a loop to go through all the digits,
//then you'd:

return Number;
}
Click to expand...

Instead of working backwards, try to work forward. To see how, assume
we call GenerateNumber("123"). GenerateNumber will enter the for loop,
finding that '1' is a digit. The result will then be 1 after the first
iteration. Then it finds that '2' is a digit, and multiplies result by
ten, giving 10, and adds the two, giving 12. The multiplication will
then produce 120, and the addition 123. It terminates when finding '\0'
is not a digit. Please do not write code before you think.

#include <cctype>

unsigned long int GenerateNumber(const char* number)
{
unsigned long int result = 0;

for (; std::isdigit(*number); ++number) { //loop through the digits
//The result is multiplied by 10 to make the last
//digit in result a zero.
result *= 10;

//Add the next digit
result += *number - '0';
}

return result;
}
 
M

Michael Schutte

Ben said:
Hi all,

I would like to know if there is an easy way to assign a string to an int.
Click to expand...

You can do it the C way (atoi()) or the other C way (sscanf()):

std::string s = "12";
int x;
if (std::sscanf(s.c_str(), "%d", &x) < 1)
std::cerr << "int parsing failed";
else
; // Success! (12 stored in x)

You have to #include <cstdio> for that.
 
W

William Xuuu

using istringstream gave me following error:
variable `std::istringstream iss' has initializer but incomplete
type

i'm not sure what it means. i'm not very familiar with
istringstream... i'm using gcc compiler version 3.2 in Linux.

Thanx!
Ben
Click to expand...

Mike has reminded you to include<sstream> first.
 

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