Header files of hashtable

#1

sir had used
#define HASHTABLE_H
#ifndef HASHTABLE_H

what does these means…?

#2

@ynikhil1999 hey Nikhil
for example
#ifndef
#ifndef < token >
/* code /
#else
/ code to include if the token is defined */
#endif
#ifndef checks whether the given token has been #defined earlier in the file or in an included file; if not, it includes the code between it and the closing #else or, if no #else is present, #endif statement. #ifndef is often used to make header files idempotent by defining a token once the file has been included and checking that the token was not set at the top of that file.
#ifndef _INCL_GUARD
#define _INCL_GUARD
#endif
for more such example go to this link
link

#3

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