Implement Trapezoidal method

/* Write C program to implement  Trapezoidal method.*/

#include<stdio.h>
#include<conio.h>
#include<math.h>

char postfix[80];
float stack[80];
char stack1[80];
int top=-1,top1=-1;

float eval(char postfix[], float x1);
void infix_postfix(char infix[]);

main()
{
float x0, xn, h, s,e1,e2;
char exp[80], arr[80];
int i,n,l=0;
clrscr();
printf("\nEnter an expression: ");
gets(exp);
puts("Enter x0, xn and number of subintervals");
scanf("%f%f%d", &x0, &xn, &n);
h=(xn-x0)/n;
if(exp[0]=='l'&& exp[1]=='o'&& exp[2]=='g')
{
 l=strlen(exp);
 for(i=0;i<l-3; i++)
   arr[0]=exp[i+3];
 arr[i]='\0';
 infix_postfix(arr);
 e1=eval(postfix,x0);
 e2=eval(postfix,xn);
 s=log(e1)+log(e2);
 for (i=1;i<=n-1;i++)
 s+=2*log(eval(postfix,x0+i*h));
}
else
{
 infix_postfix(exp);
 s=eval(postfix,x0)+eval(postfix,xn);
 for (i=1;i<=n-1;i++)
 s+=2*eval(postfix,x0+i*h);
}
printf("Value of the integral is %6.3f\n",(h/2)*s);
return(0);
}
/*Inserting the operands in a stack. */
void push(float item)
{
if(top==99)
 {
 printf("\n\tThe stack is full");
 getch();
 exit(0);
}
else
{
 top++;
 stack[top]=item;
}
return;
}
/*Removing the operands from a stack. */
float pop()
{
 float item;
 if(top==-1)
{
  printf("\n\tThe stack is empty\n\t");
  getch();
}
 item=stack[top];
 top--;
 return (item);
}
void push1(char item)
{
 if(top1==79)
{
  printf("\n\tThe stack is full");
  getch();
  exit(0);
}
else
{
 top1++;
 stack1[top1]=item;
}
 return;
}
/*Removing the operands from a stack. */
 char pop1()
{
 char item;
 if(top1==-1)
{
 printf("\n\tThe stack1 is empty\n\t");
 getch();
}
 item=stack1[top1];
 top1--;
 return (item);
}

/*Converting an infix expression to a postfix expression. */
void infix_postfix(char infix[])
{
 int i=0,j=0,k;
 char ch;
 char token;
 for(i=0;i<79;i++)
 postfix[i]=' ';
 push1('?');
 i=0;
 token=infix[i];
 while(token!='\0')
{
 if(isalnum(token))
{
 postfix[j]=token;
 j++;
}
else if(token=='(')
{
 push1('(');
}
else if(token==')')
{
 while(stack1[top1]!='(')
{
  ch=pop1();
  postfix[j]=ch;
  j++;
}
 ch=pop1();
}
else
{

 while(ISPriority(stack1[top1])>=ICP(token))
{
  ch=pop1();
/*Assigning the popped element into the postfix array. */
  postfix[j]=ch;
  j++;
}
 push1(token);
}
i++;
token=infix[i];
}
while(top1!=0)
{
 ch=pop1();
 postfix[j]=ch;
 j++;
}
postfix[j]='\0';
}

int ISPriority(char token)
{
switch(token)
{
 case '(':return (0);
 case ')':return (9);
 case '+':return (7);
 case '-':return (7);
 case '*':return (8);
 case '/':return (8);
 case '?':return (0);
 default: printf("Invalid expression");
 break;
}
return 0;
}
/*Determining the priority of elements that are approaching towards the stack. */
int ICP(char token)
{
 switch(token)
{
   case '(':return (10);
   case ')':return (9);
   case '+':return (7);
   case '-':return (7);
   case '*':return (8);
   case '/':return (8);
   case '\0':return (0);
   default: printf("Invalid expression");
   break;
}
return 0;
}
/*Calculating the result of expression, which is converted in postfix notation. */
float eval(char p[], float x1)
{
 float t1,t2,k,r;
 int i=0,l;
 l=strlen(p);
 while(i<l)
{
 if(p[i]=='x')
 push(x1);
else
 if(isdigit(p[i]))
{
 k=p[i]-'0';
 push(k);
}
else
{
 t1=pop();
 t2=pop();
switch(p[i])
{
 case '+':k=t2+t1;
 break;
 case '-':k=t2-t1;
 break;
 case '*':k=t2*t1;
 break;
 case '/':k=t2/t1;
 break;
 default: printf("\n\tInvalid expression");
 break;
}
push(k);
}
i++;
}
if(top>0)
{
 printf("You have entered the operands more than the operators");
 exit(0);
}
else
{
 r=pop();
 return (r);
}
return 0;
}

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