StringDrop

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description

StringDrop["string", n]

Gives "string" with its first n characters dropped.

StringDrop["string", -n]

Gives "string" with its last n characters dropped.

StringDrop["string", {n}]

Gives "string" with its nth character dropped.

StringDrop["string", {m, n}]

Gives "string" with characters m through n dropped.

StringDrop["string", {m, n, s}]

Drops characters m through n in steps of s.

StringDrop["string", UpTo[n]]

Drops n characters, or as many as are available.

StringDrop[{s1, s2, ...}, spec]

Gives the list of results for each of the si. Negative indices count from the end.

Examples (12)

Every input below was run against the current Mathilda build and its output recorded.

Basic examples (9)

In[1]:= StringDrop["abcdefghijklm", 4]
Out[1]= "efghijklm"

In[2]:= StringDrop["abcdefghijklm", -4]
Out[2]= "abcdefghi"

In[3]:= StringDrop["abcdefghijklm", {5, 10}]
Out[3]= "abcdklm"

In[4]:= StringDrop["abcdefghijklm", {3}]
Out[4]= "abdefghijklm"

In[5]:= StringDrop["abcdefghijklm", {1, -1, 2}]
Out[5]= "bdfhjl"

In[6]:= StringDrop[{"abcdef", "xyzw", "stuv"}, -2]
Out[6]= {"abcd", "xy", "st"}

In[7]:= StringDrop["abc", UpTo[4]]
Out[7]= ""

In[8]:= StringDrop["abcdefghijklm", {5, -4}]
Out[8]= "abcdklm"

In[9]:= StringDrop[] StringDrop::argrx: StringDrop called with 0 arguments; 2 arguments are expected.

Applications (3)

Drop the first two characters

In[10]:= StringDrop["abcdef", 2]
Out[10]= "cdef"

Drop the last two

In[11]:= StringDrop["abcdef", -2]
Out[11]= "abcd"

Drop a range

In[12]:= StringDrop["abcdef", {2, 4}]
Out[12]= "aef"

Implementation notes

Algorithm. builtin_stringdrop requires exactly two arguments (else StringDrop::argrx). It allocates a byte keep-mask initialised to true, clears the positions named by the sequence spec, and rebuilds the kept bytes in order (stringdrop_build_kept) — the complementary "keep" mask is how StringDrop is expressed as the complement of StringTake. Specs handled: an integer n (drop the first n, or the last |n| when negative), UpTo[n] (clamped to the length), {n}, {m, n} (a decreasing range drops nothing), and {m, n, s} (stepped). Negative endpoints normalise as len + k + 1. A List first argument recurses per element via evaluate(StringDrop[si, spec]).

Data structures. A bool[len] keep-mask and one output buffer.

Complexity / limits. O(len). An out-of-range position, a zero step, or a non-integer spec leaves the call unevaluated; indexing is byte-based (no UTF-8 decoding), consistent with StringTake.

Attributes: Protected.

References

See also: StringTake

• Source: src/strings/stringdrop.c
• Specification: docs/spec/builtins/string-operations.md
• Tests: tests/test_string_review.c
• Tests: tests/test_strings.c

Notes & additional examples

Notes

StringDrop is the complement of StringTake: the same sequence specification selects the characters to remove, and the survivors are concatenated in order. Internally it marks a keep-mask over the bytes and rebuilds the kept ones.

Negative indices count from the end; indexing is byte-based. A decreasing range {m, n} with m > n is empty and drops nothing; an out-of-range position leaves the call unevaluated.