## Big Numbers: Addition, Subtraction, And Multiplication

### May 31, 2011

For addition, it’s best to make a table to figure out all the signs:

|a| < |b| |
a > 0 |
b > 0 |
a + b |

|a| < |b| |
a > 0 |
b < 0 |
−(|b| − a) |

|a| < |b| |
a < 0 |
b < 0 |
−(|a| + b) |

|a| < |b| |
a < 0 |
b > 0 |
b − |a| |

|a| > |b| |
a > 0 |
b > 0 |
a + b |

|a| > |b| |
a > 0 |
b < 0 |
a − |b| |

|a| > |b| |
a < 0 |
b < 0 |
−(|a| + b) |

|a| > |b| |
a < 0 |
b > 0 |
−(|a| − b) |

Then it’s easy enough to write the addition function. `Add`

adds two big-numbers, big-digit by big-digit, carrying when necessary, starting from the least significant big-digit. `Sub`

does the same, but with a borrow instead of a carry, and calls `reduce`

to strip any leading zero big-digits in the result. The main body of the function handles zero and figures out the required operation and sign of the result:

`(define (big-plus big1 big2)`

(define (reduce xs)

(if (null? (cdr xs)) xs

(if (positive? (car xs)) xs

(reduce (cdr xs)))))

(define (add b1 b2)

(let loop ((b1 b1) (b2 b2) (c 0) (bs '()))

(cond ((null? b1)

(if (zero? c) (reverse bs) (reverse (cons 1 bs))))

((null? b2)

(let* ((sum (+ (car b1) c))

(new-c (if (<= big-base sum) 1 0)))

(loop (cdr b1) b2 new-c

(cons (modulo sum big-base) bs))))

(else (let* ((sum (+ (car b1) (car b2) c))

(new-c (if (<= big-base sum) 1 0)))

(loop (cdr b1) (cdr b2) new-c

(cons (modulo sum big-base) bs)))))))

(define (sub b1 b2)

(let loop ((b1 b1) (b2 b2) (c 0) (bs '()))

(cond ((null? b1) (reverse (reduce bs)))

((null? b2)

(let* ((diff (- (car b1) c))

(new-c (if (< diff 0) 1 0)))

(loop (cdr b1) b2 new-c

(cons (modulo diff big-base) bs))))

(else (let* ((diff (- (car b1) (car b2) c))

(new-c (if (< diff 0) 1 0)))

(loop (cdr b1) (cdr b2) new-c

(cons (modulo diff big-base) bs)))))))

(if (zero? (car big1)) big2

(if (zero? (car big2)) big1

(let* ((b1 (cdr big1)) (b2 (cdr big2))

(lt? (big-lt? (big-abs big1) (big-abs big2)))

(s1 (if (positive? (car big1)) 1 -1))

(s2 (if (positive? (car big2)) 1 -1))

(x (apply (if (= s1 s2) add sub)

(if lt? (list b2 b1) (list b1 b2))))

(len (length x)))

(if (equal? x (list 0)) x

(cons (* len (if (or (and lt? (= s2 1))

(and (not lt?) (= s1 1)))

1 -1))

x))))))

To subtract, we add the negative:

`(define (big-minus big1 big2)`

(big-plus big1 (big-negate big2)))

Internally, multiplication is similar to addition, except that the carry can be any digit, not just 0 or 1. `Times`

does the work, multiplying each digit of the multiplier, least significant digit first, with every digit of the multiplicand, with `c`

the carry and `p`

the next big-digit of the current partial product. In grade-school the partial products were built into a tableau and added all at once, after all the partial products were computed, but we find it easier to add the partial products as they are computed. `Add`

is the same as big-number addition:

`(define (big-times big1 big2)`

(define (add b1 b2)

(let loop ((b1 b1) (b2 b2) (c 0) (bs '()))

(cond ((null? b1)

(if (zero? c) (reverse bs) (reverse (cons 1 bs))))

((null? b2)

(let* ((sum (+ (car b1) c))

(new-c (if (<= big-base sum) 1 0)))

(loop (cdr b1) b2 new-c

(cons (modulo sum big-base) bs))))

(else (let* ((sum (+ (car b1) (car b2) c))

(new-c (if (<= big-base sum) 1 0)))

(loop (cdr b1) (cdr b2) new-c

(cons (modulo sum big-base) bs)))))))

(define (sign x) (if (negative? x) -1 (if (positive? x) 1 0)))

(define (times big1 big2)

(let loop ((b1 big1) (b2 big2) (zs '())

(c 0) (ps '()) (bs '()))

(cond ((null? b1) bs)

((null? b2) (let ((zs (cons 0 zs)))

(loop (cdr b1) big2 zs 0 zs

(add (reverse (if (zero? c) ps (cons c ps))) bs))))

(else (let* ((x (+ (* (car b1) (car b2)) c))

(c (quotient x big-base))

(p (modulo x big-base)))

(loop b1 (cdr b2) zs c (cons p ps) bs))))))

(if (or (big-zero? big1) (big-zero? big2)) (list 0)

(let* ((b1 (cdr big1)) (b2 (cdr big2))

(x (times b1 b2)) (len (length x)))

(cons (* len (sign (* (car big1) (car big2)))) x))))

You can see the big-number library, including the code from the previous exercise and some simple examples of today’s functions, at http://programmingpraxis.codepad.org/gAbRGnwD. We’ll do more next Tuesday.

My Haskell solution (see http://bonsaicode.wordpress.com/2011/05/31/programming-praxis-big-numbers-addition-subtraction-and-multiplication/ for a version with comments):

My Scheme solution: