C++ program to calculate the number of character in the given string

#include<iostream>
#include<conio.h>
#include<stdio.h>
#include<string.h>

int main()
{
   system("cls");

    char str[100];
    int i,j,n,k,o;

    i = 0;
    j = 0;
    k = 0;
    o = 0;

    puts("Enter any sentence ");
    gets(str);
    n = strlen(str);

    while(str[i] != '')
    {
        if((str[i] >= 65) && (str[i]<97))
        {
            j++;
        }
        if((str[j] >= 97) && (str[i] <= 122))
        {
            k++;
        }
        if(str[i] == ' ')
        {
            o++;
        }
        i++;
    }
    std::cout<<"The total number of letters = "<<n<<std::endl;
    std::cout<<"The upper case letters = "<<j<<std::endl;
    std::cout<<"The lower case letters = "<<k<<std::endl;
    std::cout<<"The white spaces = "<<o<<std::endl;

    getch();
    return 0;
}

Enter any sentence
This program is the example of FUNCTION.
The total number of letters = 40
The upper case letters = 9
The lower case letters = 24
The white spaces = 6