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4.6 Conversion of Characters and Strings

This section describes functions for converting between characters, strings and integers. format (see Formatting Strings) and prin1-to-string (see Output Functions) can also convert Lisp objects into strings. read-from-string (see Input Functions) can “convert” a string representation of a Lisp object into an object. The functions string-to-multibyte and string-to-unibyte convert the text representation of a string (see Converting Representations).

See Documentation, for functions that produce textual descriptions of text characters and general input events (single-key-description and text-char-description). These are used primarily for making help messages.

— Function: number-to-string number

This function returns a string consisting of the printed base-ten representation of number, which may be an integer or a floating point number. The returned value starts with a minus sign if the argument is negative.

          (number-to-string 256)
               ⇒ "256"
          (number-to-string -23)
               ⇒ "-23"
          (number-to-string -23.5)
               ⇒ "-23.5"

int-to-string is a semi-obsolete alias for this function.

See also the function format in Formatting Strings.

— Function: string-to-number string &optional base

This function returns the numeric value of the characters in string. If base is non-nil, it must be an integer between 2 and 16 (inclusive), and integers are converted in that base. If base is nil, then base ten is used. Floating point conversion only works in base ten; we have not implemented other radices for floating point numbers, because that would be much more work and does not seem useful. If string looks like an integer but its value is too large to fit into a Lisp integer, string-to-number returns a floating point result.

The parsing skips spaces and tabs at the beginning of string, then reads as much of string as it can interpret as a number in the given base. (On some systems it ignores other whitespace at the beginning, not just spaces and tabs.) If the first character after the ignored whitespace is neither a digit in the given base, nor a plus or minus sign, nor the leading dot of a floating point number, this function returns 0.

          (string-to-number "256")
               ⇒ 256
          (string-to-number "25 is a perfect square.")
               ⇒ 25
          (string-to-number "X256")
               ⇒ 0
          (string-to-number "-4.5")
               ⇒ -4.5
          (string-to-number "1e5")
               ⇒ 100000.0

string-to-int is an obsolete alias for this function.

— Function: char-to-string character

This function returns a new string containing one character, character. This function is semi-obsolete because the function string is more general. See Creating Strings.

— Function: string-to-char string

This function returns the first character in string. This mostly identical to (aref string 0), except that it returns 0 if the string is empty. (The value is also 0 when the first character of string is the null character, ASCII code 0.) This function may be eliminated in the future if it does not seem useful enough to retain.

Here are some other functions that can convert to or from a string:

concat
This function converts a vector or a list into a string. See Creating Strings.
vconcat
This function converts a string into a vector. See Vector Functions.
append
This function converts a string into a list. See Building Lists.
byte-to-string
This function converts a byte of character data into a unibyte string. See Converting Representations.