Julia Control

It is time to discover Julia's control flow statements. This is a group of statements used to stear the logical flow of a program or sub-program. A regular program is linear, but using control statements we can make branches. We can select which branch is executed using conditions and selectors.

The decision

In the previous example we show a "if" statement also known as conditional statement. Easy to to use and simplified syntax. Julia do not use "then" keyword or ":" symbol after the condition like in other languages. Again Julia show an elegant and simple approach. It is used for one condition or multiple conditions using "elseif" compound keyword. In python this keyword is "elsif".

Syntax:

# This is the if-elseif-else-end statement
if [first-condition]
   [furst-true block]
elseif [second-condition]
   [second-true block]
...
else
   [fasle-block - no condition is true]
end

Note: Only first condition and block is mandatory, all other conditions and corespondent blocks are optional. So we can have only one condition and one statement or block that will execute when condition evaluate to Boolean value true.

Decision Diagram

Example 1:

# Single branch conditional
if 1 == 1
   print("Of course 1==1 silly")
end

Example 2:

# Two branch conditional
print("enter any digit: a = ")
a = read(stdin, Char)

s = readline()

print("enter second digit: b = ")
b = read(stdin, Char)

s = readline()

# Two branches conditional
if a > b
   println("a > b")
else
   println("a <= b")
end

s = readline()

Homework:Open this example on-line and run it on repl.it website. Julia:decision

Ladder Statement

The if statement can have many condition and for each condition a block of code. The else statement is the alternative (optional) block that is executed when no condition is true. For each block we can also have nested if statements. This is somesing we wish to avoid if possible. It’s a good practice to not get too far from left side.

Ladder Decision

Example

# multi-branch decision ladder
print("enter any character: ")
c = read(stdin, Char)

if c in '0':'9'
   print("digit")
elseif c in  'A':'Z'
   print("uppercase letter")
elseif c in  'a':'z'
   print("lowercase letter")
else
   print("special character")
end

While Loop

While statement is used to repeat a block of code multiple times as long as one condition is true. If the condition is always true then we have an infinite loop. We can use a control variable that we increment inside the loop. In the next example we also learn two operators: "<=" and "+=" to check the value of the control variable "i". After the loop this control variable is available.

Loop Diagram

# next program will print: 1,2,3,4,5,done
i=1
while i <= 5
    print(i)
    print(",")
    i += 1
end
print("done")

For Loop

This is a repetitive statement that execute a finite number of times using a control variable. Next example is more compact:

# next program will print: 1,2,3,4,5,done
for i = 1:5
    print(i)
    print(",")
end
print("done")

Loop Control

We can early terminate a loop by using break. We can also skip execution of statements and jump back to beginning using keyword continue. Unlike while loop the control variable "i" is local inside the for loop and not available after the loop is ending.

Loop Control

Example 1:

# expected output: 3 6 9
for i in 1:10
    if i % 3 != 0
        continue
    end
    println(i)
end

Example 2:

# expected output: 1 2 3 4 5
for j in 1:1000
    println(j)
    if j >= 5
        break
    end
end

Double Loop

Julia is good with matrix. So it is possible to use two control indexes in same loop. This is useful to create pairs.

# demo for double loop
for i = 1:2, j = 3:4
    println((i, j))
end

output:


(1, 3)
(1, 4)
(2, 3)
(2, 4)