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These procedures are for modifying strings in-place. This means that the result of the operation is not a new string; instead, the original string's memory representation is modified.

— Scheme Procedure: string-set! str k chr
— C Function: scm_string_set_x (str, k, chr)

Store chr in element k of str and return an unspecified value. k must be a valid index of str.

— C Function: void scm_c_string_set_x (SCM str, size_t k, SCM chr)

Like scm_string_set_x, but the index is given as a size_t.

— Scheme Procedure: string-fill! str chr [start [end]]
— C Function: scm_substring_fill_x (str, chr, start, end)
— C Function: scm_string_fill_x (str, chr)

Stores chr in every element of the given str and returns an unspecified value.

— Scheme Procedure: substring-fill! str start end fill
— C Function: scm_substring_fill_x (str, start, end, fill)

Change every character in str between start and end to fill.

          (define y "abcdefg")
          (substring-fill! y 1 3 #\r)
          ⇒ "arrdefg"
— Scheme Procedure: substring-move! str1 start1 end1 str2 start2
— C Function: scm_substring_move_x (str1, start1, end1, str2, start2)

Copy the substring of str1 bounded by start1 and end1 into str2 beginning at position start2. str1 and str2 can be the same string.

— Scheme Procedure: string-copy! target tstart s [start [end]]
— C Function: scm_string_copy_x (target, tstart, s, start, end)

Copy the sequence of characters from index range [start, end) in string s to string target, beginning at index tstart. The characters are copied left-to-right or right-to-left as needed – the copy is guaranteed to work, even if target and s are the same string. It is an error if the copy operation runs off the end of the target string.