BMI Calculator

August 30, 2013

We distinguish between imperial and metric units by the number of arguments given to the function, using `case-lambda`:

```(define bmi   (case-lambda     ((feet inches pounds)       (let ((height (+ (* feet 12) inches)))         (* (/ pounds (* height height)) 703.0)))     ((centimeters kilograms)       (let ((meters (/ centimeters 100.0)))         (/ kilograms (* meters meters))))))```

```(define (bmi-class bmi)   (cond ((<= bmi 18.5) "underweight")         ((< bmi 25) "normal")         ((< bmi 30) "overweight")         ((< bmi 40) "obese")         (else "morbidly obese")))```

`Case-lambda` was only added to the Scheme standard in R6RS, though it has been available for years. If your Scheme doesn’t have `case-lambda`, here’s a definition from SRFI-16:

```(define-syntax case-lambda   (syntax-rules ()     ((case-lambda)      (lambda args        (error "CASE-LAMBDA without any clauses.")))     ((case-lambda       (?a1 ?e1 ...)       ?clause1 ...)      (lambda args        (let ((l (length args)))          (case-lambda "CLAUSE" args l            (?a1 ?e1 ...)            ?clause1 ...))))     ((case-lambda "CLAUSE" ?args ?l       ((?a1 ...) ?e1 ...)       ?clause1 ...)      (if (= ?l (length '(?a1 ...)))          (apply (lambda (?a1 ...) ?e1 ...) ?args)          (case-lambda "CLAUSE" ?args ?l            ?clause1 ...)))     ((case-lambda "CLAUSE" ?args ?l       ((?a1 . ?ar) ?e1 ...)       ?clause1 ...)      (case-lambda "IMPROPER" ?args ?l 1 (?a1 . ?ar) (?ar ?e1 ...)        ?clause1 ...))     ((case-lambda "CLAUSE" ?args ?l       (?a1 ?e1 ...)       ?clause1 ...)      (let ((?a1 ?args))        ?e1 ...))     ((case-lambda "CLAUSE" ?args ?l)      (error "Wrong number of arguments to CASE-LAMBDA."))     ((case-lambda "IMPROPER" ?args ?l ?k ?al ((?a1 . ?ar) ?e1 ...)       ?clause1 ...)      (case-lambda "IMPROPER" ?args ?l (+ ?k 1) ?al (?ar ?e1 ...)       ?clause1 ...))     ((case-lambda "IMPROPER" ?args ?l ?k ?al (?ar ?e1 ...)       ?clause1 ...)      (if (>= ?l ?k)          (apply (lambda ?al ?e1 ...) ?args)          (case-lambda "CLAUSE" ?args ?l            ?clause1 ...)))))```

You can run the program at http://programmingpraxis.codepad.org/frLTNeu8.

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3 Responses to “BMI Calculator”

1. Paul said

My version in Python. I cheated a little with the categories. According to specs underweight is when bmi <= 18.5. Here it is if bmi < 18.5.

CATEGORIES = zip([18.5, 25, 30, 40, 1000], ["underweight", "normal",
"overweight", "obese", "morbidly obese"])
WEIGHT_CONV = dict(zip(["kg", "lbs", "ounce"], [1, 0.45359237, 0.0283495231]))
LENGTH_CONV = dict(zip(["m", "cm", "inch", "ft"], [1, 0.01, 0.0254, 0.3048]))

def val_metric(value, conv):
"""convert to metric units (kg, m)
allowed: 5 ft 8 inch or 100 lbs 3 ounce
"""
elements = value.split()
total = 0
while elements:
total += float(elements.pop(0)) * conv[elements.pop(0)]

def bmi(weight, length):
w = val_metric(weight, WEIGHT_CONV)
l = val_metric(length, LENGTH_CONV)
bmi = w / l ** 2
for c, txt in CATEGORIES:
if bmi < c:
cat = txt
break
return bmi, cat

print "bmi=", bmi("160 lbs", "5 ft 7 inch")

2. Paul said

Hopefully this time with better formatting:

```CATEGORIES = zip([18.5, 25, 30, 40, 1000], ["underweight", "normal",
"overweight", "obese", "morbidly obese"])
WEIGHT_CONV = dict(zip(["kg", "lbs", "ounce"], [1, 0.45359237, 0.0283495231]))
LENGTH_CONV = dict(zip(["m", "cm", "inch", "ft"], [1, 0.01, 0.0254, 0.3048]))

def val_metric(value, conv):
"""convert to metric units (kg, m)
allowed: 5 ft 8 inch   or 100 lbs 3 ounce
"""
elements = value.split()
total = 0
while elements:
total += float(elements.pop(0)) * conv[elements.pop(0)]

def bmi(weight, length):
w = val_metric(weight, WEIGHT_CONV)
l = val_metric(length, LENGTH_CONV)
bmi = w / l ** 2
for c, txt in CATEGORIES:
if bmi < c:
cat = txt
break
return bmi, cat

print "bmi=", bmi("160 lbs", "5 ft 7 inch")
```
3. Jamie Hope said

Kawa has built-in support for Quantities (numbers with units), so here’s a solution using them instead of case-lambda:

```;;; Kawa knows about the units g, m, and cm, but not kg, ft, or lb, so add those.
(define-unit ft 12in)
(define-unit kg 1000g)
(define-unit lb 0.453592kg)

(define (bmi (weight ::quantity) (height ::quantity)) ::double
(let* ((weight ::quantity (+ 0.0kg weight)) ; force to kg
(height ::quantity (+ 0.0m height))  ; force to m
(bmi-with-units ::quantity (/ weight (* height height))))
(*:number bmi-with-units))) ; drop the units
```

The `bmi-class` function is the same as above.

```(bmi 95kg 190cm) => 26.31578947368421 (bmi 125lb (+ 5ft 4in)) => 21.4559633220829```