```
Author: Peter da Silva <peter@taronga.com>
Author: Donal K. Fellows <donal.k.fellows@manchester.ac.uk>
Author: Harald Oehlmann <harald.oehlmann@elmicron.de>
Author: Andreas Leitgeb <avl@logic.at>
State: Draft
Type: Project
Vote: Pending
Created: 09-Jul-2009
Post-History:
Tcl-Version: 8.7
```

# Abstract

This TIP allows the searching of lists that are grouped into collections of several elements.

# Rationale

When operating on strided lists (for example key-value lists) it's normal to
convert them between lists and arrays and back again. If it was possible to
efficiently perform a strided search of the list it would be possible to (for
example) search just the keys and ignore the values. Indeed, Tcl has a long
tradition of working with lists which are structured into groups through
**foreach** and **array get**, and this is strengthened further with
dictionaries [111] and striding sorts [326]. However, there is currently no
facility for searching such lists; this TIP proposes fixing this.

# Proposed Change

We propose adding a **-stride** option to **lsearch**, by exact analogy with the option added to **lsort** in [326], whose semantics it should closely match.

If **-stride** is supplied, the list will be treated as consisting of groups of grpSize elements.
The search will be operated within this group as it is a first level of nested lists (see *Conceptual Backround* below).

The first element of **-index** is used to seach for an item of the group.

The option **-start** always points to the beginning of the group, even if a position within the group is given.

Returned indices are the first element of the striding group(s) that is/are being indicated.

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

# Conceptual Backround

## Striding equivalent to first level of nested lists

The striding within the list is seen as the first level of list nesting. E.g.

**Nested list**:

```
set deep {{1 a A} {2 b B} {3 c C}}
```

**Flat strided list**:

```
set flat {1 a A 2 b B 3 c C}
```

Functions should operate the same way on both representation, with the only difference, that **-stride 3** must be specified in the second case.

Unfortunately, the current implementation of **lsort** is not doing this.
It interpretes **-index ""** as **-index 0**:

```
% lsort -stride 2 {A 1 A 2 A 0}
A 1 A 2 A 0
% lsort -stride 2 -index "" {B 2 B 1 A 3}
A 3 B 2 B 1
```

## Numeric position indices

Numerical positional indices (-start parameter, return value) follow the flattened list and not the grouped list. This is different to the nested list view.

Furthermore, if option **-subindices** is given and a non-empty argument for **-index**, then the group-start and index-into-group are added up. This gives compatibility with lindex, as in the no-stride case.

# Examples

In these examples, the variable *kvlist* holds the key-value list:

```
set kvlist {K1 V1 K2 V1 K1 K1}
```

Example 1: find keys even if they exist multiple times:

```
% lsearch -all -stride 2 -index 0 -exact $kvlist K1
0 4
```

Example 2: find existance of a value:

```
% lsearch -all -stride 2 -index 1 -exact $kvlist V1
0 2
```

Remark that the indexes of the first group elements are returned.
The real values are at "result+index" eq **1 3**.

Example 3: extract a sub-kv-list starting from key K2:

```
% lrange $kvlist [lsearch -stride 2 -index 0 -exact $kvlist K2] end
K2 V1 K1 K1
```

Example 4: find a group within a list:

```
% lsearch -stride 2 -exact $kvlist {K2 V1}
2
```

Example 5: find in combined strided and nested list

```
% lsearch -stride 2 -index {1 1} -exact\
{K0 {V0.0 V0.1} K1 {V1.0 V1.1}}\
V1.1
2
```

Example 6: subindices with strided list:

```
% lsearch -stride 2 -index {1 1} -subindices {1 {a A} 2 {b B}} B
3 1 (that is: 2 for the group-start plus 1 for the intra-group
index, and separately 1 for the further nested index.
% lindex {1 {a A} 2 {b B}} 3 1
B
```

to be consisten with:

```
% lsearch -index {1 1} -subindices {{1 {a A}} {2 {b B}}} B
1 1 1
% lindex {{1 {a A}} {2 {b B}}} 1 1 1
B
```

# Copyright

This document has been placed in the public domain.