# First attempt: array of hashes, please help

**URL:** <https://www.ruby-forum.com/t/first-attempt-array-of-hashes-please-help/226024>\
**Category:** Rails\
**Created:** [August 22, 2008, 7:13pm UTC](https://www.ruby-forum.com/t/first-attempt-array-of-hashes-please-help/226024 "2008-08-22T19:13:00Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![caiwangqin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/4bbf92/32.png) [@caiwangqin](https://www.ruby-forum.com/u/caiwangqin)\
**Post date:** [August 22, 2008, 7:13pm UTC](https://www.ruby-forum.com/t/first-attempt-array-of-hashes-please-help/226024/1 "2008-08-22T19:13:00Z")

</div>

Hello, I’ve been working on this project quite a bit. I’m very  
enthusiastic about learning Ruby, and have done my best to comment with  
MARKERs where the problems are or may be. The hurdle is that I need  
output to print values for each listed unit of measure per food item, in  
this case salted butter. The units are cup, tbsp, pat, and stick. I  
can only seem to get the printouts for cup or stick. Please review my  
code and help me fix it, or suggest more efficient alternatives. Thank  
you kindly.

def nutrients(food\_id)  
@food = FoodDescription.find(food\_id)  
@nutrients = []  
@nutrients = NutrientDatum.find\_all\_by\_ndb\_id(@food.ndb\_id)

```
@n = 0
@definition = []
@values = []

  @nutrient_id = @nutrients[@n].nutrient_id
  @nutrient_description =

```

NutrientDefinition.find\_by\_nutrient\_id(@nutrient\_id).description  
@definition[@n] = @nutrient\_description  
# TODO calculate for units serving

```
  @values[@n] = @nutrients[@n].nutrient_value

  @units = units(@food.ndb_id)
  @m = 1
  @composite = []
  until @m > @units.length
    @composite[@m - 1] = calculate(@food, @nutrients, @m)
    @m += 1
  end

# MARKER 1 # print @out returns @composite[0 through 8]
           # but only for the last unit, i.e. cup(1),
           # tbsp(2), pat(3), => stick(4) of butter...
           # I don't know why it's not printing 4 arrays,
           # eight hashes in each array

@out = @composite
#system("echo #{@out}")
print @out

```

end

def units(ndb\_id)  
@units = []  
@units = FoodWeight.find\_all\_by\_ndb\_id(ndb\_id)  
@units  
end

def calculate(food, nutes, unit)

```
  @n = 0
  calories = {} # MARKER 2 # may be a problem here
  energy = {}
  protein = {}
  fats = {}
  carbs = {}
  vitamins = {}
  minerals = {}
  sterols = {}
  other = {}
  @composite = []
until @n == nutes.length

  info = FoodWeight.find_by_ndb_id_and_sequence_id(food.ndb_id,

```

unit)  
results = {}  
results[:amount] = info.amount == 1.0 ?  
results[:amount] = 1 : results[:amount] =  
info.amount  
results[:description] = info.unit\_description  
results[:grams] = info.gram\_weight  
# TODO get nutrient weights per unit weight

```
  @description = NutrientDefinition.find(@n + 1).description
  @grams_unit = (nutes[@n].nutrient_value / 100 *

```

results[:grams]).to\_i  
@key = " #{results[:amount]} #{results[:description]}  
#{@grams\_unit} \n"

```
  case @description
    # Main Totals
    when "Protein" then protein[:total_protein] = @key
    when "Total lipid (fat)" then fats[:total_fat] = @key
    when "Carbohydrate, by difference" then carbs[:total_carbs] =

```

@key  
when “Energy” then  
if @description == “kcal”  
calories[:energy\_kcal] = @key  
else  
calories[:energy\_kj] = @key  
end

```
    # Carbs
    when "Starch" then carbs[:starch] = @key
    .
    .
    .
    # Minerals
    when "Calcium, Ca" then minerals[:calcium] = @key
    .
    .
    .
    # Vitamins
    when "Manganese, Mn" then vitamins[:manganese] = @key
    .
    .
    .
    # Protein and amino acids
    .
    .
    .
    # Fats
    .
    .
    .
    # Sterols
    .
    .
    .
    # Other
    .
    .
    .
    when "Theobromine" then other[:theobromine] = @key
  end
  @n += 1
end
  # TODO calculate energy

      @composite[0] = calories
      @composite[1] = energy
      @composite[2] = protein
      @composite[3] = fats
      @composite[4] = carbs
      @composite[5] = vitamins
      @composite[6] = minerals
      @composite[7] = sterols
      @composite[8] = other

@composite

