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I don't understand text formatting

General and Gameplay Programming Programming

Started by noatom February 06, 2013 06:17 PM

7 comments, last by Ectara 11 years, 2 months ago

noatom

927

Author

February 06, 2013 06:17 PM

So,I always had a big problem with text formatting,mainly because I don't understand why it's needed.

I mean I know you need string formatting to show int i = 1 to the screen as text,but why for other stuff?

For example,in one of my books there is something like this:

std::string p_apptitle;

std::string getAppTitle() { return this->p_apptitle; }

char title[255];

sprintf(title, "%s", g_engine->getAppTitle().c_str());

Why is sprintf needed? I mean apptitle is returned as a string already!

Can someone please explain the difference between sprint,printf? And why they are needed? Or just give me a link to a website or video,cause I tried finding one but I haven't found one that actually explains everything.

Bacterius

13,181

February 06, 2013 06:22 PM

printf prints (writes) to stdout, the standard output (which is usually a terminal window, and often hidden in Windows GUI programs). sprintf "prints" to a null-terminated char array (a C string) so you use that when you need to format something but don't want it ending up directly on the terminal - say you want to save it in some text file. It has nothing to do with the return value.


char title[255];
sprintf(title, "%s", g_engine->getAppTitle().c_str());

Translation: allocate a 255-character C string (which can then contain 254 characters + the null character), and write "%s" into it, using arguments g_engine->getAppTitle().c_str()). So this will just copy the application title (?) into "title". Not a very educational use of sprintf, but you get the idea, you can make it more complicated:


int a = 4;
int b = 7;
int c = a + b;
char foo[10];
sprintf(foo, "%d + %d = %d", a, b, c);
// foo now contains "4 + 7 = 11"

Yeah, this isn't exactly Python but that's how general-purpose string formatting works in C. Since you're using C++, you might want to prefer C++ alternatives (such as stringstreams and the << operator).

“If I understand the standard right it is legal and safe to do this but the resulting value could be anything.”

noatom

927

Author

February 06, 2013 07:34 PM

another question... %s and %d seem very fringe to me.Can you give me a small list or something so I know when to use %s and when to use %d? I mean now I clearly know that s is for strings and d for ints,but I remember there were some more.

And how are these called? %s and %d? Just arguments?

Bacterius

13,181

February 06, 2013 07:45 PM

another question... %s and %d seem very fringe to me.Can you give me a small list or something so I know when to use %s and when to use %d? I mean now I clearly know that s is for strings and d for ints,but I remember there were some more.

And how are these called? %s and %d? Just arguments?

No, the arguments are what you are going to substitute in their place. The %s and %d are called format flags/codes/etc.. and you can specify a lot more (say, for floating-point numbers, the number of decimals you want, or the maximum length of a string to substitute, etc...).

Here is a short introduction: http://personal.ee.surrey.ac.uk/Personal/R.Bowden/C/printf.html

For the full documentation, http://www.kernel.org/doc/man-pages/online/pages/man3/sprintf.3.html

“If I understand the standard right it is legal and safe to do this but the resulting value could be anything.”

DoctorGlow

1,813

February 06, 2013 07:57 PM

http://www.cplusplus.com/reference/cstdio/printf/?kw=printf and it's called format specifiers.

NightCreature83

5,061

February 06, 2013 10:36 PM

The C++ way of doing this is to use a stringstream which works as follows

#include <sstring>
std::stringstream sstring(""); //init to empty string
sstring << "Hello World!" << 1 << 2.0f << 3.0 << "!";
std::string helloWorldString = sstring.str();

The drawback of this for a lot of game developers is that stringstream allocates memory on the fly you do not really want to do that in code that runs often. A sprintf_s in those cases doesn't have this dynamic allocation pattern it will just work on the area you give it through the char buffer. Btw use snprintf or sprintf_s if you can as sprintf will happily write past your buffer if the string to enter is bigger than the provided buffer. These other functions will allow you to pass a maximum number of characters to write into the string and will return -1 if it errors.

Worked on titles: CMR:DiRT2, DiRT 3, DiRT: Showdown, GRID 2, theHunter, theHunter: Primal, Mad Max, Watch Dogs: Legion

Khatharr

8,814

February 07, 2013 01:43 AM


char title[255];
sprintf(title, "%s", g_engine->getAppTitle().c_str());

BUFFER OVERRUN! YAAAAAAAAAAAAAAAAAAAAY!

void hurrrrrrrr() {__asm sub [ebp+4],5;}

There are ten kinds of people in this world: those who understand binary and those who don't.

nfactorial

735

February 07, 2013 02:25 PM

The first string you pass to sprintf is the format specifier, so if you did:


sprintf( title, g_engine->getAppTitle().c_str() );

and your app title had (for example) "%d" in it (eg. "%data manager%") the sprintf would look for an integer and replace the %d with it, which would usually be some garbage from the stack. It might seem obvious in this contrived example, but can more difficult depending on what 'accidental' specifier is in the string and what is on the stack, avoiding any issues completely is always more preferable.

In the particular example you gave, there isn't any particular need to use sprintf however. It may as well be a simple string copy.

Ectara

3,097

February 07, 2013 09:00 PM

That snippet is a naive way of copying the contents of a std::string to a character array. There's probably a better function for this, like std::basic_string::copy().

http://www.cplusplus.com/reference/string/string/copy/

Calling a formatted printing function seems like overkill to just to copy a given amount of characters.

I don't understand text formatting

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