If you are new to learning programming and do not know where to start, please try Problem A “Welcome to AtCoder” from practice contest. There you can find a sample code for each language.
Also, if you are not familiar with problems in programming contests, we recommend you to try some problems in “AtCoder Beginners Selection” (https://atcoder.jp/contests/abs).

There are various approaches. Please refer to the implementation examples below. Presented later in this article are curated showcase of the sample code among these.

• Replace characters one by one: Sample code (C++)
• Print the answer one character by one: Sample code (C++) / Sample code (Python)
• Construct another answer string: Sample code (C++) / Sample code (Python)
• (Advanced) Use the standard library’s string/sequence operations: Sample code (C++, replace_if) / Sample code (Python, regular expression)

(C++) Replace characters one by one

#include <bits/stdc++.h>
using namespace std;
int main() {
  string S;
  cin >> S;
  for (int i = 0; i < S.size(); i++)
    if (S[i] != 'A')
      S[i] = '.';
  /*
  One canse also use a range-based for statement as follows:
  for (char& c : S)
    if (c != 'A')
      c = '.';
  */
  cout << S << endl;
}

(Python) Print the answer one by one

S = input()
for c in S:
  # One can also write print("A" if c == "A" else ".", end="")
  if c == "A":
    print("A", end="")
  else:
    print(".", end="")

(C++) Construct another answer string

#include <bits/stdc++.h>
using namespace std;
int main() {
  string S;
  cin >> S;
  string T = "";
  for (char c : S)
    // One can also write T += c == 'A' ? 'A' : '.'
    if (c == 'A')
      T += 'A';
    else
      T += '.';
  cout << T << endl;
}

(Python) Construct another answer string

S = input()
# Combine the list comprehension syntax and join
print("".join("A" if c == "A" else "." for c in S))