How to append float value of into a string ?

Question

I have a float variable lng = 33.785469 and lat = 78.126548. Now can I convert them to String and append in a String variable "my_location" as "your location is \nlng = 33.785469 \nlat = 78.126548". I was trying to do this with

    char charVal[5];               //temporarily holds data from vals 
    String stringVal = "";     //data on buff is copied to this string

    dtostrf(lng , 4, 2, charVal);  //4 is mininum width, 2 is precision; float value is copied onto buff
    //convert chararray to string
    for(int i=0;i<sizeof(charVal);i++)
    {
    stringVal+=charVal[i];
    }
    GPS_string = GPS_string + "Car location: \nlat:" + stringVal; 
    stringVal = "";

and it given me an error :

error: cannot convert 'String' to 'double' for argument '1' to 'char* dtostrf(double, signed char, unsigned char,

Answer 1

This has been answered in the previous question but I can repeat it here:

void loop()
{
  ...
  float latitude = 33.546600;
  float longitude = 75.456912;
  String buf;
  buf += F("your location is \nlat:");
  buf += String(latitude, 6);
  buf += F("\nlong:");
  buf += String(longitude, 6);
  Serial.println(buf);
  ...
}

An alternative is to use dtostrf() as in your snippet:

void loop()
{
  ...    
  float latitude = 33.546600;
  float longitude = 75.456912;
  const int BUF_MAX = 64;
  char buf[BUF_MAX];
  const int VAL_MAX = 16;
  char val[VAL_MAX];
  strcpy_P(buf, (const char*) F("your location is \nlat:"));
  dtostrf(latitude, 8, 6, val);
  strcat(buf, val);
  strcat_P(buf, (const char*) F("\nlong:"));
  dtostrf(longitude, 8, 6, val);
  strcat(buf, val);
  ...
}

Cheers!