Adding Machine Run 2
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- Blaze Owens
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1 Calculator Run 1
2 Adding Machine Run 2
3 Simple Adder (define TOTAL 0) (define total-message (make-message (number->string TOTAL))) (define amount-text (make-text "Amount")) (define add-button (make-button "+" (lambda (evt) (add-to-total (string->number (text-contents amount-text)))))) ; add-to-total : num -> true (define (add-to-total amt) (local [(define new-total (+ TOTAL amt))] (draw-message total-message (number->string new-total)))) (create-window (list (list total-message) (list amount-text) (list add-button))) 3
4 Why the Adder is Unlike A Calculator (define (add-to-total amt) (local [(define new-total (+ TOTAL amt))] (draw-message total-message (number->string new-total)))) Every time we have a new amt, it s added to the same original TOTAL The new total is drawn on the screen, then forgotten 4
5 Why the Adder is Unlike A Calculator (define (add-to-total amt) (local [(define new-total (+ TOTAL amt))] (draw-message total-message (number->string new-total)))) Every time we have a new amt, it s added to the same original TOTAL The new total is drawn on the screen, then forgotten We need to remember new-total for next time 5
6 set! In Advanced: changes the value of TOTAL to 17 (set! TOTAL 17) 6
7 set! In Advanced: changes the value of TOTAL to 17 (set! TOTAL 17) "Constant" definitions are no longer constant the are variable definitions A set! expression is called an assignment The value of TOTAL is the state of the program 7
8 Evaluating set! (define TOTAL 0) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (add-amt 1) (add-amt 2) 8
9 Evaluating set! (define TOTAL 0) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (add-amt 1) (add-amt 2) (define TOTAL 0) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (set! TOTAL (+ TOTAL 1)) (add-amt 2) 9
10 Evaluating set! (define TOTAL 0) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (set! TOTAL (+ TOTAL 1)) (add-amt 2) 10
11 Evaluating set! (define TOTAL 0) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (set! TOTAL (+ TOTAL 1)) (add-amt 2) (define TOTAL 0) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (set! TOTAL (+ 0 1)) (add-amt 2) 11
12 Evaluating set! (define TOTAL 0) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (set! TOTAL (+ 0 1)) (add-amt 2) 12
13 Evaluating set! (define TOTAL 0) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (set! TOTAL (+ 0 1)) (add-amt 2) (define TOTAL 0) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (set! TOTAL 1) (add-amt 2) 13
14 Evaluating set! (define TOTAL 0) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (set! TOTAL 1) (add-amt 2) 14
15 Evaluating set! (define TOTAL 0) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (set! TOTAL 1) (add-amt 2) (define TOTAL 1) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (void) (add-amt 2) To evaluate set!, change a definition and produce (void) 15
16 Evaluating set! (define TOTAL 1) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (void) (add-amt 2) 16
17 Evaluating set! (define TOTAL 1) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (void) (add-amt 2) (define TOTAL 1) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (void) (set! TOTAL (+ TOTAL 2)) 17
18 Evaluating set! (define TOTAL 1) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (void) (set! TOTAL (+ TOTAL 2)) 18
19 Evaluating set! (define TOTAL 1) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (void) (set! TOTAL (+ TOTAL 2)) (define TOTAL 1) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (void) (set! TOTAL (+ 1 2)) 19
20 Evaluating set! (define TOTAL 1) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (void) (set! TOTAL (+ 1 2)) 20
21 Evaluating set! (define TOTAL 1) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (void) (set! TOTAL (+ 1 2)) (define TOTAL 1) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (void) (set! TOTAL 3) It s important that a variable name is not replaced by its value until the value is needed 21
22 Evaluating set! (define TOTAL 1) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (void) (set! TOTAL 3) 22
23 Evaluating set! (define TOTAL 1) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (void) (set! TOTAL 3) (define TOTAL 3) (define (add-amt amt) (set! TOTAL (+ TOTAL amt))) (void) (void) 23
24 Repairing the Calculator (define (add-to-total amt) (local [(define new-total (+ TOTAL amt))] ; How do we combine two actions?... (set! TOTAL new-total) (draw-message total-message (number->string new-total))...)) 24
25 Repairing the Calculator (define (add-to-total amt) (local [(define new-total (+ TOTAL amt))] ; How do we combine two actions?... (set! TOTAL new-total) (draw-message total-message (number->string new-total))...)) For drawing, we used and to combine actions But set! doesn t return a boolean 25
