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StringPart

Status: Stable

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

Description

StringPart["string", n]
    Gives the nth character in "string".
StringPart["string", {n1, n2, ...}]
    Gives a list of the ni-th characters.
StringPart["string", m;;n;;s]
    Gives characters m through n in steps of s.
StringPart[{s1, s2, ...}, spec]
    Gives the list of results for each si.

    Negative indices count from the end.

Examples

All examples below are verified against the current Mathilda build.

In[1]:= StringPart["abcdefghijklm", 6]
Out[1]= "f"

In[2]:= StringPart["abcdefghijklm", {1, 3, 5}]
Out[2]= {"a", "c", "e"}

In[3]:= StringPart["abcdefghijklm", -4]
Out[3]= "j"

In[4]:= StringPart["abcdefghijklm", 1;;6]
Out[4]= {"a", "b", "c", "d", "e", "f"}

In[5]:= StringPart["abcdefghijklm", 1;;-1;;2]
Out[5]= {"a", "c", "e", "g", "i", "k", "m"}

In[6]:= StringPart["abcdefghijklm", -1;;1;;-2]
Out[6]= {"m", "k", "i", "g", "e", "c", "a"}

In[7]:= StringPart[{"abcd", "efgh", "ijklm"}, 1]
Out[7]= {"a", "e", "i"}

In[8]:= StringPart[{"abcd", "efgh", "ijklm"}, {1, -1}]
Out[8]= {{"a", "d"}, {"e", "h"}, {"i", "m"}}

Implementation notes

builtin_stringpart takes (string, spec) and indexes by byte (1-based; negative counts from the end via len + k + 1). The single-index path uses the helper stringpart_single, which bounds-checks k and returns a length-1 EXPR_STRING. A List spec maps stringpart_single over each index into a result List; a Span[m, n, s] spec resolves start/end/step (honouring All and negative endpoints), computes the element count, and emits the stepped characters as a List. When the first argument is itself a List of strings, the builtin recurses by constructing and evaluating an inner StringPart[si, spec] per element. Out-of-range or non-integer indices return NULL. ATTR_PROTECTED.

Attributes: Protected.

Implementation status

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

References