Count 'd' digit positive integers with 0 as a digit in C++

In this tutorial, we will be discussing a program to find the numbers having ‘d’ digits with 0 as a digit.

For this we will be provided with a number ‘d’. Our task is to count and print the number of positive integers having ‘d’ digits and 0 as one of their digit.

Example

#include<bits/stdc++.h>
using namespace std;
//counting the number of 'd' digit numbers
int count_num(int d) {
   return 9*(pow(10,d-1) - pow(9,d-1));
}
int main(){
   int d = 1;
   cout << count_num(d) << endl;
   d = 2;
   cout << count_num(d) << endl;
   d = 4;
   cout << count_num(d) << endl;
   return 0;
}

Output

0
9
2439