stack

1.Stack opertation:
i).Assign to a variable name Y the value of the third element from the top of the stack and keep the stack undisturbed.
ii) Given an arbitrary integer n pop out the top n elements. A message should be displayed if an unusual condition is encountered.
iii) Assign to a variable name Y the value of the third element from the bottom of the stack and keep the stack undisturbed.

//pro starts from here//
//Use two stacks
#include<stdio.h>
#include<stdlib.h>

#define SIZE 10

//stack structure
struct stack{
   int top;
   int a[SIZE];
};

//push: inserting element into stack

void push(struct stack *m,int data)

{
 if(m->top== (SIZE-1))   //checking for empty condition
 {
  printf("full\n");
 }
 else     //if stack is not empty
 {
  m->top++;
  m->a[m->top]=data;
 }
}

//pop: removing element from stack and it returns int value
int pop(struct stack *b)
{
 if(b->top==-1) //if stack is empty
 {
  printf("empty\n");
 }
 else // if stack contains elements
 {
  int data=b->a[b->top];
  b->top--;
  return data;
 }
}


int main()
{
 struct stack s1,temp; 
 s1.top=temp.top=-1;
 int z,i,j,x,y,n,t,c,data;
 int count;
 while(c!=3)
 {
  printf("1.insert\t2.pop\t3.break\nenter ur choice\n");
  scanf("%d",&c);
  switch(c)
  {
   case 1: printf("enter data\t");
    scanf("%d",&data);
    push(&s1,data);
    break;
   case 2: z=pop(&s1);
    printf("%d is popped out\n",z);
    break;
   case 3: break;
  }
 }
 if(s1.top<2){
  printf("stack insufficient data\n");
 }
 else
 {
            //3rd element from top
  for(i=0;i<=2;i++)
  {
   x=pop(&s1);
   push(&temp,x);
  }
 printf("\n\a3rd    TOP is %d",x);
// push(&s1,x);
 for(i=0;i<2;i++)
 {
  x=pop(&temp);
  push(&s1,x);
 }
  while(s1.top>2)   //3rd element from bottom
                {
   t=pop(&s1);
   push(&temp,t);
  }
y=pop(&s1);      \\y is element removed from stack s1
push(&s1,y);
 printf("\n3rd  BOTTOM is %d \n",y);
 while(temp.top!=-1)
 {
  t=pop(&temp);
  push(&s1,t);
    }
}
count=s1.top+1;
 printf("enterno  of ele deleted :\t");
 scanf("%d",&n);
 if(n>count)
 {
        printf("Invalid ENTRY\n");
        exit(0);
 }
 printf("\n");
 for(i=0;i<n;i++)
 {
  x=pop(&s1);
  printf("\t%d\n",x);
 }
}//END of 1st program





2. Write a program to determine if an input character string is of the form XxY. X is x string of arbitrary length using only the characters from A and B. For example, X may be ABBAB. Y is a string which is the reverse of X. Thus for the string X given above, Y is BABBA. A is any arbitrary character which is not A or B. Given ABAAxABAB the program should display a message that this string is invalid. For the string ABBAxABBA the program should write a message that it is valid. Use appropriate data structure.

//program starts here
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define SIZE 10
struct stack{
    int top;
    char a[SIZE];
};
void push(struct stack *m,char data)
{
    if(m->top==(SIZE-1))
    {
        printf("full\n");
    }
    else{
        m->top++;
        m->a[m->top]=data;
    }
}


char pop(struct stack *b)
{
    if(b->top==-1)
        printf("empty\n");
    else
    {
        char data=b->a[b->top];
        b->top--;
        return data;
    }
}
int main()
{
    struct stack s1;
    s1.top=-1;
    int i,j,k,n,len,flag=-1;
    char a[SIZE],ch;
    printf("enter string \n");
    scanf("%s",a);
    len=strlen(a);
    printf("length of the strinbg is %d\n",len);
    if(len%2==0)
    {
        printf(" FAIL\n");
        exit(0);
    }
    for(i=0;a[i]!='\0';i++)
    {
        if(a[i]=='A' ||a[i]=='B')
        {
            push(&s1,a[i]);
        }
        else{
            if(a[i]=='x')
            {
                i++;
                break;
            }
            else
            {
                printf("FAIL AT X \n");
                exit(0);
            }
        }}
    while(a[i]!='\0')
    {
        ch=pop(&s1);
        if(a[i]!=ch)
        {
            printf("FAIL AT SEC FORM\n");
            exit(0);
        }
        else
        {
            i++;
        }
    }
    printf("\tCORRECT\n");
}//END OF 2nd PROGRAM


3. Write a C program that parses Infix arithmetic expressions to Postfix arithmetic expressions using a Stack.


//program starts here
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define SIZE 20
int priority(char symbol)
{
    switch(symbol)
    {
        case '*':
        case '/':return 2;
        case '+':
        case '-':return 1;
        case '(':return 0;
    }
}
struct stack
{
    int top;
    char a[SIZE];
};
void push(struct stack *s,char data)
{
    if(s->top==(SIZE-1))
        printf("full\n");
    else
    {
        s->top++;
        s->a[s->top]=data;
    }}
char pop(struct stack *s)
{
    char ele;
    if(s->top==-1)
    {
        printf("empty\n");
        exit(0);
    }
    else
    {
        ele=s->a[s->top];
        s->top--;
        return ele;
    }}
void inf(char infix[],char postfix [])
{
    struct stack s;
    int top;
    int i,j,n,c1,c2;
    s.top=-1;
    char symbol,z;
    push(&s,'#');
    j=0;
    c1=-1;
    c2=0;
    for(i=0;i<strlen(infix);i++){
        if(infix[i]== '(')
        {
            c1++;
            break;}}
    for(i=0;i<strlen(infix);i++)
    {
        symbol=infix[i];
        if(isalnum(symbol))
        {
            postfix[j++]=symbol;
        }
        else if(symbol=='(')
        {
            push(&s,symbol);
            if(c1!=-1)
                c2++;
        }
        else if(symbol==')')
        {
            if(c1!=-1)
                c2--;
            while(s.a[s.top]!='(')
                postfix[j++]=pop(&s);
        }
        else
        {
            while(priority(s.a[s.top]) >= priority(symbol))
                postfix[j++]=pop(&s);
            push(&s,symbol);
        }
    }
    while(s.a[s.top]!='#')
    {
        if(s.a[s.top]!='(')
            postfix[j++]=pop(&s);
        else
            z=pop(&s);
    }
    postfix[j]='\0';
    if(c1!=-1&&c2!=0)
    {
        printf("invalid expression\n");
        exit(0);
    }
}
void main ()
{
    int y,i;
    char infix[20],postfix[20];
    printf("enter exp\n");
    scanf("%s",infix);
    inf(infix,postfix);
    printf("\n postfix exp is :\t%s\n\n",postfix);
}//END OF 3rd PROGRAM



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