beginnerDigit ProblemsJava

Find the First Digit of a Number

Find the leading (most significant) digit of an integer using arithmetic.

Quick Answer

Keep dividing the number by 10 until it drops below 10. What remains is a single digit — the first (leading) digit. Work with the absolute value so negatives return the same leading digit as their positive counterpart.

Problem Statement

Given an integer, return its first (leftmost, most significant) digit. For example the first digit of 5824 is 5.

Getting the last digit is easy with % 10; the first digit takes a little more work because you must remove every digit after it. Use the absolute value so -917 returns 9, and do not convert the number to a String.

Input: A single integer n.

Output: A single digit (0-9) — the leading digit of n.

Examples

Example 1
Input:  5824
Output: 5

Removing 8, 2 and 4 leaves the leading digit 5.

Example 2
Input:  -917
Output: 9

The sign is ignored; the leading digit of 917 is 9.

Constraints

  • -2,147,483,648 <= n <= 2,147,483,647
  • No String conversion

Think Before You Code

Reveal the questions to ask yourself first
  • The last digit is n % 10 — what operation isolates the first digit instead?
  • How do you know when only the leading digit is left?
  • What condition should the loop keep running under?
  • Do negatives change the leading digit?

Hints

Open them one at a time — try after each before revealing the next.

Hint 1
Dividing by 10 removes the last digit; do it again and again to peel from the right.
Hint 2
Stop when the value is a single digit — that is, while the value is 10 or more, keep dividing.
Hint 3
Once the loop ends, the value itself is the first digit. Use the absolute value to handle negatives.

Approach

Reveal the step-by-step approach

Peel digits off the right until only the leading digit remains.

  1. Take the absolute value of n.
  2. While the value is 10 or greater:
    • value = value / 10 — remove the current last digit.
  3. When the loop ends the value is between 0 and 9 — that is the first digit.
  4. Return it.

Dry Run

Walk through the example step by step

Finding the first digit of n = 5824:

step | value | value >= 10? | action
-----+-------+--------------+-------------------
1    | 5824  | yes          | value = 5824 / 10 = 582
2    | 582   | yes          | value = 582 / 10  = 58
3    | 58    | yes          | value = 58 / 10   = 5
end  | 5     | no           | first digit = 5

Solution

Reveal the full Java solution
public class FirstDigit {
    public static int firstDigit(int n) {
        long value = Math.abs((long) n);
        while (value >= 10) {
            value /= 10;
        }
        return (int) value;
    }

    public static void main(String[] args) {
        System.out.println(firstDigit(5824)); // 5
        System.out.println(firstDigit(-917)); // 9
    }
}

Where the last digit is exposed instantly by % 10, the first digit is uncovered by repeatedly discarding the last digit with / 10 until nothing but the leading digit is left. The loop condition value >= 10 is exactly "there is still more than one digit," so it halts the moment a single digit remains. The absolute value guarantees the sign never affects the result.

Time: O(d) where d is the number of digits (log10 n)Space: O(1)

Common Mistakes

  • Using n % 10, which returns the last digit rather than the first.
  • Looping while (value > 10) instead of >= 10, so an input like 10 returns 10 rather than 1.
  • Applying Math.abs to the int directly and overflowing for Integer.MIN_VALUE.

Edge Cases to Test

  • Single-digit numbers return themselves (the loop never runs).
  • n = 0 returns 0.
  • Numbers like 10 or 100 return 1, since the trailing zeros are divided away.

Interview Follow-Ups

  • How would you get the first digit in O(1) using Math.log10 and Math.pow?
  • How would you return the first two digits instead of just the first?

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