Simple C program not running as expected

[MR Tharani]

#include <stdio.h>
#include <conio.h>
#include <string.h>
 
struct Book {
           char title [200];
           char Author_name [200];
           int eidtion ,year;
           };
     struct Book b[100];
     static int i =0;
 
int add_info ();
 
int main(){
        int opt=1;
        while (opt !=0){
        printf("\n1. Add book information :");
        printf("\n2. Display list of books :");
        printf("\n3. book information :");
        printf("\n4. Add book information :");
        scanf("%d" ,&opt); 
 switch (opt){
        case 1:{
     add_info (); 
     break;
     }
 
  }
  }
          return 0;            
}
 
int add_info (){
     //gets(b[i].title);
     printf ("Enter Book title :");
     gets(b[i].title);
     printf ("Enter Author name :");
     gets(b[i].Author_name);
     printf ("Enter Book eidtion :");
     scanf("%d",&b[i].eidtion);
     printf ("Enter Year of eidtion :");
     scanf("%d",&b[i].year);
     i++;
     return 0;
     }

//as I run this program gets(b[i].title); has not executed .output on is like *** Enter Book title : Enter Book eidtion : _ both statements got printed .not asked to input in varaible b[i].title. when I used line( commented now )then it run ture manner

[weaknessforcats]

scanf stops scanning when it encounters a character tht doesn't match the type you are scanning for. In your case it looks like the enter key you used in one scanf is still in the input buffer when you reach the next scanf. That causes the second scanf to return without doing anything.

Be sure to to a getch after each scanf to remove that enter key.

[whodgson]

....also B Stroustrup recommends that C programmers should never use gets() and "consider it poisoned". ref pp1018 Programming Principles and Practice using C++.

[sudhakartyagi]

It is better you flush the standard input before you take the input form the user as

Gets reads characters from stdin and stores them as a string into b[i].title until a newline character ('\n') or the End-of-File is reached. So when it goes to read the input for the "Enter Book title :", it already find a previously stored '\n' in keyboard buffer and accepts it.

printf ("Enter Book title :");
fflush(stdin);
gets(b[i].title);

[weaknessforcats]

Unfortunately, you cannot flush an input stream because you cannot tell how much input there is. Not all of it may be entered yet.

Also, gets() converts the \n in the input to a null in the buffer hoilding the string so there is no null to trip over later.

The real problem with gets() is that you cannot know how much input will be read until a \n is encountered. And that means you cannot know how large a buffer you need to store the string. If you overrun the buffer you have corrupted memory and you will crash.

[sudhakartyagi]

Yes weaknessforcats I agree with you on

"The real problem with gets() is that you cannot know how much input will be read until a \n is encountered. And that means you cannot know how large a buffer you need to store the string. If you overrun the buffer you have corrupted memory and you will crash"

but what if I flush the buffer before presenting the message to the user to enter any input. For example

fflush(stdin);
printf ("Enter Book title :");
gets(b[i].title);

I think this will pickup the user input as is not the undesirable ones...

[weaknessforcats]

I say again you cannot use fflush to flush in input stream.

fflush can only flush your output buffer. If you use it in an input buffer, the results are undefined. Therefore, fflush(stdin) is an error.

Input streams need to be processsed and the functions that process an input stream all presume that you know what data is coming before it is entered. That is, you wil have formatted input. Now gets and scanf and the other f functions will behave correctly.

If you do not know what will be entered, then you must fetch a character at a time and make a decision for that character as to keeping it or discarding it. This can be a lot of work. So unless you are writing some professional code, stick with formatted input. That will mean that you don't need to clear out your input buffer because there will be nothing unexpected there.

[sudhakartyagi]

What if I say

while ((ch = getchar()) != '\n' && ch != EOF);

instead of fflush.