Modifying Vectors: The previous lab on vectors discussed the creating of vectors and the retrieval of data from vectors. We now consider how to modify an existing vector through two Scheme procedures.
Procedure vector-set! changes the value stored in a designated location within a vector, using the form:
(vector-set! vect index new-value)That is, within vector vect, the element at the given index is changed to the specified new-value.
As the following examples indicate, procedure vector-set! itself does not return a new value -- rather it changes parameter vect.
(define x #(3 #\a "fred" 0.31894 12/5)) x (vector-set! x 3 "susan") x (vector-set! x 1 3.141592) x (define f (lambda (y) (vector-set! y 4 15))) (f x) x (vector-set! #(1 2 3 4 5 6) 3 10) (define y #(1 2 3 4 5 6)) (vector-set! y 3 10) y
Similarly, procedure vector-fill! changes all components of a given vector to a specified value, as illustrated in the following sequence:
(define x #(3 #\a "fred" 0.31894 12/5)) x (vector-fill! x "henry") x (vector-set! x 3 "terry") xIn this example, note that Chez Scheme uses a shorthand notation when printing a vector, all of whose components are identical. In particular, Chez Scheme prints
#5("henry")to represent a vector of five components, with each component being the string "henry".
If this notation #5("henry") seems confusing, Chez Scheme can provide a more conventional output of vectors. First, type
(print-vector-length #f)In subsequent lines, vectors will be printed showing each component.
(print-vector-length #t)Convention: In writing Scheme procedures, it is common to use an exclamation point (!) at the end of a procedure name, when that procedure changes a parameter.
vector-iotathat takes a natural number
nas argument and returns a vector of length
ncontaining all of the natural numbers less than
n, in ascending order.
(vector-iota 9) ===> #9(0 1 2 3 4 5 6 7 8) (vector-iota 2) ===> #2(0 1) (vector-iota 0) ===> #0()
(define str "abcdefg") str (string-set! str 3 #\q) str (define lst '(a b c d e f g h i)) lst (set-car! lst 5) lst (set-cdr! lst '(3.14159 12/4 "string")) lstIn each case, describe how the procedure changes the first parameter.
For those that know BASIC, Pascal, C, or C++, note that Scheme's vector is analogous to an array, except that Scheme does not require that all elements in the vector have the same type. Thus, in the example #(3 #\a "fred" 0.31894 12/5), a single vector contains a character, a string, a real number, and a fraction.
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