CODEWITHSUNDEEP
clistlinked-list
ushiro_mawashi
ushiro_mawashi

Reputation: 3

Linked list trouble

Does anyone know what might be the problem with the following code? When I run it, I get the following output:

Insert a value in the list: 1
Do you want to continue? y/N: 
 1 ->

The fact is that the do-while loop executes until the scanf("%c", &ch) statement, and then it jumps out (so I cannot provide any input for the ch variable). I tried debugging with GDB and I got some weird messages: 

GI___libc_malloc (bytes=16) at malloc.c:malloc.c: No such file or directory.

Also, it says that the compiler couldn't find the vscanf.c file. Does anyone have an explanation for this strange behavior? Thanks! (The intention was to print the values of a singly linked list in reverse order.)

#include <stdio.h>
#include <stdlib.h>

struct node{
    int info;
    struct node* next;
};

struct node* head = 0;

void add_node(int value){

     struct node* current = malloc(sizeof(struct node));
     current->info = value;
     current->next = head;
     head = current;
 }

void print_node(struct node* head){

     while(head){

          printf(" %d -> ", head->info);
          head = head->next;
     }

     printf("\n");
 }

int main(void){

    int val;
    char ch;

    do {

       printf("Insert a value in the list: ");
       scanf("%d", &val);
       add_node(val);
       printf("Do you want to continue? y/N: ");
       scanf("%c", &ch);

    } while(ch == 'y' || ch == 'Y');

    printf("\n");
    print_node(head);
    return 0;
 }

Upvotes: 0

Views: 135

Answers (3)

Subbu
Subbu

Reputation: 2101

The problem you are encountering is because the once you type a value for val and then press enter , then \n still remains in the input buffer . Hence , the next scanf assumes that \n which is still in the input buffer is its input , and consumes it and then loop exits .

Other Solutions :-

1) scanf("%d%*c",&val);

This would assign the first input character to val and then anything after that would be eaten up . Hence , the \n would not go into the next scanf

2) scanf("%[^\n]%*c",&val);

This would assign the anything to the val except \n and then \n would be eaten up .

Upvotes: 0

Tarun Verma
Tarun Verma

Reputation: 329

You can check for proper input in an if-else block, and execute your code accordingly. For example, here is something I would do if I needed to check whether the user wants to continue or not:

char chTemp; //Declare a test variable to check for newline
printf("Do you want to continue? y/N: ");
if (scanf("%c%c",&ch,&chTemp) != 2 || chTemp != '\n')
{
     printf("Error in input (Integer input provided)");
}
else
{
     //Do stuff.
}

Not only will it solve your problem, but it will also check for careless integer inputs.

Upvotes: 2

Jacob Pollack
Jacob Pollack

Reputation: 3751

If you want the input to be separated by a new line (which it appears that you do) then change the format of how you are reading in your character. Change following:

scanf( "%c", &ch );

... to this:

scanf( "\n%c", &ch ); // << Note, \n to pickup newline before reading the value.

Upvotes: 2

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