05. Conditionals

Conditionals¶

Reference¶

https://github.com/JuliaComputing/JuliaBoxTutorials/tree/master/introductory-tutorials/intro-to-julia (github : JuliaComputing/JuliaBoxTutorials/introductory-tutorials/intro-to-julia/)

Topics:

1. With the if keyword
2. With ternary operators
3. With short-circuit evaluation
4. Exercises

Series¶

• http://deepstat.tistory.com/45 (01. Getting started)(in English)
• http://deepstat.tistory.com/46 (01. Getting started(한글))
• http://deepstat.tistory.com/47 (02. Strings)(in English)
• http://deepstat.tistory.com/48 (02. Strings(한글))
• http://deepstat.tistory.com/49 (03. Data structures)(in English)
• http://deepstat.tistory.com/50 (03. Data structures(한글))
• http://deepstat.tistory.com/51 (04. Loops)(in English)
• http://deepstat.tistory.com/52 (04. Loops(한글))
• http://deepstat.tistory.com/54 (05. Conditionals(한글))

With the if keyword ¶

In Julia, the syntax

if *condition 1*
    *option 1*
elseif *condition 2*
    *option 2*
else
    *option 3*
end

allows us to conditionally evaluate one of our options.

For example, we might want to implement the FizzBuzz test: given a number, N, print "Fizz" if N is divisible by 3, "Buzz" if N is divisible by 5, and "FizzBuzz" if N is divisible by 3 and 5. Otherwise just print the number itself! Enter your choice for N here:

In [1]:

N = 3

Out[1]:

3

In [2]:

if (N % 3 == 0) && (N % 5 == 0) # `&&` means "AND"; % computes the remainder after division
    println("FizzBuzz")
elseif N % 3 == 0
    println("Fizz")
elseif N % 5 == 0
    println("Buzz")
else
    println(N)
end

Fizz

In [3]:

N = 5

Out[3]:

5

In [4]:

if (N % 3 == 0) && (N % 5 == 0) # `&&` means "AND"; % computes the remainder after division
    println("FizzBuzz")
elseif N % 3 == 0
    println("Fizz")
elseif N % 5 == 0
    println("Buzz")
else
    println(N)
end

Buzz

In [5]:

N = 15

Out[5]:

15

In [6]:

if (N % 3 == 0) && (N % 5 == 0) # `&&` means "AND"; % computes the remainder after division
    println("FizzBuzz")
elseif N % 3 == 0
    println("Fizz")
elseif N % 5 == 0
    println("Buzz")
else
    println(N)
end

FizzBuzz

In [7]:

N = 1

Out[7]:

1

In [8]:

if (N % 3 == 0) && (N % 5 == 0) # `&&` means "AND"; % computes the remainder after division
    println("FizzBuzz")
elseif N % 3 == 0
    println("Fizz")
elseif N % 5 == 0
    println("Buzz")
else
    println(N)
end

1

With ternary operators ¶

For this last block, we could instead use the ternary operator with the syntax

a ? b : c

which equates to

if a
    b
else
    c
end

Now let's say we want to return the larger of two numbers. Give x and y values here:

In [9]:

x = 1
y = 2

Out[9]:

2

Using the if and else keywords, we might write:

In [10]:

if x > y
    x
else
    y
end

Out[10]:

2

and as a ternary operator, the conditional looks like this:

In [11]:

(x > y) ? x : y

Out[11]:

2

In [12]:

x = 2
y = 1

Out[12]:

1

In [13]:

if x > y
    x
else
    y
end

Out[13]:

2

In [14]:

(x > y) ? x : y

Out[14]:

2

With short-circuit evaluation ¶

We've already seen expressions with the syntax

a && b

to return true if both a and b are true. Of course, if a is false, Julia doesn't even need to know the value of b in order to determine that the overall result will be false. So Julia doesn't even need to check what b is; it can just "short-circuit" and immediately return false. The second argument b might be a more complicated expression like a function call with a side-effect, in which case it won't even be called:

In [15]:

false && (println("hi"); true)

Out[15]:

false

In [16]:

true && (println("hi"); true)

hi

Out[16]:

true

On the other hand, if a is true, Julia knows it can just return the value of b as the overall expression. This means that b doesn't necessarily need evaluate to true or false! b could even be an error:

In [17]:

x = 1
(x > 0) && error("x cannot be greater than 0")

x cannot be greater than 0

Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] top-level scope at In[17]:2

In [18]:

x = -1
(x > 0) && error("x cannot be greater than 0")

Out[18]:

false

Similarly, check out the || operator, which also uses short-circuit evaluation to perform the "or" operation.

In [19]:

true || println("hi")

Out[19]:

true

In [20]:

false || println("hi")

hi

Exercises ¶

5.1¶

Write a conditional statement that prints a number if the number is even and the string "odd" if the number is odd.

In [21]:

n = 1

Out[21]:

1

In [22]:

if n % 2 == 1
    println("odd")
else
    println(n)
end

odd

In [23]:

n = 2

Out[23]:

2

In [24]:

if n % 2 == 1
    println("odd")
else
    println(n)
end

2

5.2¶

Rewrite the code from 5.1 using a ternary operator.

In [25]:

n = 1

Out[25]:

1

In [26]:

(n % 2 == 1) ? println("odd") : n

odd

In [27]:

n = 2

Out[27]:

2

In [28]:

(n % 2 == 1) ? println("odd") : n

Out[28]:

2