TIP #326 Version 1.5: Add -stride Option to lsort

This is not necessarily the current version of this TIP.

Abstract

This TIP adds a new option, -stride, to lsort to request that a list be treated as consisting of repeated groups of elements (as opposed to sublists), and that the the groups be sorted according to a chosen element within each group.

Rationale

Flat name-value pair lists are common in Tcl - consider the output of array get and dict create or the input to foreach.

It is surprising then that there is no command to directly sort such a list by either the name or the value elements, while preserving the name-value mapping.

Doing so currently requires turning a name-value pair list into a list of sublists, using the lsort -index option to sort them, then flattening the list again, which is rather fiddly and inefficient.

While sorting name-value pair lists is no doubt the most common use case, lists containing groups of more than two elements are also reasonably common, and so providing support for groups of any size seems useful and no harder to implement.

The option format follows the Feature Request here: https://sourceforge.net/support/tracker.php?aid=747083

Proposed Change

A new option, -stride, taking one parameter, grpSize will be added to the lsort command.

If -stride is supplied, the list will be treated as consisting of groups of grpSize elements, and the groups will be sorted by either their first element or, if the -index option is used, by the element within each group given by the first index passed to -index (which is then ignored by -index).

Elements always remain in the same position within their group.

The list length must be a multiple of grpSize, which in turn must be at least 2.

Examples

 lsort -stride 2 {carrot 10 apple 50 banana 25}

returns "apple 50 banana 25 carrot 10