[C + +] string and int, float, double convert each other

Reference: http://blog.csdn.net/candadition/article/details/7342380

Converting a string type to an int, float, double type is primarily done in the following ways:

# method One: Use StringStream

StringStream has been introduced in methods that convert int or float types to string types, and can also be used as a type of string to convert to a commonly used numeric type.

Demo:

#include <iostream>
#include <sstream> // This string needs to be introduced using stringstream
using namespace std;
  
// Template function: convert string type variables to commonly used numeric types (this method has universal applicability)
template <class Type>
Type stringToNum (const string & str)
{
     istringstream iss (str);
     Type num;
     iss >> num;
     return num;
}
  
int main (int argc, char * argv [])
{
     string str ("00801");
     cout << stringToNum <int> (str) << endl;
  
     system ("pause");
     return 0;

Input Results 801

#method two: Use the Atoi (), Atil (), atof () functions-----------------are actually conversions of char types to numeric types

Note : If string s is empty with Atoi, the return value is 0. It is not possible to determine if s is 0 or null

1. Atoi (): int atoi (const char * str);

Description: Parses the C string str interpreting its content as a integral number, which is returned as anintvalu E.

parameter: str : C string beginning with the representation of a integral number.

return value: 1.  The successful conversion displays a value of type int.  2. the non-convertible string returns 0. 3. if the buffer overflows after the conversion, returnInt_maxorint_min

Demo:

#include <iostream> using namespace std; int main ()  
{ int i; char szInput [256];  
    cout<<"Enter a number: "<<endl;  
    fgets ( szInput, 256, stdin );  
    i = atoi (szInput);  
    cout<<"The value entered is :"<<szInput<<endl;  
    cout<<" The number convert is:"<<i<<endl; return 0;  
} 

Output

Enter a number:48

The value entered is:48

The number convert is:48

2.aotl (): Long int atol (const char * str);

Description: C string str interpreting its content as an integral number, which is returned as along intvalue (usage and at Oi function similar, return value is long int)

3.atof (): Double atof (const char * str);

Parameters: C string beginning with the representation of a floating-point number.

return Value:  1. Conversion successfully returns a value of type Doublel 2. Cannot convert, return 0.0. 3. Cross-border, return tohuge_val

Demo:

/ * atof example: sine calculator * /
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main ()
{
   double n, m;
   double pi = 3.1415926535;
   char szInput [256];
   printf ("Enter degrees:");
   gets (szInput);
   // char type is converted to double type
   n = atof (szInput);
   m = sin (n * pi / 180);
   printf ("The sine of% f degrees is% f \ n", n, m);
    
   return 0;
} 

C + + int type conversion String type

Reference: http://blog.csdn.net/candadition/article/details/7342092

In C + +, unlike C # or Java, you can directly use string addition to convert an int type to a string type. This type conversion in C + + requires some extra functions.

One, C + + int to string

#method one: Use the ITOA function: char * itoa (int value, char * str, int base);

description : Convert integer to String (non-standard function)

Parameters:

Value:value to being converted to a string. Str:array in memory where to store the resulting null-terminated string.  Base:numerical base used to represent the value as a string, between2and, wheremeans decimal base,hexadecimal,8octal, and2binary.

Demo:

#include <iostream>
using namespace std;
int main (int argc, char * argv [])
{
     int n = 30;
     char c [10];
     // Binary conversion
     itoa (n, c, 2);
     cout << "2->" << c << endl;
     // decimal conversion
     itoa (n, c, 10);
     cout << "10->" << c << endl;
     // Hex conversion
     itoa (n, c, 16);
     cout << "16->" << c << endl;

     system ("pause");
     return 0;
}

Output:

2-> 11110  

10-> 30  

16-> 1e  

#method two: Use Sprintf:int sprintf (char * str, const char * format, ...);

Parameter description:

% The percent symbol is printed without conversion.

b Integer converted to binary.

c Integer is converted to the corresponding ASCII character.

d Integer converted to decimal.

F times precision converted to floating point.

o Integers are converted to octal.

s Integer into string.

x integers are converted to lowercase hexadecimal.

X integers are converted to uppercase hexadecimal. 

Demo:

#include <iostream>
#include <string>
using namespace std;
int main ()
{
     int n = 30;
     char c [20]; // char * c;
     //% d decimal
     sprintf (c, "% d", n);
     cout << c << endl;
     //% o octal
     sprintf (c, "% o", n);
     cout << c << endl;
     //% X uppercase hexadecimal
     sprintf (c, "% X", n);
     cout << c << endl;
     //% cACSII characters
     sprintf (c, "% c", n);
     cout << c << endl;
    
     //% f floating point conversion
     float f = 24.678;
     sprintf (c, "% f", f);
     cout << c << endl;
     //%.2f "keep two decimal places
     sprintf (c, "% .2f", f);
     cout << c << endl;
     // Convert two numbers
     sprintf (c, "% d-%. 2f", n, f);
     cout << c << endl;

     system ("pause");
     return 0;
}

Output:

30  

36  

1E  

Note: Here is a special symbol  

24.677999  

24.68  

30-24.68 #method three: Using StringStream

Input/output string Stream class:

StringStreamprovides an interface to manipulate strings as if they were input/output streams.

#include <iostream>
#include <string>
#include <sstream> // Introduce the stringstream header file
using namespace std;
int main ()
{
     stringstream strStream;
     int a = 100;
     float f = 23.5566;
     // int and float types can be stuffed into stringstream
     strStream << a << "----" << f;
     string s = strStream.str ();
     cout << s << endl;

     system ("pause");
     return 0;
}

Output:

----23.5566  

#fourth, other

1.SPRINTF can cause buffer overflow, consider using snprintf or nonstandard asprintf

2. If it is an MFC program, you can use Cstring::format

3. If you use boost, you can use it directly: string s = Boost::lexical_cast <string> (a);

4.atoi is also non-portable.

[C + +] string and int, float, double convert each other