切り上げ

main.c

#include <bits/stdc++.h>
using namespace std;

int main() {
  int a, b; cin >> a >> b;
  cout << (a + b + 1) / 2 << endl;
}

k桁目の値

main.c

#include <bits/stdc++.h>
using namespace std;

int main() {
  int n, k; cin >> n >> k;
  cout << n / (int)pow(10, k - 1) % 10 << endl;
}

各桁の値の和

①再帰関数

main.c

#include <bits/stdc++.h>
using namespace std;

int sum_of_digits(int n) {
  if (n < 10) return n;
  return digit_sum(n / 10) + (n % 10);
}

int main() {
  int n; cin >> n;
  cout << sum_of_digits(n) << endl;
}

②while文

main.c

#include <bits/stdc++.h>
using namespace std;

int main() {
  int n; cin >> n;

  int sum = 0;
  while(n) {
    sum += n % 10;
    n /= 10;
  }

  cout << sum << endl;
}

数値の回文の形成

main.c

#include <bits/stdc++.h>
using namespace std;

int reverse(int n, int rev) {
  if (n == 0) return rev;
  return reverse(n / 10, rev * 10 + n % 10);
}

int main() {
  int n; cin >> n;
  cout << reverse(n, 0) << endl;
}