C++ 类型转换 atoi atol atof itoa ftoa

1. atoi

int atoi ( const char * str );

Convert string to integer

Parses the C string str interpreting its content as an integral number, which is returned as an int value.

/* atoi example */
#include <stdio.h>
#include <stdlib.h>

int main ()
{
  int i;
  char szInput [256];
  printf ("Enter a number: "); fgets ( szInput, 256, stdin ); i = atoi (szInput); printf ("The value entered is %d. The double is %d.\n",i,i*2); return 0; }

output

Enter a number: 73
The value entered is 73. The double is 146.

2. atol

long int atol ( const char * str );

Convert string to long integer

Parses the C string str interpreting its content as an integral number, which is returned as a long int value.

/* atol example */
#include <stdio.h>
#include <stdlib.h>

int main ()
{
  long int li;
  char szInput [256];
  printf ("Enter a long number: "); gets ( szInput ); li = atol (szInput); printf ("The value entered is %d. The double is %d.\n",li,li*2); return 0; }

output

Enter a number: 567283
The value entered is 567283. The double is 1134566.

3. atof

double atof ( const char * str );

Convert string to double

Parses the C string str interpreting its content as a floating point number and returns its value as a double.

/* 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 ); n = atof ( szInput ); m = sin (n*pi/180); printf ( "The sine of %f degrees is %f\n" , n, m ); return 0; } Output Enter degrees: 45
The sine of 45.000000 degrees is 0.707101

4. itoa

char *  itoa ( int value, char * str, int base );

Convert integer to string (non-standard function)

Converts an integer value to a null-terminated string using the specified base and stores the result in the array given by str parameter.

This function is not defined in ANSI-C and is not part of C++, but is supported by some compilers.


A standard-compliant alternative for some cases may be sprintf:

• sprintf(str,"%d",value) converts to decimal base.
• sprintf(str,"%x",value) converts to hexadecimal base.
• sprintf(str,"%o",value) converts to octal base

/* itoa example */
#include <stdio.h>
#include <stdlib.h>

int main ()
{
  int i;
  char buffer [33];
  printf ("Enter a number: "); scanf ("%d",&i); itoa (i,buffer,10); printf ("decimal: %s\n",buffer); itoa (i,buffer,16); printf ("hexadecimal: %s\n",buffer); itoa (i,buffer,2); printf ("binary: %s\n",buffer); return 0; }

Output

Enter a number: 1750
decimal: 1750
hexadecimal: 6d6
binary: 11011010110

5. ftoa

#include <sstream>

string convertDouble(double value) {
  std::ostringstream o;
  if (!(o << value))
    return ""; return o.str(); }