Program doesn't wait for user input!

This is a discussion on Program doesn't wait for user input! within the C Programming forums, part of the General Programming Boards category; Hi, I am trying to make a program that finds the Nth node in the linked list where N is ...

  1. #1
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    Program doesn't wait for user input!

    Hi, I am trying to make a program that finds the Nth node in the linked list where N is user input. When the user adds the element in the linked list, after each addition I ask whether to continue addition or stop. Here the user is expected to enter Y/N as his choice but the program skips this portion and doesn't wait for user input here.

    Code:
    #include<stdio.h>
    #include<stdlib.h>
    
    struct node 
    {
    	int nodeValue;
    	struct node *link;
    };
    
    struct node *addElement(struct node *headPtr,int elem); 
    int lengthList(struct node *headPtr);
    int Find_Nth_Node(struct node *headPtr,int number);
    
    int main()
    {
    	int num,temp,value;
    	char choice;
    	struct node *front=NULL;
    	
    	/*printf("Enter number of nodes:");
    	scanf("%d",&num);*/
            choice='y';
    	printf("\nEnter elements:\n");
    	while(choice=='y'||choice=='Y')
    	{
    		scanf("%d",&temp);
    		front=addElement(front,temp);
    		printf("\nWant to enter more?");
    		scanf("%c",&choice);         //doesn't wait for user input
    	}
    
    	printf("\nEnter the number of node whose value is desired:");
    	scanf("%d",&num);
    	if(num>lengthList(front))
    	{
    		printf("\nSorry, there are less than %d nodes in the list",num);
    		return 0;
    	}
    	value=Find_Nth_Node(front,num);
    
    	printf("\nThe value at node %d is:%d",num,value);
    	getchar();
    	return 0;
    }
    
    struct node *addElement(struct node *headPtr,int elem)
    {
    	struct node *trav_ptr,*temp;
    	temp=(struct node *)malloc(sizeof(struct node));
    	if(temp==NULL)
    	{
    		printf("\nError, no free space");
    		return 0;
    	}
    	else
    	{
    		temp->nodeValue=elem;
    		temp->link=NULL;
    	}
         
    if(headPtr==NULL)											       	                                    //If linked list is empty
    		headPtr=temp;
    	else																		//If linked list is not empty
    	{
    		trav_ptr=headPtr;
    		while(trav_ptr->link!=NULL)
    			trav_ptr=trav_ptr->link;
    		trav_ptr->link=temp;
    	}
    	return headPtr;
    }
    
    int lengthList(struct node *headPtr)
    {
    	int length=1;
    	while(headPtr->link!=NULL)
    	{
    		headPtr=headPtr->link;
    		length++;
    	}
    	return length;
    }
    
    int Find_Nth_Node(struct node *headPtr,int number)
    {
    	int ctr=0;
    	while(ctr++<number)
    		headPtr=headPtr->link;
    	return headPtr->nodeValue;
    }
    Waiting for some suggestions.

  2. #2
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    All the basic format conversions (except %c) skip white space and newlines.
    %c is different, it unconditionally reads the next character.

    In your code, this will typically be the \n left at the end of the previous %d.

    You could read this -> Cprogramming.com FAQ > Flush the input buffer

    Or you could use fgets() to read ALL input into a buffer, then use whatever (perhaps sscanf) to get from the buffer whatever interests you.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
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    Thanks for the suggestion, you are right, the choice variable was taking newline as the input and continuing the execution without user input. So i flushed the input buffer and now the program works as intended.

  4. #4
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    I see, so the %c will actually read in the "Enter" key from the %d above?

  5. #5
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    Yes, I debugged the program and choice variable contained ASCII 10(Linefeed) that caused the program to produce mysterious results.

  6. #6
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    Quote Originally Posted by gaurav_13191 View Post
    Thanks for the suggestion, you are right, the choice variable was taking newline as the input and continuing the execution without user input. So i flushed the input buffer and now the program works as intended.

    Quick and easy fix...
    Code:
    scanf(" %c",&ch);
    Note the space inserted between the " and the %.

  7. #7
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    Quote Originally Posted by CommonTater View Post
    Quick and easy fix...
    Code:
    scanf(" %c",&ch);
    Note the space inserted between the " and the %.
    I almost forgot about this method,
    every time I encounter that problem I just use fflush(stdin) .

    any difference by using fflush or add blackspace behind it?

  8. #8
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    Quote Originally Posted by Blane View Post
    I almost forgot about this method,
    every time I encounter that problem I just use fflush(stdin) .

    any difference by using fflush or add blackspace behind it?
    Unless you're using that braindead old Turbo C or Turbo C++ compiler fflush(stdin) is some serious bad juju... that can cause data loss on buffered input streams.

  9. #9
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    Quote Originally Posted by Blane View Post
    any difference by using fflush or add blackspace behind it?
    After an exhaustive internet search, making multiple phone calls, and then writing a few letters; I was rewarded with this little gem hidden away by the all powerful mods to ensure us mere mortals would never find it:

    FAQ-fflush(stdin)

    Quote Originally Posted by anduril462 View Post
    Now, please, for the love of all things good and holy, think about what you're doing! Don't just run around willy-nilly, coding like a drunk two-year-old....
    Quote Originally Posted by quzah View Post
    ..... Just don't be surprised when I say you aren't using standard C anymore, and as such,are off in your own little universe that I will completely disregard.
    Warning: Some or all of my posted code may be non-standard and as such should not be used and in no case looked at.

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