Pick lines by their position. Line numbering starts at 1, so keep odd lines returns lines 1, 3, 5 and so on, while keep even lines returns lines 2, 4, 6 and so on. Swap odd and even exchanges each adjacent pair, so line 1 and line 2 trade places, line 3 and line 4 trade places, and so on; if there is a final line with no partner it stays where it is. The counts show how many lines went in and how many came out.
Useful for splitting interleaved data such as alternating labels and values, separating question and answer lines, or reordering paired records. Everything runs locally in your browser — your text never leaves your device.