how can you write a program that returns the last integer of the inputted number from the keyboard by the user using recursion?

Why do you ever want to do that? Because it is your homework assignment?

You need a few pieces:

  • Input routine to collect user number
  • Process to split that number into individual digits
  • Recursive call that takes the result of the above operation as input and returns a digit in the terminating case

Recursion can be tricky ... especially when you first try it ... so look up lots of examples where recursion is used.

For a popular example, see 'quick sort'

The following (inefficent design) on a related example may give you some ideas on how to get started ...

/* getLastCharRecursive.c */

#include <stdio.h>
#include <string.h>
#include <ctype.h> /* re. tolower ... */

/*
    this efficient demo uses recursive calls ...
    string passed in MUST BE NON-empty
*/
char getLastDig( const char* s )
{
    char c = s[0];
    if( *(s+1) == 0 ) return c;
    getLastDig( s+1 );
}

/* a handy utility for many C student coding problems ... */
char takeInChar( const char* msg )
{
    char chr;
    fputs( msg, stdout ); fflush( stdout );
    chr = getchar();
    if( chr != '\n' )
        while( getchar() != '\n' ) ; /* flush stdin ... */
    return chr;
}


int main()
{
    char buf[1024], c, *p;

    do
    {
        for( ; ; )
        {
            fputs( "Enter several char's: ", stdout );
            fflush( stdout );

            fgets( buf, sizeof(buf) , stdin );
            p = strchr( buf, '\n' );
            if( p ) *p = 0;
            else while( getchar() != '\n' ); /* flush stding */

            printf( "You entered '%s'\n", buf );

            /* ensure something entered before breaking ... */
            if( buf[0] ) break; /* if( buf[0] != 0 ) break; */
            /* else...    */
            printf( "You *must* enter something ... "
                    "try again ...\n");
        }

        c = getLastDig( buf );

        printf( "The last char entered (before 'Enter pressed') "
                "was: '%c'\n", c );
    }
    while( tolower( takeInChar( "More (y/n) ? " )) != 'n' );

    fputs( "\nPress 'Enter' to continue/exit ... ", stdout );
    fflush( stdout );
    getchar();
    return 0;
}