Keys and values – looping

To isolate the keys of a dictionary, use the keys function ki = keys(d3), with ki being a KeyIterator object, which we can use in a for loop as follows:

for k in keys(d3) 
    println(k) 
end 

Assuming d3 is again d3 = Dict(:A => 100, :B => 200), this prints out A and B. This also gives us an alternative way to test if a key exists with in. For example, :A in keys(d3) returns true and :Z in keys(d3) returns false.

If you want to work with an array of keys, use collect(keys(d3)), which returns a two-element Array{Symbol,1} that contains :A and :B. To obtain the values, use the values function: vi = values(d3), with vi being a ValueIterator object, which we can also loop through with for:

for v in values(d3) 
    println(v) 
end 

This returns 100 and 200, but the order in which the values or keys are returned is undefined.

Creating a dictionary from arrays with keys and values is trivial because we have a Dict constructor that can use these; as in the following example:

keys1 = ["J.S. Bach", "Woody Allen", "Barack Obama"] and 
values1 =  [ 1685, 1935, 1961] 

Then, d5 = Dict(zip(keys1, values1)) results in a Dict{String,Int64} with three entries as follows:

"J.S. Bach"    => 1685 
"Woody Allen"  => 1935 
"Barack Obama" => 1961 

Working with both the key and value pairs in a loop is also easy. For instance, the for loop over d5 is as follows:

for (k, v) in d5 
       println("$k was born in $v")  
   end

This will print the following output:

J.S. Bach was born in 1685
Barack Obama was born in 1961
Woody Allen was born in 1935  

Alternatively, we can use an index in the tuple:

for p in d5 
  println("$(p[1]) was born in $(p[2])")  
end 

Here are some more neat tricks, where dict is a dictionary:

• Copying the keys of a dictionary to an array with a list comprehension:

arrkey = [key for (key, value) in dict] 

This is the same as collect(keys(dict)).

• Copying the values of a dictionary to an array with a list comprehension:

arrval = [value for (key, value) in dict] 

This is the same as collect(values(dict))