Double to string

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

Ked gave you ecvt()

It actually does something like this:

Code:

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

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

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.

Code:

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; 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

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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:

Code:

---

; 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

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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.

Well, i suppose you won't need that parameter :p

Code:

---

; 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

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