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Tag: dictionary

Learning Swift – Collections [Dictionary]

January 23rd, 2017
Hello friends,

Now that we have learned arrays and sets, it’s time to finish collections by learning dictionaries.

Dictionaries. What do you think about when you hear the word dictionary. Well, I think about the dictionaries, the English dictionaries we used when we were kids to find the meaning of words. I use them still. Just that the dictionary is now on the internet.

Think about how we used to search a meaning of the word, for example, “rock” in the dictionary. We will straight away go to the r section of the book, then search for a word starting with “ro” and finally zero in on our word rock. Read its meaning and close.

We wouldn’t start with words starting with a, followed by b, and so on to reach the words starting with “r”. That is going to take a lot of time. Am I right? We will just straight away jump to the words starting with “r”

One important thing to note here is that rock is the key and its meaning is the value. We find the key, we find the meaning.

Just like those English dictionaries, dictionaries in swift are generally pairs/associations of keys and values. These keys and values can be of the same type or different. Also, every value is associated with a key, which is now an identifier for that value.

Like sets, dictionaries also have no ordering.

A dictionary is written as [“key” : “value”]

Example, a dictionary of antonyms : [“hot” : “cold”, “dark” : “light”, “big” : “small”]

Here “hot’ will be a key and “cold” its value.

The swift dictionary type is written using Dictionary[“Key” : “Value”]. This can also be written as [“Key”: “Value”]. The latter form is widely used.

Coming back to the playground, let’s learn how to initialize and use these dictionaries.

The first method, just like in arrays and sets, uses the initializer syntax.

Here we will use the antonyms example.
```
var antonyms = [String : String]()
```
This will generate variable antonyms which can store keys of type String and values also of type string. Currently, it is initialized as an empty dictionary.

Let’s print this and see
```
print(antonyms)
```
This symbol here [:] shows the dictionary is empty.

Now comment this line and the print line and again initialize the dictionary as empty.
```
var antonyms = [:]
```
This throws an error “Empty collection literal requires an explicit type”

This happens because the compiler doesn’t know the type of key and values. So we cannot use this way to initialize a dictionary.

When we uncomment our first line. Then remove “var” from this one the error vanishes.

Let’s add few antonyms to our dictionary. And remove the comment from print statement.
```
antonyms = ["hot" : "cold", "dark" : "light", "big" : "small"]
```
We can see that antonyms now has these values and is not empty.

Moving on to the second method to initialize dictionary using Dictionary Literal.

Here we will initialize a grocery list dictionary
```
var groceryList : [String:Int] = ["potatoes" : 4, "carrots" : 6, "onions" : 10]
```
[String: Int] shows that the groceryList is a dictionary with keys of type string and values of type Integer.

There is another point to note here, we don’t need to specify the dictionary type as swift can infer by itself.
```
var groceryList = ["potatoes" : 4, "carrots" : 6, "onions" : 10]
```
Well, now we know how to initialize the dictionary. so let’s have a look at the operations we can perform on it.

The first operation is “Insert”

We use subscript syntax to insert into dictionaries. In this method, we will use a new key having the same type as Keys and assign to it new value having the same type as Values.

Now we will add tomatoes and squash to our dictionary.
```
groceryList["tomatoes"] = 12
groceryList["squash"] = 2
```
See potatoes and squash are added to the dictionary.

Next operation is “Remove”

We will remove squash from our groceryList.

One way is to use subscript syntax and assign the key which we want to delete, a null value.
```
groceryList["squash"] = nil
```
Now when we print the groceryList and click on the show result, we see that no squash exists in the dictionary.

Another method is to use removeValue(forKey:) function. For this one let’s remove carrots from our groceryList.
```
groceryList.removeValue(forKey: "carrots")
```
Now when you print the groceryList and click on show result, you can see carrots are removed from the dictionary.

Next operation is “count” to know the size of our dictionary.
```
print(groceryList.count)
```
The answer is three as expected.

The next and last operation is “Update”.

Let’s print the groceryList once again. You see tomatoes have a value of 12. We will change it to 6.

Again one way is to use the subscript syntax and assign the new value to a key.
```
groceryList["tomatoes"] = 6
```
Print the groceryList again. Click on show result button. See the value for tomatoes is now 6.

Next method is to use updateValue(_:forKey:) function. Let’s change onions to 5 from 10.
```
groceryList.updateValue(5, forKey: "onions")
```
Print the groceryList again. Click on show result button. See the value for onions is now 5.

This completes our dictionaries and collections. You can find the source code for this post here.
Learning Swift – Collections [Sets]

January 20th, 2017
Hello Friends,

Megha here.

In our previous post we studied arrays.

And today we will be learning SETS.

What are sets?

A set is just like an array. Only the difference being that sets have distinct values and no ordering.

example: A set of first four letters of an alphabet. [“a”, “b”, “c”, “d”]

How do we initialize a set in Swift?

We do the initialization using initializer syntax:

Let’s initialize a set containing ages of our friends.
```
var age = Set<Int>()
```
var age followed by Set, Int in open brackets, ending with function brackets.

When we print age, we see that it’s empty.

So, we need to add in the ages using
```
age = [38, 65, 97]
```
Now when we print age, we see it has three numbers 38, 65, 97.

The second method of initialization is using array literal.

For this one, we will use an array of colors.
```
var color : Set<String> = ["Red", "Green", "Yellow"]
```
The variable color is declared as a set if string values are written using Set<String>. The variable color can only save String values as we specified the type as String. Since we used three colors to initialize the set, color set is not empty.

In arrays, we used [String], but here we use Set<String> as Set type can not be inferred only with String in double brackets, word Set is very important.

