Why is my static stack not working? - c

Question

static stack implementation
this is also not deleting according to the lifo principle
static stack implementation:
it is not taking name for the second time
this is the new code now tell me why is it not working
please help
typedef struct student {
char name[20];
int roll;
int age;
} mystruct;
#define size 40
int top;
static mystruct s[size];
void push()
{
if (top == size - 1) {
printf("\noverflow"); //
} else {
printf("\nenter the name of the student");
gets(s[top].name);//not taking name for d 2 time
printf("\nenter the roll number");
scanf("%d", &s[top].roll);
printf("\nenter the age of the student");
scanf("%d", &s[top].age);
++top;
}
}
void pop()
{
if (top == -1)
{
printf("\nunderflow");
} else {
printf("%s", s[top].name);
printf("%d", s[top].roll);
printf("%d", s[top].age);
printf("\npopped");
--top;
}
}
void display()
{
int i;
if (top == -1) {
printf("\nstack is empty");
} else {
for (i = top; i > 0; i--) {
printf("\nthe name of the student is%s", s[top].name);
}
printf("\nthe roll no of the student is%d", s[top].roll);
printf("\nthe age of the student is%d", s[top].age);
}
}
main()
{
top = -1;
char ch;
while (1) {
printf("\nwelcome to static stack menu");
printf("\n1.PUSH\n2.POP\n3.DISPLAY\n0.EXIT");
printf("\nplease enter your choice\n");
ch = getche();
if (ch == '0') {
break;
}
switch (ch) {
case '1':
push();
break;
case '2':
pop();
break;
case '3':
display();
break;
default:
printf("choice not valid");
break;
}
}
}

Answer 1

The first problem I noticed was that top is initialized to -1. Trying to access the member data of s[top] when top is initialized to -1 will result in unpredictable behavior.
I would suggest changing the line
top = -1;
to
top = 0;
That changes the basic assumption you have made in push, pop, and display about when the stack is empty and when it is full. Instead of checking if ( top == -1 ), you have to now check if (top == 0 ). Instead of checking if ( top == size - 1 ), you have to now check if ( top == size ).
In pop, you have to use top-1 instead of top.
The for loop in display is not scoped correctly. You need to use:
for (i = top-1; i >= 0; i--) {
printf("\nthe name of the student is %s", s[i].name);
printf("\nthe roll no of the student is %d", s[i].roll);
printf("\nthe age of the student is %d", s[i].age);
}
Also, reading the options for the menu and reading the subsequent input is little bit tricky.
After you read the menu option, you have to make sure that you eat up all the input until the next newline. Otherwise, gets() will read everything after your menu option until the end of the line. If you typed 1 for the menu and then typed Return/Enter, the name will be automatically accepted as "\n". Hence, I suggest the lines:
printf("\nwelcome to static stack menu");
printf("\n1.PUSH\n2.POP\n3.DISPLAY\n0.EXIT");
printf("\nplease enter your choice\n");
ch = fgetc(stdin);
/* Skip till the end of line is read. */
while ( fgetc(stdin) != '\n' );
Also, after you read the age of the object, you have to eat everything up to the newline. Otherwise, the newline character is read in as the choice for the next menu option.
scanf("%d", &s[top].age);
/* Skip till the end of line is read. */
while ( fgetc(stdin) != '\n' );
Here's the fully working file. I have replaced gets by fgets and getche by fgetc.
#include <stdio.h>
#include <string.h>
typedef struct student {
char name[20];
int roll;
int age;
} mystruct;
#define size 40
int top;
static mystruct s[size];
void push()
{
if (top == size) {
printf("\noverflow"); //
} else {
printf("\nenter the name of the student: ");
fgets(s[top].name, 20, stdin);//not taking name for d 2 time
// The newline character is part of s[top].name when fgets is
// finished. Remove that.
s[top].name[strlen(s[top].name)-1] = '\0';
printf("\nenter the roll number: ");
scanf("%d", &s[top].roll);
printf("\nenter the age of the student: ");
scanf("%d", &s[top].age);
/* Skip till the end of line is read. */
while ( fgetc(stdin) != '\n' );
++top;
}
}
void pop()
{
if (top == 0)
{
printf("\nunderflow");
} else {
printf("%s, ", s[top-1].name);
printf("%d, ", s[top-1].roll);
printf("%d", s[top-1].age);
printf("\npopped");
--top;
}
}
void display()
{
int i;
if (top == 0) {
printf("\nstack is empty");
} else {
for (i = top-1; i >= 0; i--) {
printf("\nthe name of the student is %s", s[i].name);
printf("\nthe roll no of the student is %d", s[i].roll);
printf("\nthe age of the student is %d", s[i].age);
}
}
}
main()
{
top = 0;
char ch;
while (1) {
printf("\nwelcome to static stack menu");
printf("\n1.PUSH\n2.POP\n3.DISPLAY\n0.EXIT");
printf("\nplease enter your choice\n");
ch = fgetc(stdin);
/* Skip till the end of line is read. */
while ( fgetc(stdin) != '\n' );
if (ch == '0') {
break;
}
switch (ch) {
case '1':
push();
break;
case '2':
pop();
break;
case '3':
display();
break;
default:
printf("choice, %c, not valid", ch);
break;
}
}
}

Answer 2

You need to change getche() to getchar()
Note: getche() is a non-standard function.

Answer 3

Maybe this will be useful http://www.delorie.com/djgpp/doc/libc/libc_385.html
Pay attention to implementation note:
"If you can detect the situation when one of the conio functions is called for the very first time since program start, you could work around this problem by calling the gppconio_init function manually"
or just replace it with getchar(). And there meaned conio included.