Transformation from char to float

Posted on 2003-10-29
Last Modified: 2010-07-27
I try to read a number from a char variable. The value of the char is something like "$GPGGA,11111.11,...". I would like to transform the "11111.11" to his float value (11111.11). I am using the BCB6. Is there any command in C++ to do it?

Question by:rsch1973
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Expert Comment

ID: 9641830
There is the function in Delphi.
function StrToFloat(const S: string): Extended;

I think that there is the same function in BCB
LVL 22

Expert Comment

ID: 9641857
One simple way woul dbe to use strchr() to find the first comma, then strchr() from there+1 to find the nexxt one,
then strncpy to extract the number to another string variable, then sscanf or atof() on that.


Expert Comment

ID: 9641872
Sample from MSDN:

/* SSCANF.C: This program uses sscanf to read data items
 * from a string named tokenstring, then displays them.

#include <stdio.h>

void main( void )
   char  tokenstring[] = "15 12 14...";
   char  s[81];
   char  c;
   int   i;
   float fp;

   /* Input various data from tokenstring: */
   sscanf( tokenstring, "%s", s );
   sscanf( tokenstring, "%c", &c );
   sscanf( tokenstring, "%d", &i );
   sscanf( tokenstring, "%f", &fp );

   /* Output the data read */
   printf( "String    = %s\n", s );
   printf( "Character = %c\n", c );
   printf( "Integer:  = %d\n", i );
   printf( "Real:     = %f\n", fp );

String    = 15
Character = 1
Integer:  = 15
Real:     = 15.000000

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LVL 11

Expert Comment

ID: 9642018
In addition to sscanf, atof will convert a pointer to a C-style string (char * at a null-terminated string). You can use grg99's suggestion to find the beginning of the string you want to convert and then call atof:

char * theLine;
// you read the value into theLine here
const char * comma;
comma = strchr(theLine, ',');
float value = atof(comma);

Hope this helps, -bcl
LVL 86

Expert Comment

ID: 9642141

#include <string>
#include <sstream>
#include <iostream>
#include <stdlib.h>

using namespace std;

void main () {

    float f;
    char* pc;
    string s;
    stringstream ss;

    ss << "$GPGGA,11111.11,...";

    getline ( ss, s, ','); // extract 'til 1st ','

    cout << s << endl;

    getline ( ss, s, ','); // extract 'til 2nd ','

    cout << s << endl;

    f = (float) strtod ( s.c_str (), &pc); // convert to 'float'

    cout << f << endl;

Accepted Solution

antoinebf earned 250 total points
ID: 9642176
What follows is a nice way of doing it with STL:

#include <sstream>
#include <string>
    float fValue(0);
    std::string myString("$GPGGA,11111.11,...");
    std::string::size_type pos1 = myString.find_first_of(',', 0);
    if (pos1 != std::string::npos)
        std::string::size_type pos2 = myString.find_first_of(',', pos1 + 1);
        std::string subString(myString.substr(pos1 + 1, pos2 != std::string::npos ? pos2 - pos1 - 1 : std::string::npos));
        std::istringstream iss(subString);
        iss >> fValue;

You can obviously play with the condition to make sure you convert only if possible, or if other conditions are met, but the basic idea here is the use the right shift operator.



Expert Comment

ID: 9642189
Or try this:

#include <string>
#include <iostream>
#include <sstream>
#include <stdlib.h>

using namespace std;

void main () {
  double d;
  string val("$GGG-1111.11TZR$");
  size_t pos = val.find_first_of("+-0123456789");

  if(pos==string::npos) {//not found
    d = 0.0;
  } else {
//1.Solution => use atof
  d= atof(val.c_str()+pos);

//2.Solution => use stringstream
  stringstream sio;
  sio << val.substr(pos);
  sio >> d;
  cout << "Double: " << d << endl;

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