This initialization can be done using only “Set” as Swift can infer the type of elements of the set because of its type inference property.

First commenting this initialization, we write
```
var color : Set = ["Red", "Green", "Yellow"]
```
Next, we work upon the “Operations on Set”

The first operation is “Inserting an element into the set“. For this, we will insert color “Blue” in the set “color”.
```
color.insert("Blue")
```
We can see here in the result bar that “Blue” has been added to set “color”

To confirm, we can print color.

A difference from the array can be seen here, we have not specified the location where we want the blue to be placed. Because unlike arrays, sets are unordered lists. So blue is added at a random position.

The subsequent function is “Removing an element from the set“. For this, we will remove color “Green” from the set.
```
color.remove("Green")
```
When we print the set now, the result is devoid of “Green”.

Now we move on to “Contain function: to check whether an element is inside the set”. Again for this first, we will check whether alphabet “a” is inside set and then check whether “Red” is inside the set.
```
print(color.contains("a"))
```
We can see the result here in the result are is “false” as “a” is not inside set color.
```
print(color.contains("Red"))
```
Red is inside set the color and so the result is true.

Next is the method to count the number of elements in the set.
```
print(color.count)
```
Clearly, the answer is 3 as seen here.

Apart from all these operations, there are certain operations that are performed on sets as we have studied in maths.

Let’s see one by one all the operations:

Consider two sets :

A = { “a”, “e”, “i”, “o”, “u” }

B = { “l”, “a”, “u”, “r”, “e”, “n” }

The two circles here denote sets A and B.

Since { “a”, “e”, “u”} is common in two sets, we place them in the intersecting of two circles.

{ “a”, “e”, “u”} is also the result of intersection of sets A and B. The yellow color is used to highlight An intersection B

Union operation on the sets leads to a set of all elements from A and B.

So here union of set A and B will have {“a”, “e”, “i”, “o”, “u”, “l”, “r”, “n”} with { “a”, “e”, “u”} only once. As we have seen earlier sets cannot have repeated values, hence no repeated values here.

On this slide, the yellow color is used to show the union of sets A and B.

Symmetric Difference gives values in each set but not in both. Here they will be { “o”, “i”, “l”, “r”, “n”}

Subtracting B from A will give values in A which are not in B. ie { “o”, “i” }

Now we will perform all these operations in Swift.

We have taken two sets:
```
var vowels : Set = ["a", "e", "i", "o", "u"] 

var nameLetters : Set = ["l", "a", "u", "r", "e", "n"]
```
The first operation is Intersection
```
print(nameLetters.intersection(vowels))
```
The result is { “a”, “e”, “u”} as expected

Next is Union
```
print(nameLetters.union(vowels))
```
The result is all letters in both sets : [“a”, “i”, “r”, “n”, “u”, “e”, “l”, “o”]

For Symmetric Difference
```
print(nameLetters.symmetricDifference(vowels))
```
we get [“o”, “l”, “r”, “i”, “n”] as expected

The last is subtraction
```
print(nameLetters.subtracting(vowels))
```
The result is [“l”, “r”, “n”] as expected.

With this we complete Sets. In next post, we will cover Dictionaries, the last of collections.

The code for this post is available here.
Learning Swift – Collections [Arrays]

January 11th, 2017
Hello Friends,

A very Happy New Year to all of you.

Sorry for such a huge delay in publishing this post.

My new year resolution is to keep up at least one post every week starting today.

In this post, we delve into collections.

A collection can be defined as an object containing multiple elements.

Swift has three collection types namely Array, Set and Dictionary.

Array

An Array is an ordered list of elements of same data type. An example: Square of integers between 1 and 10.

In swift, the array can be initialized in three ways:
```
//Method 1
var oddNumber = Array<Int>()
oddNumber = [3, 5, 7]
```
In the first method, Array specifies the type of object being initialized and Int specifies the type of items stored in the array.

Presently the array oddNumber is empty. Therefore we assign some values of Integer type to it.
```
//Method 2
var price = [Double]()

price.append(14.6)
price.append(56.3)
price.append(14.6)

print(price)
```
In the second method, the square brackets ensure compiler knows the price is an array and double defines the type of the items in that array.

append() method is used to add items to an array. This method always adds elements to the end of the array.
```
//Method 3
//var house : [String] =["Gryffindor", "Hufflepuff"]
var house = ["Gryffindor", "Hufflepuff"]
```
Here we use the array literal to initialize the array. [String] is used to indicate an array of string.

Array Operations
```
//Operations on array

//append
house.append("Ravenclaw")

//insert
house.insert("Slytherin", at: 1)

//remove
house.remove(at: 1)

print(house)

//count
print(house.count)

//retrieve
print(house[2])
```
Append, Insert, Remove, Count, and Retrieve are the operations that can be performed on arrays.

Append is used to add items at the end of an array. Here we are adding “Ravenclaw” to the array “house”. Ravenclaw is added to the end of the array.

Insert is used to add items at a desired position in the array.

“insert(newElement: Element, at: Int)

Here newElement is the element we want to add in the array. And after at: we specify the index at which element will be added.

In our example, we are adding “Slytherin” and index 1. Array index start from 0, so Slytherin gets added after Gryffindor which is at 0. Rest elements are shifted one place down.

Remove is used to delete items in the array.

“remove(at: Int)”

Here we just need to specify the index of the element we want to delete.

In our example we are deleting “Slytherin” which is at 1.

Count returns the total number of items in the array.

We can retrieve any element of the array through its index. Here we want to access Ravenclaw. So we use the index of Ravenclaw.

You can find the code for this project here

We will study sets and dictionaries in the following posts.

Happy New Year once again.