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Programming in Emacs Lisp

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1.8.1 Arguments' Data Types

The type of data that should be passed to a function depends on what kind of information it uses. The arguments to a function such as + must have values that are numbers, since + adds numbers. Other functions use different kinds of data for their arguments.

For example, the concat function links together or unites two or more strings of text to produce a string. The arguments are strings. Concatenating the two character strings abc, def produces the single string abcdef. This can be seen by evaluating the following:

(concat "abc" "def")

The value produced by evaluating this expression is "abcdef".

A function such as substring uses both a string and numbers as arguments. The function returns a part of the string, a substring of the first argument. This function takes three arguments. Its first argument is the string of characters, the second and third arguments are numbers that indicate the beginning and end of the substring. The numbers are a count of the number of characters (including spaces and punctuations) from the beginning of the string.

For example, if you evaluate the following:

(substring "The quick brown fox jumped." 16 19)

you will see "fox" appear in the echo area. The arguments are the string and the two numbers.

Note that the string passed to substring is a single atom even though it is made up of several words separated by spaces. Lisp counts everything between the two quotation marks as part of the string, including the spaces. You can think of the substring function as a kind of `atom smasher' since it takes an otherwise indivisible atom and extracts a part. However, substring is only able to extract a substring from an argument that is a string, not from another type of atom such as a number or symbol.

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