Control Flow

In computer science, control flow (or flow of control) is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. [1]

This page provides syntax for some of the common control flow methods in Julia . Each section includes an example to demonstrate the described methods.

Use Cases and Syntax

• Test if a specified expression is true or false

• Short-circuit evaluation

  • Test if all of the conditions are true x && y

  • Test if any of the conditions are true x || y

  • Test if a condition is not true !z

• Conditional evaluation

  • if statement

  • if-else

  • if-elseif-else

  • ?: (ternary operator)

Conditional Statements

Input:

# conditions.jl
# Demonstrates use of if statement

x, y, z = 100, 200, 300
println("x = $x, y = $y, z = $z")

# Test if x equals 100
if x == 100
  println("$x equals 100")
end

# Test if y does not equal z
if !(y == z)
   println("$y does not equal $z")
end

# Test multiple conditions
if x < y < z
  println("$y is less than $z and greater than $x")
end

# Test multiple conditions using "&&"
if x < y && x < z
  println("$x is less than $y and $z")
end

# Test multiple conditions using "||"
if y < x || y < z
  println("$y is less than $x or $z")
end

# if-else statement
if x < 100
  println("$x less than 100")
else
  println("$x is equal to or greater than 100")
end

# Same logic as above but using the ternary or 
# base three operator (?:)
println(x < 100 ? "$x less than 100 again" : "$x equal to or greater than 100 again")

# if-elseif-else statement
if y < 100
   println("$y is less than 100")
elseif y < 200
  println("$y is less than 200")
elseif y < 300
  println("$y is less than 300")
else
  println("$y is greater than or equal to 300")
end

Output:

x = 100, y = 200, z = 300
100 equals 100
200 does not equal 300
200 is less than 300 and greater than 100
100 is less than 200 and 300
200 is less than 100 or 300
100 is equal to or greater than 100
100 equal to or greater than 100 again
200 is less than 300

Loops

• Repeat a block of code a specified number of times or until some condition is met.

• while loop

• for loop

• Use break to terminate loop

Input:

# Demonstrates use of loops                                                                                    

i = 1

# while loop for incrementing i by 1 from 1 to 3
while i <= 3
  println("while: $i")
  global i += 1     # updating operator; equivalent to i = i + 1
end

# for loop
for j = 1:3
  println("for: $j")
end

for j in 1:3
  println("for again: $j")
end

# nested for loop
for j = 1:3
  for k = 1:3
    println("nested for: $j * $k = $(j*k)")
  end
end

Output:

while: 1
while: 2
while: 3
for: 1
for: 2
for: 3
for again: 1
for again: 2
for again: 3
nested for: 1 * 1 = 1
nested for: 1 * 2 = 2
nested for: 1 * 3 = 3
nested for: 2 * 1 = 2
nested for: 2 * 2 = 4
nested for: 2 * 3 = 6
nested for: 3 * 1 = 3
nested for: 3 * 2 = 6
nested for: 3 * 3 = 9

Comparison Operators and Functions

Operator	Example
Equality	x == y or isequal(x, y)
Inequality	x != y or !isequal (x, y)
Less than	x < y
Less than or equal to	x <= y
Greater than	x > y
Greater than or equal to	x >= y

Input:

# compare.jl                                                                                                 
# Demonstrate comparison operators                                                                               

# Assign values to variables using parallel assignment                                                           
c1, c2, c3, c4 = 25, 50, 75, 50
println("c1 = $(c1), c2 = $(c2), c3 = $(c3), c4 = $(c4)")

# Output results of different comparison operations                                                             
 
# Testing equality                                                                                               
println("  c1 = c3 is $(c1 == c3)")
println("  c2 = c4 is $(isequal(c2, c4))")

# Changing values using abbreviated assignment operators                                                        
c1 *= 3    	# Shorthand for c1 = c1 * 3                                                                       
c4 += 1    	# Shorthand for c4 = c4 + 1                                                                       

println("c1 = $(c1), c2 = $(c2), c3 = $(c3), c4 = $(c4)")
 
# Testing less than and greater than
println("  c1 < c2 is $(c1 < c2)")
println("  c4 <= c2 is $(c4 <= c2)")
println("  c1 > c2 is $(c1 > c2)")
println("  c3 >= c2 is $(c3 >= c2)")

Output:

c1 = 25, c2 = 50, c3 = 75, c4 = 50
  c1 = c3 is false
  c2 = c4 is true
c1 = 75, c2 = 50, c3 = 75, c4 = 51
  c1 < c2 is false
  c4 <= c2 is false
  c1 > c2 is true
  c3 >= c2 is true

References

1. Wikipedia contributors. (n.d.). Control flow. In Wikipedia. Retrieved May 1, 2024, from https://en.wikipedia.org/wiki/Control_flow

Resources

• Julia Documentation: Manual - Control Flow

• Think Julia: Chapter 5 - Conditionals and Recursion

• Think Julia: Chapter 7 - Iteration