Question

I need to write recursive function Repl
takes as input an expression in e in Expr and returns an expression in Expr
wherein each number is replaced by the number 1.
For example, if e is the expression
((((9 + 5) ∗ 2) ∗ (2 + (4 ∗ 6))))
then Repl(e) is the expression
((((1 + 1) ∗ 1) ∗ (1 + (1 ∗ 1))))

Can anybody help me how to go about this?
Iterative one is easy to write but how to write it recursively?

Answer 1

It is not clear why you would want a recursive solution for this problem, but the solution is relatively straightforward. Here is pseudocode:

string replace(string s, bool seenDigit) {
    if (s == "") {
        // The string is empty : we are done
        return "";
    }
    if (s[0] is digit) {
        if (seenDigit) {
            // This is a second, third, etc. digit in a multi-digit chain
            // It has been replaced with "1" already, so we cut it out
            return replace(s.substring(1), true);
        } else {
            // This is the first digit in a chain of one or more digits
            // Replace it with "1", and tell the next level that we've
            // done the replacement already
            return "1"+replace(s.substring(1), true);
        }
    } else {
        // Non-digits do not get replaced
        return s[0] + replace(s.substring(1), false);
    }
}

s[0] means the first character; string+string denotes concatenation.