5th Week of Swift — for in, while, and optional

More than one condition

var age = 18
var weight = 45
if var age < 30 {
   if var weight < 50 {
      var message = "你是我的傳說"
}}
else {}

&&

var age = 18
var weight = 45
if var age < 30 && var weight < 50 {
   var message = "你是我的傳說"
}
else {
   var message = "太老又太胖"
}

||

OR, when one of condition satisfied, the program can work)

var age = 35
var weight = 45
if age < 30 || weight < 50 {
   var message = "你是我的傳說"
}
else {}

!

means opposite

The order of reading code

• From left to right

var age = 35
var weight = 45
var height = 160
if age < 30 || weight < 50 && height < 170 {
   var message = "你是我的傳說"
}
else {}

• In this situation, not taller than 170, either age < 30 or weight < 50 is accepted.

LOOP

How to express var sum = 1 + 2 + 3 + 4+….+ 1000 ??

→ 1..< 3 , including 1, 2

→1…3, including 1, 2, 3

1. for + name of constant + in + a range of number or other set operation

var sum = 0
for i in 1...1000 {
   sum = sum + i
}

Practice

Q1: sum up 1 to 1000 and only with even number (hint: for in &)

var sum = 0
for i in 1...1000 {
   if i % 2 == 0
   sum = sum + i
}

2. while ( suitable for those the variable is not clear)

• Variable has to be declared in advance, while “for in” is not essential

var number = 0
while number < 10 {
   number = number - 1
}

Practice

Q1: 1² + 2² + 3² +…+99²

Q2:2²+4²+6²+…+100²

Comment

• for single line: //
• more than one line comment: /*, */

Optional

• Can be lack of information
• nil: means empty

var age: Int?
var age:Int? = nil

• Read the information in optional: add!

var age:Int? = 18
age = age! + 1

• Check whether there is information in optional

var age:Int? = 18
if age != nil {
   age = age! + 1
}

Optional binding

if + let + constant number + = + optional

var age: Int? = 18
if let ageNumber = age {
   age = ageNumber + 1
}
else {
}

• Can use the same name in optional binding

var age:Int? = 18
if let age = age {
   age = age + 1 
else {
}

Implicitly Unwrapped Optionals

• Without using ! to unwrapp the optionals
• Suitable for in which the situation are most with information in optionals

var age:Int! = 18
age = age + 1

• But it could happen when the optional is empty, there will be crash
• So by using if to check is safer ( optional binding is unnecessary)

var age:Int! = 3
if age != nil {
   var number = age * 10
}