Javascript reduce(), passing in initial value

Question

I'm working through an exercise using the underscore library to perform a count of item occurrences.

Specifically, I have this array:

products = [
       { name: "Sonoma", ingredients: ["artichoke", "sundried tomatoes", "mushrooms"], containsNuts: false },
       { name: "Pizza Primavera", ingredients: ["roma", "sundried tomatoes", "goats cheese", "rosemary"], containsNuts: false },
       { name: "South Of The Border", ingredients: ["black beans", "jalapenos", "mushrooms"], containsNuts: false },
       { name: "Blue Moon", ingredients: ["blue cheese", "garlic", "walnuts"], containsNuts: true },
       { name: "Taste Of Athens", ingredients: ["spinach", "kalamata olives", "sesame seeds"], containsNuts: true }
    ];

And am trying to use map(), flatten() and reduce() to determine the number of times each ingredient shows up.

When I use the code below, ingredientCount returns undefined values:

var ingredientCount = { "{ingredient name}": 0 };

    _(products).chain()
    .map(function(obj) {return obj.ingredients;})
    .flatten().reduce(function(memo, x) {
      return memo[x] = (memo[x] || 0) + 1;
    }, ingredientCount)
    .value();

However, if I remove the second argument to reduce() and include the initial object in the function body, everything works as expected, i.e.:

var ingredientCount = { "{ingredient name}": 0 };

_(products).chain()
.map(function(obj) {return obj.ingredients;})
.flatten().reduce(function(memo, x) {
  return ingredientCount[x] = (ingredientCount[x] || 0) + 1;
})
.value();

Can somebody help explain why this is the case?

Thanks!

I don't understand why you are using reduce...the reduce function will return asingle value what you want is teh sum of all ingredients individually something like this "{"artichoke":1,"sundried tomatoes":2,"mushrooms":2,"roma":1,"goats cheese":1,"rosemary":1,"black beans":1,"jalapenos":1,"blue cheese":1,"garlic":1,"walnuts":1,"spinach":1,"kalamata olives":1,"sesame seeds":1}" — Cyril Cherian
– Cyril Cherian, Commented Oct 17, 2015 at 0:39
@Cyril—the accumulator can be an Object (i.e. the value can be an object reference). — RobG
– RobG, Commented Oct 17, 2015 at 0:49
Yes you are right but when i applied Jaromanda answer to the above problem i got a single result...not what is expected..your answer works fine. — Cyril Cherian
– Cyril Cherian, Commented Oct 17, 2015 at 0:56

Jaromanda X · Accepted Answer · 2015-10-17 00:30:42Z

1

return memo[x] = (memo[x] || 0) + 1; returns a number for the next iteration of the loop to use as memo.

instead do

memo[x] = (memo[x] || 0) + 1;
return memo;

if you really want to do it in a single statement

return memo[x] = (memo[x] || 0) + 1, memo;

edited Oct 17, 2015 at 0:30

answered Oct 17, 2015 at 0:29

Jaromanda X

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2 Comments

Barmar Over a year ago

This is the second question I've seen today where someone made essentially the same mistake with reduce.

Jaromanda X Over a year ago

I think there's some sort of mass shared consciousness thing happening on SO - you see clusters of almost the same problem come up occasionally. the batch of fizzbuzz and tic-tac-toe questions is almost due :p

RobG · Accepted Answer · 2015-10-17 00:45:19Z

Always interesting to see a plain JS alternative:

var counts = p.reduce(function(acc, prod) {
  prod.ingredients.forEach(function(i) {
    acc[i] = (acc[i] || 0) + 1;
  });
  return acc;
},{});

which seems lot clearer, though in a general case it's better to use:

  if (!acc.hasOwnProperty(i)) acc[i] = 0;
  acc[i]++

for the counter part.

products = [
       { name: "Sonoma", ingredients: ["artichoke", "sundried tomatoes", "mushrooms"], containsNuts: false },
       { name: "Pizza Primavera", ingredients: ["roma", "sundried tomatoes", "goats cheese", "rosemary"], containsNuts: false },
       { name: "South Of The Border", ingredients: ["black beans", "jalapenos", "mushrooms"], containsNuts: false },
       { name: "Blue Moon", ingredients: ["blue cheese", "garlic", "walnuts"], containsNuts: true },
       { name: "Taste Of Athens", ingredients: ["spinach", "kalamata olives", "sesame seeds"], containsNuts: true }
    ];
    
function countIngredients(p) {
  var counts = p.reduce(function(acc, prod) {
    prod.ingredients.forEach(function(i) {
      acc[i] = (acc[i] || 0) +1;
    })
    return acc;
  },{})
  return counts;
}

document.write(JSON.stringify(countIngredients(products)));

Cyril Cherian · Accepted Answer · 2015-10-17 01:04:56Z

I would do it like this in underscore

    products = [
           { name: "Sonoma", ingredients: ["artichoke", "sundried tomatoes", "mushrooms"], containsNuts: false },
           { name: "Pizza Primavera", ingredients: ["roma", "sundried tomatoes", "goats cheese", "rosemary"], containsNuts: false },
           { name: "South Of The Border", ingredients: ["black beans", "jalapenos", "mushrooms"], containsNuts: false },
           { name: "Blue Moon", ingredients: ["blue cheese", "garlic", "walnuts"], containsNuts: true },
           { name: "Taste Of Athens", ingredients: ["spinach", "kalamata olives", "sesame seeds"], containsNuts: true }
        ];
    ingredientCount = {};

    _(products).chain()
        .map(function(obj) {return obj.ingredients;}).flatten().each(function(memo, x) {
       ingredientCount[memo] = (ingredientCount[memo] || 0) + 1;
    });

    document.write(JSON.stringify(ingredientCount));

    //this will produce this "{"artichoke":1,"sundried tomatoes":2,"mushrooms":2,"roma":1,"goats cheese":1,"rosemary":1,"black beans":1,"jalapenos":1,"blue cheese":1,"garlic":1,"walnuts":1,"spinach":1,"kalamata olives":1,"sesame seeds":1}"

<script src="https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.8.3/underscore.js"></script>

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