# W1154 For Loop

Within these castle walls be forged Mavens of Computer Science ...
— Merlin, The Coder

## Introduction

The for loop is a construct which enables us to execute a segment of code a defined number of times. It is thus the most appropriate choice when we know in advance how to calculate the number of iterations prior to beginning the loop. While this construct is not strictly necessary, it provides syntactic sugar by clarifying intent.

## For Loops

for loops execute for a defined count of iterations, as such, the test condition is implied within the definition. Through each iteration, a loop control variable is adjusted toward its final goal. When that goal is reached, the loop exits.

Both while and repeat-while loops require the explicit definition of a loop control variable which must be explicitly incremented (or otherwise adjusted) and explicitly checked each time through the loop to determine if the loop has completed. In contrast, a for loop handles this work for us implicitly by defining the loop control variable, incrementing (or otherwise adjusting) it automatically, and checking for the completion condition.

The actual implementation is identical to that of a while loop.

## Intervals

Assume two numbers, a and b, such that b > a. An interval is the set of numbers lying between these two numbers. Both a and b are the endpoints of the interval, the lower bound and the upper bound respectively. An open interval does not include either of the endpoints, a half-open interval include just one of its endpoints, while a closed interval includes both of its endpoints.

### Interval Notation

Notation Interpretation Description
(a, b) a < x < b An open interval; both endpoints are excluded.
(a, b] a < x ≤ b A half-open interval; the lower bound is excluded.
[a, b) a ≤ x < b A half-open interval; the upper bound is excluded.
[a, b] a ≤ x ≤ b A closed interval; both endpoints are included.

### Interval Classifications

An empty interval has no elements. Examples include:

• (b, a), (b, a], [b, a), [b, a]
• (a, a), (a, a], [a, a)

A degenerate interval has a single element. An example is:

• [a, a]

## Swift For Loops

Because iterating over a discrete interval is very common, Swift supports several different ways of implementing for loops.

### Swift Ranges

A range in Swift represents either a closed interval or a half-open interval. In both cases, one may iterate of the range.

#### ClosedRange

A ClosedRange includes both the lower bound and the upper bound. It's written using ... between the lower and upper bounds. For example, the equivalent of [4, 7] is:

```4 ... 7
```

An example of a loop using a ClosedRange is:

```for x in lowerBound...upperBound {
print(x);
}
```

#### Range

A Range includes the lower bound but excludes the upper bound. It's written using ..< between the lower and upper bounds. For example, the equivalent of [4, 7) is:

```4 ..< 7
```

An example of a loop using a Range is:

```for x in lowerBound..<upperBound {
print(x);
}
```

### Swift Strides

A stride in Swift represents either a sequence based upon a closed interval or a half-open interval.

#### Closed Stride

A closed stride includes the lower bound and may include the upper bound, stepping as specified. It uses the keyword through for the upper bound and is written as stride(from:4, through:7, by:1).

An example of a loop using a closed stride is:

```for x in stride(from:lowerBound, through:upperBound, by:1) {
print(x);
}
```

#### Half-Open Stride

A half-open stride includes the lower bound but excludes the upper bound, stepping as specified. It uses the keyword to for the upper bound and is written as stride(from:4, to:7, by:1).

An example of a loop using a half-open stride is:

```for x in stride(from:lowerBound, to:upperBound, by:1) {
print(x)
}
```

#### Stride By

Strides do not necessarily have to increment by 1; by can be any arbitrary value. A positive stride iterates upward; a negative stride iterates downward. One advantage of strides is that they may use floating point values.

## The Special Meaning of "_", or I Don't Care

Sometimes, we don't care about the value of a variable. For example, there are occasions where we want to execute something a certain number of times but we don't need to keep track of the specific iteration. The `_` (underscore) character is intended for this purpose.

For example, if we want to print "Hello, World!" exactly one hundred times, we could type the following:

```for _ in 1 ... 100 {
print("Hello, World!")
}
```

## Key Concepts

Key Concepts • The for loop is a construct which enables us to execute a segment of code a defined number of times.
• Both while and repeat-while loops require the explicit definition of a loop control variable which must be explicitly adjusted and explicitly checked each time through the loop. In contrast, a for loop handles this work implicitly.
• The actual implementation is identical to that of a while loop.
• Intervals: Assume two numbers, a and b, such that b > a. An interval is the set of numbers lying between these two numbers.
• Both a and b are the endpoints of the interval, the lower bound and the upper bound respectively.
• An open interval does not include either of the endpoints
• A half-open interval include just one of its endpoints
• A closed interval includes both of its endpoints
• An empty interval has no elements
• A degenerate interval has a single element
• Swift defines two range types:
• A ClosedRange includes both the lower bound and the upper bound and is written using ...
• A Range include the lower bound but excludes the upper bound and is written using ..<
• A half-open stride includes the lower bound and may include the upper bound and is written using stride(from:a, to:b, by:s)
• A closed stride includes the lower bound and may include the upper bound and is written using stride(from:a, through:b, by:s) Coming Soon M1154-20 requires understanding of functions described in W1205. Therefore, that unit should be moved earlier and then M1154-20 into that unit.

## Exercises

Exercises •  M1154-10  Complete  Merlin Mission Manager  Mission M1154-10.
•  M1154-20  Complete  Merlin Mission Manager  Mission M1154-20.
•  X1154  Using Draw.IO create a single XML file named "Flowcharts.drawio" with one page for each flowchart below. Create a new subdirectory, J1154, in your Journals directory. Upload the file to the directory via SFTP. Be sure to push the file to your GitHub repository. When done, tag the repository as J1154.Final.
1. Finding the greatest number of three
2. Finding the GCD of two numbers using Euclid's Algorithm
3. Determining whether or not a number is prime