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More efficient?

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Started by geekalert April 30, 2006 10:41 AM

10 comments, last by deadimp 17 years, 11 months ago

geekalert

193

Author

April 30, 2006 10:41 AM

I just have a quick question: out of A and B, would you say that A is more efficient? And safer? Thanks. A:


char* i = szFile + lstrlen(szFile);
while((i--) != szFile && (*i) != '\\');


for(int i = 0; i < lstrlen(szFile); i++)
{
    if(szFile == '\\')
        break;
}

All of the code above is preparing for the line:


lstrcpy(lpOutput, szFile + i);    // if B
lstrcpy(lpOutput, i + 1);         // if A

A JPEG is worth a thousand and twenty four DWORD's. ;)
Note: Due to vacationing my website will not be updated till late-August. Of course I still have internet, but who wants to program during a vacation?

Anonymous

April 30, 2006 10:46 AM

Quote:All of the code above is preparing for the line:

lstrcpy(lpOutput, szFile + i); // if B

lstrcpy(lpOutput, i + 1); // if A

Well then there is a scope issuse in B in respect to i.

Anonymous

April 30, 2006 10:49 AM

Can't you use one of these funcs which are standard
char *strchr(const char *string, int c)
char *strrchr(const char *string, int c)
char *strstr(const char *s1, const char *s2)
char *strpbrk(const char *s1, const char *s2)

Anonymous

April 30, 2006 10:50 AM

I think A and B don't even do the same!
A searches for the last \ in the string while the for loop searches for the first occurence.

Anonymous

April 30, 2006 10:52 AM

Quote:Original post by geekalert
I just have a quick question: out of A and B, would you say that A is more efficient? And safer? Thanks.

A:

char* i = szFile + lstrlen(szFile);
while((i--) != szFile && (*i) != '\\');

B:

for(int i = 0; i < lstrlen(szFile); i++)
{
if(szFile == '\\')
break;
}

All of the code above is preparing for the line:

lstrcpy(lpOutput, szFile + i); // if B
lstrcpy(lpOutput, i + 1); // if A

int i;
for(i = 0; i < lstrlen(szFile); i++)
{
if(szFile == '\\')
break;
}
lstrcpy(lpOutput, szFile + i); // if B

This should work though (don't know about the logic, but the syntax is right). Don't use A, it's quite hard to see what it does.

geekalert

193

Author

April 30, 2006 10:52 AM

oops. B was supposed to be 'reversed':

for(int i = lstrlen(szFile); i > -1; i--)

sorry for any confusion

Anonymous

April 30, 2006 11:03 AM

extern char* szFile;
char* start;
extern char* lpOuput;
start = *strrchr(szFile, '\\');//find last occurance in the string
if(start)//check its valid
{
strcpy(lpOutput,start);
}
else
{
//error character not found in string;
}

Anonymous

April 30, 2006 11:08 AM

hmmmm

start = *strrchr(szFile, '\\');//find last occurance in the string

should read

start = strrchr(szFile, '\\');//find last occurance in the string

Zahlman

1,682

April 30, 2006 01:46 PM

If you don't have the knowledge to test for yourself which is faster, then you *definitely* shouldn't be worrying about it. As for safety, they're both *horrible*, and anyway, the C standard library provides the exact same thing. In a way that is pretty much guaranteed to be at least as fast if not faster, because the compiler implementation can replace it with pre-prepared assembly and doesn't have to look at your code and figure out "oh, he just wants a strrchr()".

But even then, any kind of C string manipulation is horrible for safety. If you at all have the option of using C++, you can write something like:

std::string output(filename, 0, filename.rfind('\\') + 1);// +1 is if you want to keep the backslash.

and it will *still* probably be about as fast as whatever you were going to write "the hard way".

But all of that efficiency concern *completely and utterly misguided* anyway, because it will take orders of magnitude more time for the OS to open the file than any of this string processing.

Driv3MeFar

1,080

April 30, 2006 06:06 PM

Quote:Original post by geekalert

B:
for(int i = 0; i < lstrlen(szFile); i++){    if(szFile == '\\')        break;}

just a quick note: in a for loop (or any loop, really), you pretty much always want to do something like this:

int len = lstrlen(szFile);for(int i = 0; i < len; i++){    if(szFile == '\\')        break;}

in C++, the strlen() function has O(N) complexity, which is not something you want to do every iteration of a loop.

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