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reverse-digits

Reversing the digits of a number flips its order: 12345 → 54321, 3.14 → 41.3 (reverse of "314"). Common in interview problems ("reverse a number without converting to string"), palindrome checks, and recreational math (Kaprekar's constant: 6174). The ZTools Reverse Digits tool handles integers, decimals, negatives, and bulk lists. Pure-arithmetic mode (the interview-question style) and string-reverse mode both available — pick which you want.

Use cases​

Solve the classic interview question​

"Reverse a number using arithmetic only (no string conversion)." Tool runs both modes side-by-side so you can verify your code.

Palindrome detection​

A number is a palindrome if reversing leaves it unchanged. 121 → 121 (yes), 1234 → 4321 (no).

Kaprekar routine demo​

For any 4-digit number with non-identical digits, repeatedly: sort digits descending, sort ascending, subtract → eventually reach 6174 (Kaprekar's constant). Reversal is part of the digit-shuffle.

Generate test data with consistent transformation​

Bulk-reverse a column of IDs / numbers for a test fixture.

How it works​

  1. Paste number(s) — Single value or list. Decimals and negatives supported.
  2. Pick mode — Arithmetic (modulo 10 / divide by 10) or string-reverse (treat as digit string).
  3. Configure — Preserve sign (default), keep leading zeros (string mode), reverse decimal point (yes/no).
  4. Read result — Reversed value(s). Stats: input, output, palindrome flag.

Examples​

Input: 12345

Output: 54321.

Input: 120

Output: 21 (arithmetic — leading zero of reverse drops) or 021 (string mode).

Input: −42

Output: −24 (sign preserved) or 24- (sign appended) — depends on mode.

Input: 3.14

Output: 41.3 (reverse "314" then place decimal correctly).

Input: 121 (palindrome check)

Output: 121 → 121. Palindrome: yes.

Frequently asked questions​

Why does 120 become 21?

In arithmetic reversal, 0 × 100 + 2 × 10 + 1 = 21 (leading zero from "021" drops). In string mode, you can preserve "021" — toggle "keep leading zeros".

Negative number handling?

−42 → −24 (sign preserved, digits reversed). Some interpretations append the minus instead — toggle the convention.

Decimal handling?

3.14 reversed: "314" → "413" → 41.3 (decimal placed at the same offset from the end). For exact symmetry, use a palindrome test directly.

Maximum size?

JavaScript numbers safe up to 2^53. Above, use BigInt for arbitrary-precision integer reversal.

Privacy?

All client-side.

Tips​

  • For palindrome detection, the simplest method is reverse + compare — the tool flags it automatically.
  • For BigInt-sized numbers (>2^53), the arithmetic mode silently loses precision. Toggle BigInt mode.
  • For interview prep, run both arithmetic and string mode and verify they match — catches off-by-one errors in your code.

Try it now​

The full reverse-digits runs in your browser at https://ztools.zaions.com/reverse-digits — no signup, no upload, no data leaves your device.

Open the tool ↗

Last updated: 2026-05-06 · Author: Ahsan Mahmood · Edit this page on GitHub