```

end

== Sample output  
…  
sodium 1 stick 772  
calcium 1 stick 3  
zinc 1 stick 178  
iron 1 stick 0  
copper 1 stick 0  
magnesium 1 stick 1  
fluoride 1 stick 2  
phosphorus 1 stick 2823  
cholesterol 1 stick 11  
phytosterols 1 stick 0  
caffeine 1 stick 2  
theobromine 1 stick 27  
…

---

<div class="post-metadata">

**Author:** ![caiwangqin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/4bbf92/32.png) [@caiwangqin](https://www.ruby-forum.com/u/caiwangqin)\
**Post date:** [August 22, 2008, 8:15pm UTC](https://www.ruby-forum.com/t/first-attempt-array-of-hashes-please-help/226024/2 "2008-08-22T20:15:21Z")

</div>

2 problems, 1 big, one not so significant

An @ sign declares an instance variable, that is, it persists through  
the  
entire class. You have @composite as a class variable, so every time you  
set  
it to something, it erases what you had it at last. You have every  
variable  
declared as an instance variable when you would most likely be fine if  
none  
of them were.

Here’s an instance variable example: you have a person object with a  
first\_name and last\_name attribute. You would use first\_name and  
last\_name  
as instance variables because they tell you something about the state of  
the  
particular instance of the object. You could have a method called  
full\_name  
that returns @first\_name + " " + @last\_name.

The second thing I noticed was

until @m \> @units.length

The more elegant ruby way to do this is:

@units.length.each do |val|  
composite[val-1] = …  
end

I suggest you pick up a copy of Programming Ruby by the pragmatic  
programmers. I don’t know how I would have gotten by without it when I  
was  
learning.

Hope this helps

On Fri, Aug 22, 2008 at 1:13 PM, Jesse C. \<

---

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**Author:** ![caiwangqin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/4bbf92/32.png) [@caiwangqin](https://www.ruby-forum.com/u/caiwangqin)\
**Post date:** [August 22, 2008, 8:15pm UTC](https://www.ruby-forum.com/t/first-attempt-array-of-hashes-please-help/226024/3 "2008-08-22T20:15:21Z")

</div>

Sorry, I made a mistake:

An @ sign declares an instance variable, that is, it persists through  
the  
entire class.

An instance variable persists through the entire INSTANCE of a class, a  
class variable (denoted by @@) persists through all instances of a  
class.

---

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**Author:** ![caiwangqin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/4bbf92/32.png) [@caiwangqin](https://www.ruby-forum.com/u/caiwangqin)\
**Post date:** [August 22, 2008, 8:27pm UTC](https://www.ruby-forum.com/t/first-attempt-array-of-hashes-please-help/226024/4 "2008-08-22T20:27:04Z")

</div>

WOW! I was tinkering for hours, and made only this change, and presto!

```
  composite = []
  until @m > @units.length
    composite[@m - 1] = calculate(@food, @nutrients, @m)
    @m += 1
  end

```

I’ve got access to values for each unit per food item in the whole  
database.

Thanks !!

Joe K wrote:

> 2 problems, 1 big, one not so significant
> 
> An @ sign declares an instance variable, that is, it persists through  
> the  
> entire class. You have @composite as a class variable, so every time you  
> set  
> it to something, it erases what you had it at last. You have every  
> variable  
> declared as an instance variable when you would most likely be fine if  
> none  
> of them were.
> 
> Here’s an instance variable example: you have a person object with a  
> first\_name and last\_name attribute. You would use first\_name and  
> last\_name  
> as instance variables because they tell you something about the state of  
> the  
> particular instance of the object. You could have a method called  
> full\_name  
> that returns @first\_name + " " + @last\_name.
> 
> The second thing I noticed was
> 
> until @m \> @units.length
> 
> The more elegant ruby way to do this is:
> 
> @units.length.each do |val|  
> composite[val-1] = …  
> end
> 
> I suggest you pick up a copy of Programming Ruby by the pragmatic  
> programmers. I don’t know how I would have gotten by without it when I  
> was  
> learning.
> 
> Hope this helps
> 
> On Fri, Aug 22, 2008 at 1:13 PM, Jesse C. \<

---

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**Author:** ![caiwangqin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/4bbf92/32.png) [@caiwangqin](https://www.ruby-forum.com/u/caiwangqin)\
**Post date:** [August 23, 2008, 1:55pm UTC](https://www.ruby-forum.com/t/first-attempt-array-of-hashes-please-help/226024/5 "2008-08-23T13:55:05Z")

</div>

You’re very welcome. I strongly urge you though to get your variable  
types  
straightened out before things get out of hand. A little labor in  
getting  
all these fixed now will prevent tons of problems in the future. All  
these  
instance variables may also have an impact on performance as well since  
they  
persist in memory for the life of the instance (it wouldn’t be something  
that would apparent until you are getting lots of concurrent hits). A  
perfect example is below. You have a counter there (@m) that persists  
for  
the entire life of the instance when ideally, you want to destroy it  
when  
the loop is finished. Read up on ruby variable types and scope.

