Miért nem jó ez a logaritmusos keresés C++ -ban?

#include <iostream>

#include <time.h>

#include <stdlib.h>

#include <algorithm>

using namespace std;

int main()

{

srand(time(0));

int N=20, tomb[N];

for (int i=0; i<N; i++) {tomb[i]=rand()%20+1;}

sort (tomb,tomb+N);

int alsohatar=0;

int felsohatar=N-1;

int keresett=13;

bool benne=false;

while (benne==false && felsohatar-1!=alsohatar)

{

int ujhatar= (alsohatar+felsohatar)/2;

if (tomb[ujhatar]<keresett)

{

alsohatar=ujhatar;

}

else if (tomb[ujhatar]>keresett)

{

felsohatar=ujhatar;

}

else if (tomb [ujhatar]==keresett)

{

benne==true;

}

}

for (int i=0; i<N; i++) cout<<tomb[i]<<" ";

if (benne==true) cout<<"Benne van!"<<endl;

return 0;

}