Reverse a sentence using stack in C - c

Question

i want to reverse a sentence using stack in c.
eg. how are you => you are how.
i have written the following program
#include<stdio.h>
#include<conio.h>
struct rev
{
char *var;
struct rev *next;
};
struct rev *top=NULL;
struct rev *temp,*test;
main()
{
int start, j = 0;
char *p="Hi, How are you? Hope everything is fine";
char *s;
char *t;
*t = '\0';
s = p;
while(1) {
if(*p == ' '|| *p == '\0') {
printf("Inside if \n");
*(t + j) = '\0';
printf("This is t %s\n",t);
if(top == NULL) {
temp = (struct rev *)malloc(sizeof(struct rev));
temp->next = NULL;
temp->var=t;
top = temp;
} else {
temp = (struct rev *)malloc(sizeof(struct rev));
printf("This is going in stack %s\n", t);
temp->var = t;
temp->next = top;
top = temp;
printf("This is top %s\n", top->var);
}
j = 0;
} else {
*(t+j) = *p;
printf("%c\n", *p);
j++;
}
if(*p == '\0') {
break;
}
//printf("%c",*p);
p++;
}
struct rev *show;
show = top;
while(show != NULL) {
printf("%s\n", show->var);
show = show->next;
}
getch();
}
It is storing correctly but on traverse it is giving only the final element.
i am not be able to figure it out what is the problem.
Here is my output window:-

Answer 1

Read complete line. Tokenize string on whitespace, pushing each word to the stack. Pop the stack while printing.
Useful functions:
fgets
strtok
Complete solution can be done in less than 20 lines (including structure definitions, header files, etc.)
You also have a problem with undefined behavior in your code. You have a pointer p which points to a constant array of characters (all string literals are constant arrays of characters). Then you try to modify that constant array.
You might want something like this instead:
char arr[] = "Some string here";
char *p = arr;
And you have another case of undefined behavior as well: You have the pointer t which is not initialized. You then continue to dereference it. I would say that you are lucky to not get a crash.
You also don't update t in the loop, which you probably should.

Answer 2

First of all your char *t is just a pointer, make it point to malloced memory and then go ahead... I dont understand how the code is even running ... You are doing *(t + j) when t is actually pointing to junk.
On first look You are overwriting t... after parsing a string. i.e you set j = 0 and overwrite previously stored string and your struct rev hold a pointer to this t hence
you are getting you? you? you? as output. instead of having char *var in struct rev point to t .. your char *var point to a malloced memory and do strcpy or strtok
I just did a rough modification of your code and it worked for me on linux + gcc ... Here's the code:
#include<stdio.h>
#include <stdlib.h>
struct rev
{
char *var;
struct rev *next;
};
struct rev *top=NULL;
struct rev *temp,*test;
main()
{
int start, j = 0;
char *p="How are you?";
char *s;
char *t;
t = malloc(1000);
if (t == NULL) {
//OUT OF MEMORY
exit(1);
}
s = p;
while(1) {
if(*p == ' '|| *p == '\0') {
printf("Inside if \n");
*(t + j) = '\0';
printf("This is t %s\n",t);
if(top == NULL) {
temp = (struct rev *)malloc(sizeof(struct rev));
temp->next = NULL;
temp->var = malloc(100);
if (temp->var == NULL) {
//OUT OF MEMORY
exit(1);
}
strcpy(temp->var, t);
top = temp;
} else {
temp = (struct rev *)malloc(sizeof(struct rev));
printf("This is going in stack %s\n", t);
temp->var = malloc(100);
if (temp->var == NULL) {
//OUT OF MEMORY
exit(1);
}
strcpy(temp->var, t);
temp->next = top;
top = temp;
printf("This is top %s\n", top->var);
}
j = 0;
} else {
*(t+j) = *p;
printf("%c\n", *p);
j++;
}
if(*p == '\0') {
break;
}
//printf("%c",*p);
p++;
}
struct rev *show;
show = top;
while(show != NULL) {
printf("%s\n", show->var);
show = show->next;
}
//getch();
}
Here's the output:
H
o
w
Inside if
This is t How
a
r
e
Inside if
This is t are
This is going in stack are
This is top are
y
o
u
?
Inside if
This is t you?
This is going in stack you?
This is top you?
you?
are
How
PS: I dont undertand in which part of the code you are implementing push|pop ... You are using list and telling you want a stack. Stack and List are 2 different data-structures.

Answer 3

You can start by initializing variable t to point to a valid memory address.

Answer 4

It seems that the logic of "show" is written incorrectly-
show = top;
while(show != NULL) {
printf("%s\n", show->var);
show = show->next;
}
pointing show to top and printing "show->next" is incorrect way of popping out from stack.

Answer 5

Here is an example of pop function-
int pop(STACK **head) {
if (empty(*head)) {
fputs("Error: stack underflow\n", stderr);
abort();
} else {
STACK *top = *head;
int value = top->data;
*head = top->next;
free(top);
return value;}}