CODEWITHSUNDEEP
cstackreverse
user4252523
user4252523

Reputation: 69

Unable to reverse a simple string using Stack's Pop() function

I have a simple stack program which is trying to reverse a string. I am able to 'display' the reversed string, but when it comes to storing the character returned by the Pop() function, I am failing to do so. What I am doing below is that in the main() after I have pushed the string to the stack, I try to use re-use str[] to store the reversed string. Since it is an array, I should be able to modify it. However, when I do so, for some strange reason the for-loop runs only ONE time. I am guessing it is doing so because in pop() *head = NULL in second iteration and hence it fails.

I am failing to run the for-loop for more than once. My questions is what is preventing me from storing the reversed string?

1) Why is the for-loop running only one time when I use str[i]?

2) Am I using str[i] in some strange way which is causing this to happen?

char ret = '\0';
void Push(Node **head, char data)
{
    Node *temp = (Node*)malloc(sizeof(Node));
    temp->data = data;
    temp->next = NULL;
    if (*head == NULL)
    {
        *head = temp;
        return;
    }

    temp->next = *head;
    *head = temp;
    return;
}

void PrintAll(Node *temp)
{
    if(temp == NULL) return;
    printf("%c, ", temp->data);
    PrintAll(temp->next);
}
char Pop(Node **head)
{
    if(*head == NULL)  
    {
        return '\0';
    }
    Node *temp = *head;
    ret = temp->data;
    *head = (*head)->next;
    free (temp);
    return ret;
}

int main()
{
    Node *head = NULL;

    char str[] = "helloworld";
    printf("Original String is:%s\n", str);
    printf("Length is:%lu\n", strlen(str));

    for (int i = 0; i <=strlen(str); i++)
    {
        Push(&head, str[i]);
    }

    printf ("String has been pushed to the stack.\n");
    PrintAll(head);
    printf("\nString has been printed. Now popping:%ld.\n", strlen(str));


    //Problem starts here in this for-loop
    for (int i = 0; i <=strlen(str); i++)
    {
        str[i] = Pop(&head); //When these next lines are enabled, the for-loop runs only once!
        printf("%c, ", str[i]); //Pop() is returning the value from a global variable 'ret'
        //printf("%c ", Pop(&head)); //if two previous lines are disabled, string is displayed in reverse

    }
    printf("\n");
    printf ("String has been popped from the stack.\n");
    return 0;
}

Upvotes: 0

Views: 89

Answers (1)

Barmar
Barmar

Reputation: 780974

The last thing you pushed onto the stack was the null terminator of the string. So the first thing you popped was this null terminator. When you assigned str[i] = Pop(&head);, you made the first character the null terminator. As a result, strlen(str) is now 0, and the loop stops on the next iteration because 1 <= 0 is not true.

You shouldn't push the null terminator onto the stack. The first loop should use i < strlen(str) rather than i <= strlen(str) (a more efficient way would be str[i] != 0).

And when popping, you shouldn't use strlen() of the resulting string, since you don't know yet how long it will be. You should call Pop(&head) until it returns '\0', which indicates that you've emptied the stack.

int main()
{
    Node *head = NULL;

    char str[] = "helloworld";
    printf("Original String is:%s\n", str);
    printf("Length is:%lu\n", strlen(str));

    for (int i = 0; str[i] != 0; i++)
    {
        Push(&head, str[i]);
    }

    printf ("String has been pushed to the stack.\n");
    PrintAll(head);
    printf("\nString has been printed. Now popping:%ld.\n", strlen(str));

    for (int i = 0; str[i] = Pop(&head); i++)
    {
        printf("%c, ", str[i]); //Pop() is returning the value from a global variable 'ret'
    }
    printf("\n");
    printf ("String has been popped from the stack.\n");
    return 0;
}

Upvotes: 1

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