Having troulbe grasping simple boolean loop if/else program in C++

I need a little help figuring out a couple of parts from a C++ assignment. I am asked to write a program as follows:

Write a program that accepts input from the keyboard (with the input terminated by pressing the Enter key) and counts the number of letters (A-Z and a-z), numerical digits (0-9), and other characters. Input the string using cin and use the following looping structure to examine each character in the string with an "if" statement and multiple "else if" statements.

char s[50];
int i;

. . .

i = 0;
while (s[i] != 0) { // a string is terminated with a null (0) value

. . .

i++;
}

Your program should make use of the relational operators (e.g., == < > <= >= !=) to determine whether a particular character is a letter, number, or other character. You may only #include and may not use any other include files.

The program should have an output similar to the following:

Enter a continuous string of characters with no blank spaces (example: aBc1234!@#$%)

enter your string: aBc1234!@#$%

your string has 12 total characters
3 letters
4 numerical characters
5 other characters

Here is an example program that counts lower case letters:

// PROG07.CPP example

#include <iostream>
using namespace std;


int main()
{
char s[50];
int i;
int lowercase = 0;

//get string from the user
cout << "Enter a continuous string of characters with no blanspaces\n"
cout << "(example: aBc1234!@#$%)" << endl << endl;
cout << "Enter your string: ";
cin >> s;
cout << endl;

// loop through the string, lower case letters
// note, strings (character arrays) have an invisible
// zero value at their end
i = 0;
while (s[i] != 0) // while the character does not have ASCII code zero
{
if ((s[i] >= 'a' && s[i] <= 'z'))
lowercase++;
i++;
}

cout << "Your string has " << lowercase << " lower case letters" << endl;

// including the next line for Dev-C++:
system("pause"); // not needed for CodeBlocks

return 0;
}

So far, I have come up with this:

#include <iostream>
using namespace std;

int main()
{
char s[50];
int i;
int lowercase, uppercase, numChars, otherChars = 0;

cout << "Enter a continuous string of characters" << endl;
cout << "(example: aBc1234!@#$%)" << endl;
cout << "Enter your string: ";
cin >> s;
cout << endl;

while (s[i] != 0) // while the character does not have ASCII code zero
{
if ((s[i] >= 'a' && s[i] <= 'z'))
    lowercase++;
    i++;
}
while (s[i] != 0)
{
if ((s[i] >= 'A' && s[i] <= 'Z'))
    uppercase++;
    i++;
}
cout << lowercase + uppercase << " letters" << endl;
i = 0;
while (s[i] != 0)
{
if ((s[i] >= '0' && s[i] <= '9'))
    numChars++;
    i++;
}

cout << numChars << " numerical characters" << endl;

return 0;
}

Any help would be greatly appreciated.