seq2slice

Convert a subsequence string to a Slice object.

Usage

var seq2slice = require( '@stdlib/slice/seq2slice' );

seq2slice( str, len, strict )

Converts a subsequence string to a Slice object, where len specifies the maximum number of elements allowed in the slice.

var s = seq2slice( ':5', 10, false );
// returns <Slice>

var v = s.start;
// returns 0

v = s.stop;
// returns 5

v = s.step;
// returns 1

A subsequence string has the following format:

<start>:<stop>:<increment>

where

  • If an increment is not specified, the default increment is 1. An increment of zero is not allowed.
  • The start index is inclusive.
  • The stop index is exclusive.
  • Both start and stop indices are optional. If not provided, start and stop default to index extremes. Which extremes correspond to which index depends on whether the increment is positive or negative.
  • Both start and stop can be negative; in which case, the corresponding index is resolved by subtracting the respective value from the provided length len.
  • Both start and stop can use the end keyword (e.g., end-2::2, end-3:, etc), which supports basic subtraction and division.
  • The end keyword resolves to the provided length len. Thus, :-1 is equivalent to :end-1, :-2 is equivalent to :end-2, and so on and so forth. The exception is when performing a division operation when the increment is less than zero; in which case, end is equal to len-1 in order to preserve user expectations when end/d equals a whole number and slicing from right-to-left. The result from a division operation is rounded down to the nearest integer value.
var s = seq2slice( 'end:2:-1', 10, false );
// returns <Slice>

var v = s.start;
// returns 9

v = s.stop;
// returns 2

v = s.step;
// returns -1

s = seq2slice( 'end-2:2:-1', 10, false );
// returns <Slice>

v = s.start;
// returns 8

v = s.stop;
// returns 2

v = s.step;
// returns -1

s = seq2slice( 'end/2:2:-1', 10, false );
// returns <Slice>

v = s.start;
// returns 4

v = s.stop;
// returns 2

v = s.step;
// returns -1

When strict is true, the function throws an error if a subsequence string resolves to a slice exceeding index bounds.

var s = seq2slice( '10:20', 10, true );
// throws <RangeError>

Notes

  • When len is zero, the function always returns a Slice object equivalent to 0:0:<increment>.
  • When strict is false, the resolved slice start is clamped to the slice index bounds (i.e., [0, len)).
  • When strict is false, the resolved slice end is upper bound clamped to len (i.e., one greater than the last possible index).
  • When the increment is negative, the resolved slice end value may be null, thus indicating that a non-empty slice should include the first index.
  • The function ensures that results satisfy the convention that :n combined with n: is equivalent to : (i.e., selecting all elements). This convention matches Python slice semantics, but diverges from the MATLAB convention where :n and n: overlap by one element.
  • Unlike MATLAB, but like Python, the subsequence string is upper-bound exclusive. For example, in Python, 0:2 corresponds to the sequence {0,1}. In MATLAB, 1:3 corresponds to {1,2,3}.

Examples

var seq2slice = require( '@stdlib/slice/seq2slice' );

var s = seq2slice( ':', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 5. step: 1.'

s = seq2slice( '2:', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 2. stop: 5. step: 1.'

s = seq2slice( ':3', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 3. step: 1.'

s = seq2slice( '2:4', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 2. stop: 4. step: 1.'

s = seq2slice( '1:4:2', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 1. stop: 4. step: 2.'

s = seq2slice( '2::2', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 2. stop: 5. step: 2.'

s = seq2slice( ':-2', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 3. step: 1.'

s = seq2slice( ':-1:2', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 4. step: 2.'

s = seq2slice( '-4:-1:2', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 1. stop: 4. step: 2.'

s = seq2slice( '-5:-1', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 4. step: 1.'

s = seq2slice( '::-1', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 4. stop: null. step: -1.'

s = seq2slice( ':0:-1', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 4. stop: 0. step: -1.'

s = seq2slice( '3:0:-1', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 3. stop: 0. step: -1.'

s = seq2slice( '-1:-4:-2', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 4. stop: 1. step: -2.'

s = seq2slice( ':end', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 5. step: 1.'

s = seq2slice( ':end-1', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 4. step: 1.'

s = seq2slice( ':end/2', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 2. step: 1.'

s = seq2slice( 'end/2::-1', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 2. stop: null. step: -1.'

s = seq2slice( 'end-2::-1', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 3. stop: null. step: -1.'

s = seq2slice( 'end/2:', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 2. stop: 5. step: 1.'

See Also

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