Question

1 x 2⁰ +1 x 2¹ + 1 x 2² + 0 x 2³ +1 x 2⁴ = 1 + 2 x (1 + 2 x (1 + 2 x (0 + 2 x 1) ) )

Recall b[0] = 1, b[1] = 1, b[2] = 1,b[3] = 0, b[4] = 1

/* to convert a binary representation to a decimal one*/

int dec, b[5] = {1, 1, 1, 0, 1};
dec = b[4];
for (int i = 3; i >= 0; i--)
{
    dec=2 * dec + b[i];  //horner's scheme
}
cout << dec << endl;

I tried to write this code again in C language, but it’s not working correctly:

#include<stdio.h>

int main()
{
    int B[5];
    int x, s, s1;

    for(int i = 1;i <= 5; i++)
    {
        printf("Enter %d. digit of binary number", i);
        scanf("%d", &B[i]);
    }


    s = B[5];   /*this part for reverse the array*/
    B[5] = B[1];
    B[1] = s;

    s1 = B[4];
    B[4] = B[2];
    B[2] = s1; 

    x = B[4];

    for (int i = 3; i >= 0; i--)
    {
        x = 2 * x + B[i];  
    }

    printf("%d", x);
    scanf("%d");
}

Answer 1

This is your initial loop where you initialise the array B

    int B[5];
    int x,s,s1;
    for(int i=1;i<=5;i++) {
    printf("Enter %d. digit of binary number",i);
    scanf("%d",&B[i]);}

which is bad as you are accessing element B[5] which is outside the bounds of your array anyway, but also you are never initialising B[0] which you use in your second loop

    x=B[4];
    for (int i=3;i>=0;i--)
    {
        x=2*x+B[i];  
    }

try changing your first loop to

    for(int i=0;i<5;i++) {
        printf("Enter %d. digit of binary number",i);
        scanf("%d",&B[i]);
    }

and see if this gives the result you expect.

Also this code

    s=B[5];   /*this part for reverse the array*/
    B[5]=B[1];
    B[1]=s;

    s1=B[4];
    B[4]=B[2];
    B[2]=s1;

has problems, because you declared B as an array of 5 integers and since arrays have zero based indices, the only values for an index that are valid are from 0 to 4. If you want to reverse the array correctly replace your code with

    s=B[4];   /*this part for reverse the array*/
    B[4]=B[0];
    B[0]=s;

    s1=B[3];
    B[3]=B[1];
    B[1]=s1;

You must not access B[5] as this is outside the bounds of your array !!!!