# How to Square Every Digit in C++

## The challenge

You are asked to square every digit of a number and concatenate them.

For example, if we run 9119 through the function, 811181 will come out, because 9^{2} is 81 and 1^{2} is 1.

**Note:** The function accepts an integer and returns an integer

## The solution in C++

Option 1:

```
int square_digits(int n) {
int a = 1;
int m = 0;
while (n > 0) {
int d = n % 10;
m += a * d * d;
a *= d <= 3 ? 10 : 100;
n /= 10;
}
return m;
}
```

Option 2:

```
#include <string>
int square_digits(int num) {
std::string s = std::to_string(num);
std::string ans;
for(char c: s){
int i = c - '0';
ans += std::to_string(i * i);
}
return std::stoi(ans);
}
```

Option 3:

```
int square_digits(int num) {
int total = 0;
int mul = 1;
while(num) {
int d = num % 10;
total += d * d * mul;
mul *= (d > 3 ? 100 : 10);
num /= 10;
}
return total;
}
```

## Test cases to validate our solution

```
#include <random>
#include <string>
Describe(Square_Every_Digit)
{
It(Sample_tests)
{
Assert::That(square_digits(3212), Equals(9414), ExtraMessage("Incorrect answer for n=3212" ));
Assert::That(square_digits(2112), Equals(4114), ExtraMessage("Incorrect answer for n=2112" ));
Assert::That(square_digits(0), Equals(0), ExtraMessage("Incorrect answer for n=0" ));
Assert::That(square_digits(13579), Equals(19254981), ExtraMessage("Incorrect answer for n=13579"));
Assert::That(square_digits(24680), Equals(41636640), ExtraMessage("Incorrect answer for n=24680"));
}
It(Random_tests)
{
for(size_t i = 0; i < 100; i++){
int d = generate_random_num();
int expected = refe(d);
auto msg = [d]() { return "Incorrect answer for n=" + std::to_string(d); };
Assert::That(square_digits(d), Equals(expected), msg);
}
}
private:
int refe(int d){
std::string s = std::to_string(d);
std::string ans;
for(char c: s){
int i = c - '0';
ans += std::to_string(i * i);
}
return std::stoi(ans);
}
std::default_random_engine engn { std::random_device{}() };
std::uniform_int_distribution<> d_pick {0, 10000};
int generate_random_num(){
return d_pick(engn);
}
};
```