26 Making the Adder Remember Totals Also new in Advanced: the begin form (define (add-to-total amt) (local [(define new-total (+ TOTAL amt))] (begin (set! TOTAL new-total) (draw-message total-message (number->string new-total))))) 26
27 Making the Adder Remember Totals Also new in Advanced: the begin form (define (add-to-total amt) (local [(define new-total (+ TOTAL amt))] (begin (set! TOTAL new-total) (draw-message total-message (number->string new-total))))) The begin form Evaluates its first expression Throws away the result Goes away when only one expression is left begin works with any number of expressions Run 27
28 Evaluating begin (define TOTAL 3) (define (running-total amt) (begin (set! TOTAL (+ TOTAL amt)) TOTAL)) (running-total 10) 28
29 Evaluating begin (define TOTAL 3) (define (running-total amt) (begin (set! TOTAL (+ TOTAL amt)) TOTAL)) (running-total 10) (define TOTAL 3)... (begin (set! TOTAL (+ TOTAL 10)) TOTAL) 29
30 (define TOTAL 3)... (begin (set! TOTAL (+ TOTAL 10)) TOTAL) Evaluating begin 30
31 (define TOTAL 3)... (begin (set! TOTAL (+ TOTAL 10)) TOTAL) (define TOTAL 3)... (begin (set! TOTAL (+ 3 10)) TOTAL) Evaluating begin 31
32 (define TOTAL 3)... (begin (set! TOTAL (+ 3 10)) TOTAL) Evaluating begin 32
33 (define TOTAL 3)... (begin (set! TOTAL (+ 3 10)) TOTAL) (define TOTAL 3)... (begin (set! TOTAL 13) TOTAL) Evaluating begin 33
34 Evaluating begin (define TOTAL 3)... (begin (set! TOTAL 13) TOTAL) 34
35 Evaluating begin (define TOTAL 3)... (begin (set! TOTAL 13) TOTAL) (define TOTAL 13)... (begin (void) TOTAL) 35
36 Evaluating begin (define TOTAL 13)... (begin (void) TOTAL) 36
37 Evaluating begin (define TOTAL 13)... (begin (void) TOTAL) (define TOTAL 13)... (begin TOTAL) 37
38 Evaluating begin (define TOTAL 13)... (begin TOTAL) 38
39 Evaluating begin (define TOTAL 13)... (begin TOTAL) (define TOTAL 13)... TOTAL 39
40 Evaluating begin (define TOTAL 13)... TOTAL 40
41 Evaluating begin (define TOTAL 13)... TOTAL (define TOTAL 13)
42 More Calculator Buttons Run 42
43 Implementing More Calculator Buttons... ; op-button : string (num num -> num) -> button (define (op-button label OP) (make-button label (lambda (evt) (change-total OP (string->number (text-contents amount-text)))))) ; change-total : (num num -> num) num -> true (define (change-total OP amt) (local [(define new-total (OP TOTAL amt))] (begin (set! TOTAL new-total) (draw-message total-message (number->string new-total))))) (create-window (list (list total-message) (list amount-text) (list (op-button "+" +) (op-button "-" -) (op-button "*" *) (op-button "/" /)))) 43
44 The Digit Buttons Run Now two pieces of state: The running total The number we re typing, so far 44
45 ... (define WORKING 0) Implementing Digit Buttons ; digit-button : num -> button (define (digit-button n) (make-button (number->string n) (lambda (evt) (add-digit n)))) ; add-digit : num -> true (define (add-digit n) (begin (set! WORKING (+ n (* WORKING 10))) (draw-message total-message (number->string WORKING)))) ; change-total : (num num -> num) num -> true (define (change-total OP amt) (local [(define new-total (OP TOTAL amt))] (begin (set! TOTAL new-total) (set! WORKING 0) (draw-message total-message (number->string new-total)))))... 45
46 Infix Operations Run A normal calculator uses infix (algebra-like) order New piece of state: The operation to perform when the number is ready 46
47 ... (define PREV-OP +) Implementing Infix Operations ; op-button : string (num num -> num) -> button (define (op-button label OP) (make-button label (lambda (evt) (begin (change-total PREV-OP WORKING) (set! PREV-OP OP) true))))... (create-window (list (list total-message) (map digit-button (7 8 9)) (map digit-button (4 5 6)) (map digit-button (1 2 3)) (map digit-button (0)) (list (op-button "+" +) (op-button "-" -) (op-button "*" *) (op-button "/" /) (op-button "=" (lambda (tot new) new))))) 47
48 Multiple Calculators Run How can we keep the calculators from using the same TOTAL? 48
49 Multiple Calculators Run How can we keep the calculators from using the same TOTAL? Easy use local 49
50 Implementing Multiple Calculators (define (make-calculator) (local [(define TOTAL 0) (define WORKING 0)...] (create-window (list (list total-message) (map digit-button (7 8 9)) (map digit-button (4 5 6)) (map digit-button (1 2 3)) (map digit-button (0)) (list (op-button "+" +) (op-button "-" -) (op-button "*" *) (op-button "/" /) (op-button "=" (lambda (tot new) new))))))) (make-calculator) (make-calculator) 50
51 When to use State Use state and set! when a function needs to remember something about previous calls, and you have no control over the callers 51
52 When NOT to use State The following is a unacceptable use of set! (define REV empty) (define (reverse-list l) (cond [(empty? l) REV] [(cons? l) (begin (set! REV (cons (first l) REV)) (reverse-list (rest l)))])) (reverse-list ( )) Recursive calls build on earlier results, but we control all of the recursive calls It produces the wrong result when you call it a second time 52
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