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#include<stdio.h>

#include<conio.h>

#include<math.h>

char post_fix[80];

float stack[80];

char stack1[80];

int top=-1,top1=-1;

float eval(char post_fix[], float x1);

void infix_post_fix(char infix[]);

main()

{
    
    float x0, xn, h, s,e1,e2, e3;
    
    char exp[80], arr[80];
    
    int i,n,l=0;
    
    clrscr();
    
    printf("nEnter an expression: nn");
    
    gets(exp);
    
    puts("nnEnter x0, xn and number of sub-intervals: nn");
    
    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]=";
        
        infix_post_fix(arr);
        
        e1=eval(post_fix,x0);
        
        e2=eval(post_fix,xn);
        
        e3=4*eval(post_fix, x0+h);
        
        s=log(e1)+log(e2)+log(e3);
        
        for (i=3;i<=n-1;i+=2)
        
        s+=4*eval(post_fix,x0+i*h)+2*eval(post_fix, x0+(i-1)*h);
        
    }
    
    else
    
    {
        
        infix_post_fix(exp);
        
        s=eval(post_fix,x0)+eval(post_fix,xn)+4*eval(post_fix, x0+h);
        
        for (i=3;i<=n-1;i+=2)
        
        s+=4*eval(post_fix,x0+i*h)+2*eval(post_fix, x0+(i-1)*h);
        
    }
    
    printf("nnThe value of integral is %6.3fn",(h/3)*s);
    
    return(0);
    
}

/*Inserting the operands in a stack. */

void push(float item)

{
    
    if(top==99)
    
    {
        
        printf("ntThe 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("ntThe stack is emptynt");
        
        getch();
        
    }
    
    item=stack[top];
    
    top–;
    
    return (item);
    
}

void push1(char item)

{
    
    if(top1==79)
    
    {
        
        printf("ntThe 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("ntThe stack1 is emptynt");
        
        getch();
        
    }
    
    item=stack1[top1];
    
    top1–-;
    
    return (item);
    
}

/*Converting an infix expression to a postfix expression. */

void infix_post_fix(char infix[])

{
    
    int i=0,j=0,k;
    
    char ch;
    
    char token;
    
    for(i=0;i<79;i++)
    
    post_fix[i]=' ';
    
    push1('?');
    
    i=0;
    
    token=infix[i];
    
    while(token!=")
    
    {
        
        if(isalnum(token))
        
        {
            
            post_fix[j]=token;
            
            j++;
            
        }
        
        else if(token=='(')
        
        {
            
            push1('(');
            
        }
        
        else if(token==')')
        
        {
            
            while(stack1[top1]!='(')
            
            {
                
                ch=pop1();
                
                post_fix[j]=ch;
                
                j++;
                
            }
            
            ch=pop1();
            
        }
        
        else
        
        {
            
            while(ISPriority(stack1[top1])>=ICP(token))
            
            {
                
                ch=pop1();
                
                post_fix[j]=ch;
                
                j++;
                
            }
            
            push1(token);
            
        }
        
        i++;
        
        token=infix[i];
        
    }
    
    while(top1!=0)
    
    {
        
        ch=pop1();
        
        post_fix[j]=ch;
        
        j++;
        
    }
    
    post_fix[j]=";
    
}

/*Determining the priority of elements that are placed inside the stack. */

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("nnInvalid expression");
        
    }
    
    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 ":return (0);
        
        default: printf("nnInvalid expression");
        
    }
    
    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("ntInvalid expression");
                
            }
            
            push(k);
            
        }
        
        i++;
        
    }
    
    if(top>0)
    
    {
        
        printf("You have entered the operands more than the operatorsnn");
        
        exit(0);
        
    }
    
    else
    
    {
        
        r=pop();
        
        return (r);
        
    }
    
    return 0;
    
}

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