Create Symmetric List

Three common cases. Test data: palindromic sequences exercise edge cases in algorithms that walk lists from both ends, like two-pointer routines. Design and patterns: symmetric values feed gradient stops, animation keyframes that reverse cleanly, and knitting or bead patterns. And puzzle construction: crossword-style grids and rhythm patterns in music notation often need mirrored sequences, which is tedious to type by hand and trivial to generate.

Full mirror: [A,B,C] → [A,B,C,C,B,A]. The center is duplicated, so reading from middle outward you see C, C, B, B, A, A. Length 2n. Palindrome mode: [A,B,C] → [A,B,C,B,A]. The center C appears only once. This is a true lexical palindrome – reads the same forwards and backwards. Length 2n-1.

Mirrors on both ends: [A,B,C] → [C,B,A,B,C,B,A]. The reversed list is prepended AND appended (with the original sandwiched in the middle, with the original’s first element shared). Useful when you want a pattern that looks symmetric AND extends in both directions. Length 3n-2.

Yes. After generating the output, the tool checks whether items[i] === items[n-1-i] for all i. Full-mirror mode always produces valid palindromes. Palindrome mode always produces valid palindromes. Bi-mirror mode produces valid palindromes when the input itself is a palindrome – otherwise the result is symmetric but not palindromic.

Bi-mirror prepends the reverse of the input. For input [A,B,C]: reversed is [C,B,A], then concatenated with input minus first → [C,B,A,B,C]. The result reads C-B-A-B-C – that IS a palindrome. Actually for any input, the result is symmetric around the center. Sorry, bi-mirror always produces palindromes too. Disregard the prior caveat.

Input separators just need to split the text into items. Output formats include all the separators PLUS JSON (compact and pretty) and CSV (RFC 4180 escaping). Custom field lets you type any string (multi-character, including escaped sequences like " - ").

50,000 items. The tool throws an error if the mirrored list would exceed this. Input cap depends on mode: full mirror 25,000 input items (× 2 = 50k out); palindrome 25,001; bi-mirror 16,667 (× 3 – 2 = 49,999).

Yes – copy the output, paste back into the input, click Create again. [A,B,C] → [A,B,C,C,B,A] → [A,B,C,C,B,A,A,B,C,C,B,A]. Each nesting doubles the length (in full mode).

One item per row. Items containing commas, double-quotes, or newlines are wrapped in "..." with internal quotes doubled (RFC 4180). Useful for importing into Excel / Google Sheets.

No. All processing happens locally in your browser – nothing is sent to a server, logged, or stored, and the tool keeps working offline once the page has loaded.