Binary Search TestCase Not Returning a Value - c++

Question

So I am trying to solve the following question
Input Format is
N
x x x x x ...
q
y y y y y ...
N=size of array
x,x,x ... are elements of array
q=no of queries
y,y,y .. are queries to be searched in the array using binary search
Here is My code
#include <iostream>
#include<algorithm>
using namespace std;
int main()
{
int N,q;
cin>>N;
int a[N];
for(int i=1;i<=N;i++)
{
cin>>a[i];
}
cin>>q;
int b[q];
for(int i=0;i<q;i++)
{
cin>>b[i];
}
int len=sizeof(a)/sizeof(a[1]);
sort(a,a+len);
int beg=1,end=N;
for(int j=0;j<q;j++)
{
beg=1;end=N;
while(beg<=end)
{
int mid=(beg+end)/2;
if(b[j]==a[mid])
{
cout<<mid<<endl;
break;
}
else if(b[j]<a[mid])
{
end=mid-1;
}
else
beg=mid+1;
}
}
return 0;
}
My code is giving the following output which is wrong
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
for the input
100
100 99 98 97 96 95 94 93 92 91 90 89 88 87 86 85 84 83 82 81 80 79 78 77 76 75 74 73 72 71 70 69 68 67 66 65 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
20
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
correct output is
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

Answer 1

Arrays are 0-based.
Arrays are not variable-length in C++.
You have a wrong update here:
else if(b[j]<a[mid])
{
end=mid-1;
}
The end is non-inclusive.
You will also want to keep going until (beg<end) not beg<=mid - otherwise mid will simply equal both.
Here's C++ version that fixes all of the above and uses iterators instead of indexes. Iterators remove the ambiguity (base-0 vs base-1) and make it very explicit that a range is [begin, end), by contract.
Live ON Coliru
#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
using namespace std;
int main() {
int N;
cin >> N;
std::vector<int> a(N);
std::copy_n(std::istream_iterator<int>(std::cin), N, a.begin());
int q;
cin >> q;
std::vector<int> b(q);
std::copy_n(std::istream_iterator<int>(std::cin), q, b.begin());
sort(a.begin(), a.end());
for (auto query : b) {
auto beg = a.begin();
auto end = a.end();
while (beg < end) {
auto mid = beg + (end-beg) / 2;
if (query == *mid) {
cout << *mid << endl;
break;
} else if (query < *mid) {
end = mid;
} else beg = mid + 1;
}
}
}
Prints
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

Answer 2

Problem 1
Your program has undefined behavior due to accessing a using an out of bounds index in the following loop.
for(int i=1;i<=N;i++)
{
cin>>a[i];
}
That loop needs to be changed to use a 0 based index.
for(int i = 0; i < N; i++)
{
cin >> a[i];
}
Problem 2
For similar reasons, the initial value of beg needs to be 0, not 1.
Problem 3
You are comparing with values of a[mid] but you are outputting mid. The output also needs to be a[mid].
Problem 4
else if(b[j]<a[mid])
{
end=mid-1;
}
needs to be
else if(b[j]<a[mid])
{
end=mid;
}
With the above changes, the program works as expected in my environment. Here's the updated program:
#include <iostream>
#include<algorithm>
using namespace std;
int main()
{
int N,q;
cin>>N;
int a[N];
for(int i=0;i<N;i++)
{
cin>>a[i];
}
cin>>q;
int b[q];
for(int i=0;i<q;i++)
{
cin>>b[i];
}
int len=sizeof(a)/sizeof(a[1]);
sort(a,a+len);
int beg=0,end=N;
for(int j=0;j<q;j++)
{
beg=0;end=N;
while(beg<end)
{
int mid=(beg+end)/2;
if(b[j]==a[mid])
{
cout << a[mid] << endl;
break;
}
else if(b[j]<a[mid])
{
end=mid;
}
else
beg=mid+1;
}
}
return 0;
}
See it working at https://ideone.com/wgF2IS.

Answer 3

Array start with 0 index.so make sure for N elements your loop must start with zero index.
for(int i = 0; i < N; i++)
{
cin >> a[i];
}
Similarly, for same reason assign beg '0' value and end 'N-1' value.