Julia control flow
last modified October 20, 2023
In this article we talk about the flow control in Julia.
When a Julia program is run, the code is executed from top to bottom. The flow of the program can be altered with various keywords, including if/elseif/else, while and for.
The if condition
The if keyword is used to check if an expression is true. If it is true, a statement is then executed. The statement can be a single statement or a compound statement. A compound statement consists of multiple statements enclosed by the block. A block is code enclosed by curly brackets.
r = rand(-5:5)
println(r)
if r > 0
println("The r variable is positive")
end
A random number is generated. If the number is greater than zero, we print a message to the terminal.
r = rand(-5:5)
The rand function generates a random integer between -5 .. 5.
if r > 0
println("The r variable is positive")
end
Using the if keyword, we check if the generated number is greater than zero. The
if keyword is followed by an expression that evaluates to a boolean value. If
the boolean value is true then the block is executed. In our case, the string
The r variable is positive is printed to the terminal. If the random
value is negative, nothing is done. The block is terminated with
end keyword.
$ julia simple.jl -4 $ julia simple.jl 1 The r variable is positive $ julia simple.jl 4 The r variable is positive
The else condition
We can use the else keyword to create a simple branch. If the
expression following the if keyword evaluates to false, the block
following the else keyword is automatically executed.
r = rand(-5:5)
println(r)
if r > 0
println("The n variable is positive")
else
print("The r variable is negative or zero")
end
Either the block following the if keyword is executed or the block
following the else keyword.
The elseif condition
We can create multiple branches using elseif. It tests for another
condition if and only if the previous condition was not met.
Note that we can use multiple elseif keywords in our tests.
r = rand(-5:5)
println(r)
if r > 0
println("The r is positive")
elseif r == 0
println("The r is zero")
else
println("The r is negative")
end
If the first condition evaluates to true, e.g. the random value is greater than
zero, the first block is executed and the remaining two blocks are skipped. If
the first condition is not met, then the second condition following the
elseif keyword is checked. If the second condition evaluates to
true, the second block is executed. If not, the third block following the else
keyword is executed. The else block is always executed if the previous
conditions were not met.
$ julia main.jl 4 The r is positive $ julia main.jl 5 The r is positive $ julia main.jl 0 The r is zero $ julia main.jl -1 The r is negative
The while loop
The while keyword is used to create a cycle. The statements inside
the while loop are executed until the expression evaluates to false.
vals = [22, 34, 12, 32, 4]
let i = length(vals)
mysum = 0
while i != 1
i -= 1
mysum = mysum + vals[i]
end
println("The sum is: ", mysum)
end
We want to calculate the sum of all values in the vals array. We
utilize the while loop. We determine the length of the list with
length function. The while loop is executed over and over again,
until the i is equal to one. In the body of the loop, we decrement the counter
and calculate the sum of values.
$ julia whilestm.jl The sum is: 100
The break keyword
The break keyword is used to interrupt the cycle if needed.
while true
r = rand(1:30)
print("$r ")
if r == 22
break
end
end
println()
In our example, we print random integer numbers. If the number equals to 22, the
cycle is interrupted with the break keyword. The while true creates
an endless cycle. The break is used to jump out of this endless cycle.
$ julia breakstm.jl 15 7 24 27 7 22
The continue keyword
The continue statement is used to interrupt the current cycle
without jumping out of the whole cycle. It initiates a new cycle.
let num = 0
while num < 1000
num = num + 1
if num % 2 == 0
continue
end
print("$num ")
end
end
println()
In the example we print all numbers smaller than 1000 that cannot be divided by number 2 without a remainder.
if num % 2 == 0
continue
end
When the number can be divided by 2 without a remainder, we call continue and the cycle is interrupted and a new one is started. As a consequence, the number is not printed.
The for loop
The for/in keywords are used to iterate over a sequence. The
sequence can be a collection, a string, or a range.
words = ["sky", "summer", "winter", "war", "rock"]
for word in words
println(word)
end
println("---------------------------------")
for word = words
println(word)
end
println("---------------------------------")
word = "falcon"
for e in word
println(e)
end
println("---------------------------------")
for e in range(1,5)
println(e)
end
The program uses a for loop to iterate over sequences.
words = ["sky", "summer", "winter", "war", "rock"]
We define an array of words.
for word in words
println(word)
end
We go over the array element by element with for/in and print each
element to the console.
for word = words
println(word)
end
Julia has also an alternative syntax: for/=.
word = "falcon"
for e in word
println(e)
end
Here we iterate over string characters.
for e in range(1,5)
println(e)
end
We go over a range of values created with range.
$ julia main.jl sky summer winter war rock --------------------------------- sky summer winter war rock --------------------------------- f a l c o n --------------------------------- 1 2 3 4 5
In this article we have worked with program control flow in Julia.