---

<div class="post-metadata">

**Author:** ![caiwangqin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/4bbf92/32.png) [@caiwangqin](https://www.ruby-forum.com/u/caiwangqin)\
**Post date:** [August 22, 2008, 10:01pm UTC](https://www.ruby-forum.com/t/first-attempt-array-of-hashes-please-help/226024/6 "2008-08-22T22:01:28Z")

</div>

On Aug 22, 2008, at 2:27 PM, Jesse C. wrote:

> database.
> 
> > @units.length.each do |val|  
> > composite[val-1] = …  
> > end
> > 
> > On Fri, Aug 22, 2008 at 1:13 PM, Jesse C. \<

At first, I was just going to comment on this:

Since @m seems to be a value from 1 to @units.length, the better way  
would be something like:

(removed\_email\_address@domain.invalid).each do |m|  
composite[m-1] = calculate(@food, @nutrients, m)  
end

But then I had to go back to your original post to see why you weren’t  
iterating over @units rather than the index.

So here’s a longer commentary:

On Aug 22, 2008, at 1:13 PM, Jesse C. wrote:

> you kindly.
> 
> def nutrients(food\_id)  
> @food = FoodDescription.find(food\_id)  
> @nutrients = []

No need to initialize this as the next line already does that.

> @nutrients = NutrientDatum.find\_all\_by\_ndb\_id(@food.ndb\_id)

Perhaps have some ActiveRecord associations (note that’s I’m guessing  
at some of these):

class FoodDescription  
belongs\_to :ndb  
delegates :nutrient\_data, :to =\> :ndb  
end  
class Ndb  
has\_many :nutrient\_data  
end  
class NutrientDatum  
belongs\_to :ndb  
end

Then you can say:

@nutrients = @food.nutrient\_data

> @n = 0  
> @definition = []  
> @values = []
> 
> ```
> @nutrient_id = @nutrients[@n].nutrient_id
> @nutrient_description =  
> 
> ```
> 
> NutrientDefinition.find\_by\_nutrient\_id(@nutrient\_id).description

It looks like you could also define the relation:  
class NutrientDatum  
belongs\_to :nutrient

# attributes: nutrient\_value

end  
class Nutrient  
has\_one :nutrient\_definition  
delegates :description, :to =\> :nutrient\_definition  
end  
class NutrientDefinition  
belongs\_to :nutrient

# attributes: description

end

> ```
> @definition[@n] = @nutrient_description
> # TODO calculate for units serving
> 
> @values[@n] = @nutrients[@n].nutrient_value
> 
> @units = units(@food.ndb_id)
> 
> ```

Yikes! Like Joe K said, you need to get your variable types  
straightened out. The units() method already has access to @food, it  
creates/assigns @units, and then you assign that object to @units  
again here!

> ```
> # tbsp(2), pat(3), => stick(4) of butter...
> 
> ```
> 
> @units = FoodWeight.find\_all\_by\_ndb\_id(ndb\_id)  
> @units  
> end

Another association:  
class FoodWeight  
belongs\_to :nutrient\_datum, :foreign\_key =\> ‘ndb\_id’  
end  
class NutrientDatum  
has\_many :food\_weights  
end

class FoodDescription  
delegates :food\_weights, :to =\> :ndb  
end

Then you can just refer to:  
def units  
@food.food\_weights  
end

> ```
> carbs = {}
> results[:amount] = info.amount == 1.0 ?
> results[:amount] = 1 : results[:amount] =  
> 
> ```
> 
> info.amount  
> At the very least, this ought to be:  
> results[:amount] = info.amount == 1.0 ? 1 : info.amount  
> But the chance that info.amount is exactly 1.0 is low (floating point  
> comparisons being notoriously prone to small errors and 1.0 will  
> behave like 1 anywhere it’s likely to matter in an application such as  
> this).

So it’s just:  
results[:amount] = info.amount

> ```
> results[:description] = info.unit_description
> results[:grams] = info.gram_weight
> 
> ```

If you just want to initialize results, you could more compactly say:

```
   results = { :amount => info.amount,
               :description => info.unit_description,
               :grams => info.gram_weight }

```

> ```
> case @description
> end
> .
> .
> when "Theobromine" then other[:theobromine] = @key
> end
> 
> ```

I’d probably back this all with a model (that might not be in the  
database) that would know the nutrient classification. You also seem  
to avoid using Strings as hash keys. You could have something like:

```
 case classification_for(@description)
 when :protein
   protein[@description] = @key
 when :fats
   fats[@description] = @key
 when :carbs
   carbs[@description] = @key

```

… WAIT! That seems awfully repetitious. If composite weren’t an  
array, but a hash, then you could have:

```
 composite[classification_for(@description)][@description] = @key

```

Assuming an implementation such as:  
def classification\_for(description)  
case description  
when “Protein”  
:protein  
when “Total lipid (fat)”  
:fats  
when “Carbohydrate, by difference”, ‘Starch’  
:carbs  
…  
end

Can you start to see why this would be good to add as an attribute of  
the NutrientDefinition?

