Converting numbers to strings and string

Converting numbers to strings and strings to numbers

This question is asked quite often, so here is a way of doing it using stringstreams:

number to string

int Number = 123;//number to convert int a string
string Result;//string which will contain the result

stringstream convert; // stringstream used for the conversion

convert << Number;//add the value of Number to the characters in the stream

Result = convert.str();//set Result to the content of the stream

//Result now is equal to "123"

string to number

string Text = "456";//string containing the number
int Result;//number which will contain the result

stringstream convert(Text); // stringstream used for the conversion initialized with the contents of Text

if ( !(convert >> Result) )//give the value to Result using the characters in the string
    Result = 0;//if that fails set Result to 0
//Result now equal to 456

Simple functions to do these conversions

template <typename T>
string NumberToString ( T Number )
{
	stringstream ss;
	ss << Number;
	return ss.str();
}

template <typename T>
T StringToNumber ( const string &Text )//Text not by const reference so that the function can be used with a 
{                               //character array as argument
	stringstream ss(Text);
	T result;
	return ss >> result ? result : 0;
}

Last edited on

firedraco (6243)

Looks good...but I have a question...does ss >> result filter out "junk" characters, i.e. if you put in "9kf84.85" into a double will it give you 984.85 or not do anything (giving you 0).

Also, 0 could theoretically be a valid value for what they are putting in...I would either ask them to specify a default value to return or throw an exception.

Bazzy (6281)

does ss >> result filter out "junk" characters

no, it stops at the 1^st invalid character:
"9kf84.85" becomes 9

Also, 0 could theoretically be a valid value for what they are putting in...

Notice that "simple" is underlined, those are just examples

Bazzy (6281)

A better string to number function:

template <typename T>
T StringToNumber ( const string &Text, T defValue = T() )
{
    stringstream ss;
    for ( string::const_iterator i=Text.begin(); i!=Text.end(); ++i )
        if ( isdigit(*i) || *i=='e' || *i=='-' || *i=='+' || *i=='.' )
            ss << *i;
    T result;
    return ss >> result ? result : defValue;
}

leaves only numeric characters, sign and scientific notation (for doubles)
so that StringToNumber("3 ran 8 dom 9 chars 0.45e+3",0.0) will produce 3890450

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Tilpo (8)

And how would a function that returns a default value if any text except for mathimatical operators are found look like?

Bazzy (6281)

The last version I provided leaves + and - in the string before converting it to a number (as a number can be in the form +123.456e-7), if you leave in the string other characters the result would be truncated
eg: "123/456" = 123

For the edits above: I've just noticed that I forgot to pass the string as reference

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namelij (3)

int result;
string text("123");
stringstream convert(text);

if(!(convert >> text))
result = 0;

cout << result;

why my answer is 3688360, when the text is "123" or "456"

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Bazzy (6281)

Your error is here:

1
2

if(!(convert >> text))
result = 0;

You are getting the number back in the string, fix it in this way:

1
2

if(!(convert >> result))
result = 0;

helios (17563)

Well, I've been using these, which've worked quite well, for me:

template <typename T>
long atoi(const std::basic_string<T> &str){
	std::basic_stringstream<T> stream(str);
	long res;
	return !(stream >>res)?0:res;
}
template <typename T>
std::basic_string<T> itoa(long n,unsigned w=0){
	std::basic_stringstream<T> stream;
	if (w){
		stream.fill('0');
		stream.width(w);
	}
	stream <<n;
	return stream.str();
}

namelij (3)

Bazzy,
thank you!

chiwing (154)

 for (int i= 65; i<=90;i++ )
	{
      convert << i;                   //error C2065: 'convert' : undeclared identifier
      drive_ID = convert.str(); //error C2065: 'convert' : undeclared identifier
      drive_ID = i + ":\\" ;       // error C2228: left of '.str' must have class/struct/union
 drive_type = GetDriveType(drive_ID);
      if (drive_type != 3) continue;
      
	  filename = drive_ID + ":\\Program Files\\JuniorT\\254.txt" ; 
      
      exist = CreateFile(filename, GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, 0, NULL);
              CloseHandle(exist);

i encount compile error of it
how to solve it?

Last edited on

Bazzy (6281)

You need to declare 'convert' before using it

chiwing (154)

#include "stdafx.h"
#include <windows.h>
#include <math.h>
#include <cstdlib>
#include <stdlib.h>
#include <stdio.h>
#include <iostream>
#include <string>
#include <string.h>

#include <WINERROR.H>
#include <stddef.h>
#include <stdlib.h>
#include <wchar.h>
#include <tchar.h>

string drive_ID ;
for (int i= 65; i<=90;i++ )
{
      stringstream convert;
      convert << i;    //*error C2079: 'convert' uses undefined class 'std::basic_stringstream<_Elem,_Traits,_Alloc>'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>,
1>            _Alloc=std::allocator<char>
1>        ]
warning C4552: '<<' : operator has no effect; expected operator with side-effect
*/

      drive_ID = convert.str();  //error C2228: left of '.str' must have class/struct/union  
      drive_ID = i + ":\\"        ;   
      drive_type = GetDriveType(drive_ID);
      if (drive_type != 3) continue;

       filename = drive_ID + ":\\Program Files\\JuniorT\\254.txt" ; 
      
      exist = CreateFile(filename, GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, 0, NULL);
              CloseHandle(exist);

after adding stringstream convert;

still have error
how to amend~
thanks

Last edited on

Bazzy (6281)

Did you #include<sstream> ?

jsmith (5804)

And need std::stringstream too

Duthomhas (13201)

Bazzy +1

chiwing (154)

may i ask how to convert number 324545678 to wstring ?

Bazzy (6281)

try with wstringstream:

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2
3

wstringstream wss;
wss << 324545678;
wstring ws = wss.str();

Last edited on

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Converting numbers to strings and strings to numbers

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