Algosim documentation: part

part

Returns a part of an object.

Syntax

Description

If X is a linear container and R a range, then part(X, R) returns the part of X that is specified by R. R can be an index, a range(a, b), or a list of indices and ranges of any length and composition.

In this case X can be a vector, a matrix (row-major order), a list, a structure, a pixmap (row-major order), or a binary data object.

If X contains n elements, then indices 1..n counts from the start of X, while indices −n..−1 counts from the end of X. Positive and negative indices can be freely mixed; for instance, range(2, −2) means the range from the second element to the next to last element. If b (being either positive or negative) refers to an element preceding a in X, then range(a, b) means all indices from b to a in reverse order.

range(a, b, k) produces the sequence from a to b (possibly in reverse) but taking only every kth index.

If X is a planar container (such as a matrix or a pixmap), then part(X, R1, R2) returns the part of X with horizontal indices described by R1 and vertical indices described by R2.

Examples

v ≔ ❨4, 1, 0, 2, 5, 7, −3, 4, 7, 8❩
(4, 1, 0, 2, 5, 7, −3, 4, 7, 8)
part(v, 4)
(2)
part(v, '(4, 5, 1, 2))
 ⎛2⎞
 ⎜5⎟
e⎜4⎟
 ⎝1⎠

part(v, '(1, range(5, 8)))
 ⎛4 ⎞
 ⎜5 ⎟
e⎜7 ⎟
 ⎜−3⎟
 ⎝4 ⎠

part(v, '(1, range(5, −1)))
 ⎛4 ⎞
 ⎜5 ⎟
 ⎜7 ⎟
e⎜−3⎟
 ⎜4 ⎟
 ⎜7 ⎟
 ⎝8 ⎠

part(v, '(1, range(−1, −5)))
 ⎛4 ⎞
 ⎜8 ⎟
 ⎜7 ⎟
e⎜4 ⎟
 ⎜−3⎟
 ⎝7 ⎠

part(v, range(1, −1, 2))
 ⎛4 ⎞
 ⎜0 ⎟
e⎜5 ⎟
 ⎜−3⎟
 ⎝7 ⎠

A ≔ ❨❨5, 1, 2, 3, 6❩, ❨1, 2, 3, 6, 7❩, ❨4, 1, 2, 3, 8❩, ❨0, 1, 2, 6, 1❩❩
⎛5  1  2  3  6⎞
⎜1  2  3  6  7⎟
⎜4  1  2  3  8⎟
⎝0  1  2  6  1⎠

part(A, range(3, 6))
2
3
6
1

part(A, range(−1, 1, 2))
1
2
0
3
1
7
3
1
3
1

part(A, '(1, 4, 5), range(−1, −3))
⎛0  6  1⎞
⎜4  3  8⎟
⎝1  6  7⎠

See also