Dictionaries

What comes to your mind when you think of a dictionary? Today in software or online dictionary you type a word and get its meaning in seconds. When I was small we had paper books, in each page there were printed lot of words and its meaning, even then we could find meaning of a word within one or two minutes. That’s because dictionary was a ordered set of words with which we could pin point the word were looking with ease. In programming languages dictionaries are designed in similar fashion, it could book looked up with ease.

In a language dictionary, you can think of a looked up word as a key or index, the meaning that is entered against the word as a value. Take a look at the code below, type it and execute it:

spell = Dict(
    1 => "one",
    2 => "two",
    3 => "three"
)

Output:

Dict{Int64,String} with 3 entries:
=> "two"
=> "three"
=> "one"

In the above example we create a dictionary using the Dict() function, the keys are 1, 2 and 3, we have given their English names as their values. Now if you want to get the spelling of 3, you can get it as shown:

number = 3
spell[number]

Output:

"three"

One who knows arrays can argue this can be achieved with the following code:

spell = ["one", "two", "tree"]
spell[3]

that’s correct, but take the example as shown below:

prices = Dict(
    "mango" => 50,
    "bananna" => 10,
    "samosa" => 15,
    "briyani" => 80
)

Output:

Dict{String,Int64} with 4 entries:
  "mango"   => 50
  "samosa"  => 15
  "briyani" => 80
  "bananna" => 10

above we have created a dictionary with prices of items, now say is it better for one to access price of a mango by giving prices[1], or prices["mango"]?

Let’s say we went to shop and bought two Briyani parcels and four Samosas, I’m starting to feel hungry now 😋. We can represent our purchase as shown:

purchase = Dict(
    "briyani" => 2,
    "samosa" => 4
)

Output:

Dict{String,Int64} with 2 entries:
  "samosa"  => 4
  "briyani" => 2

Just to know more about dictionaries, this is how you would list keys of a dictionary:

keys(purchase)

Output:

Base.KeySet for a Dict{String,Int64} with 2 entries. Keys:
  "samosa"
  "briyani"

This is how you will list values contained in the dictionary:

values(purchase)

Output:

Base.ValueIterator for a Dict{String,Int64} with 2 entries. Values:
  4
  2

The length() function works in a dictionary and returns the count of key value pairs in it.

length(purchase)

Output:

typeof() seems to be a universal function, and it works on dictionaries as well:

typeof(purchase)

Output:

Dict{String,Int64}

Now let’s calculate our bill, type the program below and see what happens:

total = 0

for (item, quantity) in purchase
    total += prices[item] * quantity
end

total

Output:

So let me explain how the program works. First we have assigned a variable total to 0 here total = 0.

Next look at this statement:

for (item, quantity) in purchase

look how we unravel the key value pair as (item, quantity). Since we are using a for loop, for every purchase, the key gets stored in variable called item and value get’s stored in variabe called quantity. Now for the first iteration it would would be like this:

total += prices[item] * quantity

Which would translate into total = 0 + (prices["briyani"] * 2), which will be reduced to total = 80 * 2, hence total will become 160, and the loop goes on and on till all the purchase elements are fetched and the total calculated, and we get a grand total of 220

Since we know list comprehension, we can write the above code as shown below:

sum([prices[item] * quantity for (item, quantity) in purchase])

Output:

Neat isn’t it?