Digit Processing: Numbers as Sequences
n % 10 and n / 10 peel digits one by one — digit sums, digit counts, digit max, and rebuilding numbers.
Peeling digits with % and /
An integer is a sequence of digits you can process without any string:
long long n; in >> n;
long long digitSum = 0;
while (n > 0) {
digitSum += n % 10; // last digit
n /= 10; // drop last digit
}
n % 10 = last digit; n / 10 = everything before it. Two operations,
every digit problem becomes a loop problem.
The classic digit tasks
| Task | How |
| --- | --- |
| Digit sum | accumulate n % 10 |
| Digit count | count iterations |
| Largest digit | track max of n % 10 |
| Digit product | accumulate product (beware 0 digits) |
| Reverse digits | rev = rev * 10 + n % 10 |
Worked example: is the number a palindrome?
long long n; in >> n;
long long m = n, rev = 0;
while (m > 0) { rev = rev * 10 + m % 10; m /= 10; }
out << (rev == n ? "PALIN" : "NO") << "\n";
Note we kept n untouched in a copy m — the loop destroys what it
peels. Forgetting the copy is a classic bug.
Overflow and negatives
Reversing 19-digit numbers can overflow long long — read constraints
first. And for negative inputs, decide the convention (usually work with
abs(n)) before looping.