Double to string

[kedaman]

http://www.cplusplus.com/ref/cstdlib/ecvt.html

[jim mcnamara]

Ked gave you ecvt()

It actually does something like this:

char t[20];
double z=10;
sprintf(t,"%f",z);

[kedaman]

Jim, whats so cool about using sprintf, it has to interpret the "%f" at runtime right?

[jim mcnamara]

No. the compiler takes all of the stuff that is pre-defined like %f and provides a vector directly to the conversion routine. If the string is _T or WCHAR then it gets even messier and uses another routine.

If you play around like this: "%5.2f", then the code doesn't go directly to conversion at runtime. You can look at the asm c++ generates and see this. Change the compile options to produce an asm list.

Any numeric -> string conversion involves a fair amount of overhead. Formatting in general is very expensive. You can look at this as just formatting with type conversion.

[kedaman]

; 138  : 	char t[20];
; 139  : 	double z=10;
; 140  : 	sprintf(t,"%f",z);  

	lea	eax, DWORD PTR _t$[esp+20]
	push	1076101120				; 40240000H
	push	0
	push	OFFSET FLAT:??_C@_02JBAA@?$CFf?$AA@	; `string'
	push	eax
	call	_sprintf
	add	esp, 16					; 00000010H

Not that i understand why the parameters but _sprintf gets called and only other reference in the listing is
EXTRN _sprintf:NEAR
same thing here:

; 138  : 	char t[20];
; 139  : 	double z=10;
; 140  : 	sprintf(t,"%5.2f",z);  

	lea	eax, DWORD PTR _t$[esp+20]
	push	1076101120				; 40240000H
	push	0
	push	OFFSET FLAT:??_C@_05MPII@?$CF5?42f?$AA@	; `string'
	push	eax
	call	_sprintf
	add	esp, 16					; 00000010H

[jim mcnamara]

That's interesting. The last time I tried this , there were big differences in the entry point call. I wonder why. Sorry.

I don't have an answer, unless the compiler or the _sprintf entry point have changed since I last played with it.

We were testing performance and found the "%5.2f" was slower than "%f" for several hundred thousand iterations. So, we looked at the asm, and there was a load of difference. We were converting hundreds of millions of IBM BCD's to doubles, doing some calculations, then to strings and inserting them into a DB. The strings had to be a fixed format.

[kedaman]

Well, i suppose you won't need that parameter

; 138  : 	char t[20];
; 139  : 	sprintf(t,"%5.2f");  

	lea	eax, DWORD PTR _t$[esp+20]
	push	OFFSET FLAT:??_C@_05MPII@?$CF5?42f?$AA@	; `string'
	push	eax
	call	_sprintf
	add	esp, 8