AnsiString to double

[davman2002]

I am an experienced C++ Programmer but this is my first experience with Borland. I need to know how to convert a string variable to a double. This is the code that I have but it does not work

AnsiString Value1 = txtValue1->Text.ToDouble();
AnsiString Value2 = txtValue2->Text.ToDouble();

DoCalculations.Subtract(Value1,Value2);

debugger says cannot convert AnsiString to double

Please help

 

[Ramone]

There is a function in the math.h librairy that converts a string to a double but I don't know if it is what you are looking for...

From Borland C++5.0 Help files:

Syntax

#include <math.h>
double atof(const char *s);
long double _atold(const char *s);

Description

Converts a string to a floating-point number.

atof converts a string pointed to by s to double; this function recognizes the character representation of a floating-point number, made up of the following:
An optional string of tabs and spaces
An optional sign
A string of digits and an optional decimal point (the digits can be on both sides of the decimal point)
An optional e or E followed by an optional signed integer

The characters must match this generic format:
[whitespace] [sign] [ddd] [.] [ddd] [e|E[sign]ddd]
atof also recognizes +INF and -INF for plus and minus infinity, and +NAN and -NAN for Not-a-Number.
In this function, the first unrecognized character ends the conversion.
_atold is the long double version; it converts the string pointed to by s to a long double.
The functions strtod and _strtold are similar to atof and _atold; they provide better error detection, and hence are preferred in some applications.

Return Value

atof and _atold return the converted value of the input string.
If there is an overflow, atof (or _atold) returns plus or minus HUGE_VAL (or _LHUGE_VAL), errno is set to ERANGE (Result out of range), and _matherr (or _matherrl) is not called.

Example:

#include <stdlib.h>
#include <stdio.h>

int main(void)
{
float f;
char *str = &quot;12345.67&quot;;

f = atof(str);
printf(&quot;string = %s float = %f\n&quot;, str, f);
return 0;
}

Borland C++ 5.0 Programmer's Guide

 

[2ffat]

If you use atof you have to convert AnsiString to a character array with c_str().

float f;
AnsiString Str = "12345.67";

f = atof(Str.c_str());

For AnsiString values you can also try StrToFloat. When using this it is a good idea to use it in a try ... catch clause just in case the String does not contain a usable value

double Value1, Value2; // You defined these as Strings not doubles
try
{
    // Try to convert from string to double
    Value1 = StrToFloat(txtValue1.c_str());
    Value2 = StrToFloat(txtValue2.c_str());
}
catch (...)
{
     // Warn the user about the error
}

James P. Cottingham

When a man sits with a pretty girl for an hour, it seems like a minute. But let him sit on a hot stove for a minute and it's longer than any hour. That's relativity.
[tab][tab]Albert Einstein explaining his Theory of Relativity to a group of journalists.

 

[Oinbones]

To convert an AnsiString into (long) Double the method AnsiString::ToDouble() is the right choice.

The declaration of the method is (C++Builder Help)

double __fastcall ToDouble() const;

Consequently, the method returns a DOUBLE value. What you try by

AnsiString Value# = txtValue#->Text.ToDouble();

is to assert a double value to an AnsiString! The compiler of course moans. (You could also try AnsiString1= .5; for example with the same compiler error.) - Right is that txtValue->Text is a valid AnsiString to be applied the method ToDouble on.

Therefore one solution could look like:

float value1= txtValue1->Text.ToDouble();
float value2= txtvalue2->Text.ToDouble();

DoCalculations.Subtract(value1,value2);

That' s all!

----------------------------------------------------------
Also being a beginner with C++Builder (4.0) I have a problem to apply the method FloatToStrF for the vice versa conversion AnsiString -> float. However, I could go around by using &quot;old&quot; char *, sprintf function and conversion char * -> AnsiString! - The problem here is that the compiler does not want to know the necessary constants like sffGeneral.

 

[butthead]

Memo2->Lines->Strings [0] = FloatToStrF(result, ffCurrency, 18, 2);

Memo2->Lines->Strings [1] = FloatToStrF(result, ffNumber, 18, 2);

Memo2->Lines->Strings [2] = FloatToStrF(result, ffExponent, 6, 2);