How to convert a floating point number to a string with max. 2 decimal digits in Delphi - delphi

Question

How can I convert a floating point number to a string with a maximum of 2 decimal digits in Delphi7?
I've tried using:
FloatToStrF(Query.FieldByName('Quantity').AsFloat, ffGeneral, 18, 2, FS);
But with the above, sometimes more than 2 decimal digits are given back, ie. the result is: 15,60000009

Answer 1

Use ffFixed instead of ffGeneral.
ffGeneral ignores the Decimal parameter.
When you use ffGeneral, the 18 is saying that you want 18 significant decimal digits. The routine will then express that number in the shortest manner, using scientific notation if necessary. The 2 is ignored.
When you use ffFixed, you are saying you want 2 digits after the decimal point.
If you are wondering about why you sometimes get values that seem to be imprecise, there is much to be found on this site and others that will explain how floating-point numbers work.
In this case, AsFloat is returning a double, which like (most) other floating-point formats, stores its value in binary. In the same way that 1/3 cannot be written in decimal with finite digits, neither can 15.6 be represented in binary in a finite number of bits. The system chooses the closest possible value that can be stored in a double. The exact value, in decimal, is:
15.5999999999999996447286321199499070644378662109375
If you had asked for 16 digits of precision, the value would've been rounded off to 15.6. But you asked for 18 digits, so you get 15.5999999999999996.

Answer 2

If you really mean what you write (MAX 2 decimal digits) and does not mean ALWAYS 2 decimal digits, then the two code snippets in the comments won't give you want you asked for (they will return a string that ALWAYS has two decimal digits, ie. ONE is returned as "1.00" (or "1,00" for Format depending on your decimal point).
If you truly want an option with MAX 2 decimal digits, you'll have to do a little post-processing of the returned string.
FUNCTION FloatToStrMaxDecimals(F : Extended ; MaxDecimals : BYTE) : STRING;
BEGIN
Result:=Format('%.'+IntToStr(MaxDecimals)+'f',[F]);
WHILE Result[LENGTH(Result)]='0' DO DELETE(Result,LENGTH(Result),1);
IF Result[LENGTH(Result)] IN ['.',','] THEN DELETE(Result,LENGTH(Result),1)
END;
An alternative (and probably faster) implementation could be:
FUNCTION FloatToStrMaxDecimals(F : Extended ; MaxDecimals : BYTE) : STRING;
BEGIN
Result:=Format('%.'+IntToStr(MaxDecimals)+'f',[F]);
WHILE Result[LENGTH(Result)]='0' DO SetLength(Result,PRED(LENGTH(Result)));
IF Result[LENGTH(Result)] IN ['.',','] THEN SetLength(Result,PRED(LENGTH(Result)))
END;
This function will return a floating point number with MAX the number of specified decimal digits, ie. one half with MAX 2 digits will return "0.5" and one third with MAX 2 decimal digits will return "0.33" and two thirds with MAX 2 decimal digits will return "0.67". TEN with MAX 2 decimal digits will return "10".
The final IF statement should really test for the proper decimal point, but I don't think any value other than period or comma is possible, and if one of these are left as the last character in the string after having stripped all zeroes from the end, then it MUST be a decimal point.
Also note, that this code assumes that strings are indexed with 1 for the first character, as it always is in Delphi 7. If you need this code for the mobile compilers in newer Delphi versions, you'll need to update the code. I'll leave that exercise up to the reader :-).

Answer 3

i use this function in my application:
function sclCurrencyND(Const F: Currency; GlobalDegit: word = 2): Currency;
var R: Real; Fact: Currency;
begin
Fact:= power(10, GlobalDegit);
Result:= int(F*Fact)/Fact;
end;