need help to develop a C++ program?

To read a line of text from the keyboard, just use cin.getline(). This will allow any length of text typed, and when the user presses the Enter key, you get the line of text.

Just use the STL string class 'std::string', which has a length() function. So, you will know the length of the string.

Converting to lower case is done in a tricky way. see this example:

std::string myStr = "This is a test.";

transform(myStr.begin(), myStr.end(), myStr.begin(), tolower);

To remove not-letter characters, there may be a tricky way to do this. But, a simple way would be as follows:

std::string newStr;

for (int i=0; i<myStr.length(); ++i)

{

char ch = myStr[i];

if (isalpha(ch)) newStr += ch;

}

Now the string newStr has the text with only alphabetic characters.