> ```
> @composite[5] = vitamins
> @composite[6] = minerals
> @composite[7] = sterols
> @composite[8] = other
> 
> ```
> 
> @composite  
> end

If you want to produce the output in a particular order, you can  
always iterate over an ordered array of keys.

[:calories, :energy, :protein, :fats, …].each do |kind|  
puts kind  
composite[kind].each do |description,text|  
puts “#{description} #{text}”  
# or something like:  
# puts “%-30s %s”%[description, text]  
end  
end

> cholesterol 1 stick 11  
> phytosterols 1 stick 0  
> caffeine 1 stick 2  
> theobromine 1 stick 27  
> …

In addition to thinking (learning) about variable types, you should  
also look into the complexity that ActiveRecord associations can cover  
for you. (Note that were I’ve reopened the model classes several  
times, you’d generally have all the associations in one place in each  
model.)

-Rob

Rob B. [http://agileconsultingllc.com](http://agileconsultingllc.com)  
removed\_email\_address@domain.invalid

---

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**Author:** ![caiwangqin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/4bbf92/32.png) [@caiwangqin](https://www.ruby-forum.com/u/caiwangqin)\
**Post date:** [August 23, 2008, 2:28pm UTC](https://www.ruby-forum.com/t/first-attempt-array-of-hashes-please-help/226024/7 "2008-08-23T14:28:39Z")

</div>

Hrm, you mean like Thursday when pulling 500k records took 3.5 hours?

Also, I’ve changed the large case statement to check against the  
nutrient ids instead of their string descriptions. Much more accurate.

I started learning rails with RailsSpace, and I was perplexed over the  
usage of ‘object’ instead of ‘@object’.

Is there a quick rule of thumb when instance variables are preferred  
over their non-delimited counterpart?

Joe K wrote:

> You’re very welcome. I strongly urge you though to get your variable  
> types  
> straightened out before things get out of hand. A little labor in  
> getting  
> all these fixed now will prevent tons of problems in the future. All  
> these  
> instance variables may also have an impact on performance as well since  
> they  
> persist in memory for the life of the instance (it wouldn’t be something  
> that would apparent until you are getting lots of concurrent hits). A  
> perfect example is below. You have a counter there (@m) that persists  
> for  
> the entire life of the instance when ideally, you want to destroy it  
> when  
> the loop is finished. Read up on ruby variable types and scope.

---

<div class="post-metadata">

**Author:** ![caiwangqin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/4bbf92/32.png) [@caiwangqin](https://www.ruby-forum.com/u/caiwangqin)\
**Post date:** [August 24, 2008, 7:25pm UTC](https://www.ruby-forum.com/t/first-attempt-array-of-hashes-please-help/226024/8 "2008-08-24T19:25:21Z")

</div>

On Aug 23, 2008, at 8:28 AM, Jesse C. wrote:

> Is there a quick rule of thumb when instance variables are preferred  
> over their non-delimited counterpart?

You should only use an instance variable when it holds some part of  
the state of the object. Most normal computation and data  
manipulation should use simple local variables. It’s also very useful  
to obtain the data from the object that “owns” it when it is needed  
rather than carrying it around in a local variable outside the object.

In many applications, the total amount of data far exceeds the amount  
of code that manipulates it. This is one reason that focussing on a  
good data structure first will often yield great dividends later.

-Rob

> > they  
> > persist in memory for the life of the instance (it wouldn’t be  
> > something  
> > that would apparent until you are getting lots of concurrent hits). A  
> > perfect example is below. You have a counter there (@m) that persists  
> > for  
> > the entire life of the instance when ideally, you want to destroy it  
> > when  
> > the loop is finished. Read up on ruby variable types and scope.

Rob B. [http://agileconsultingllc.com](http://agileconsultingllc.com)  
removed\_email\_address@domain.invalid

---

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**Author:** ![caiwangqin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/4bbf92/32.png) [@caiwangqin](https://www.ruby-forum.com/u/caiwangqin)\
**Post date:** [August 24, 2008, 7:36pm UTC](https://www.ruby-forum.com/t/first-attempt-array-of-hashes-please-help/226024/9 "2008-08-24T19:36:09Z")

</div>

Thank you 🙂 I’ve just started reading the PickAxe, alongside The  
Rails Way. Also the weekly paycheck will be good incentive to get up to  
speed and stay on the ball. thanks again.