Control Flow

While Loops

Julia’s while loop syntax is similar to that of many other languages:

while predictate
    # do something
end

As with all languages, it is important to make sure that the predicate has a teminating condition so that you don’t end up in an infinite loop.

i = 1
while i < 10
    print(string(i) * " ")
    i *= 2
end

1 2 4 8 

(Note the * above; in Julia, string concatenation is performed with this operator, as opposed to the more standard +.)

Break Statements

Julia’s while loops also support break statements, which stop and exit the loop when evaluated. To include a break, include the keyword break:

while predicate
    # do something
    if predicate
        break
    end
end

An example of this is given below.

println("Without break statement:")
i = 3
while i < 25
    print(string(i) * " ")
    i += 3
end

println("\nWith break statement:")
i = 3
while i < 25
    print(string(i) * " ")
    if i % 18 == 0
        break
    end
    i += 3
end

Without break statement:
3 6 9 12 15 18 21 24 
With break statement:
3 6 9 12 15 18 

Continue Statements

Julia also supports continue statements, which skip the rest of the loop evaluation in the current loop when evaluated. When using continue statements, it is very important that your increment is before the continue predicate, or else you will end up in an infinite loop.

while predicate
    # do something
    increment
    if predicate
        continue
    end
    # do something
end

Consider the example below, which only prints odd numbers.

i = 0
while i < 10
    i += 1
    if i % 2 == 0
        continue
    end
    print(string(i) * " ")
end

1 3 5 7 9 

For Loops

Julia’s for loop syntax is similar to that of Python. It uses the in operator to iterate through the elements of an array or other iterable.

for element in array
    # do something
end

for loops support the use of break and continue statements as well. For ranges, it is simple to create an array of integers using : syntax, e.g. 1:10. These ranges are inclusive at both ends and default to a step size of +1. It is also possible to specify a different step size using a second ::

low:step:high

For example, the range 10:-2:1 is ${10, 8, 6, 4, 2}$. An example for loop is given below.

for i in 5:5:100
    if i % 20 == 0
        continue
    end
    print(string(i) * " ")
    if i % 17 == 0
        break
    end
end

5 10 15 25 30 35 45 50 55 65 70 75 85 

Exercises

Exercise 2.3.1: Create a while loop that prints all of the letters in the phrase “data science is fun” but which are not in the word “string”.

Hint: Create an array of the letters and iterate through the index. You can check if a substring occurs in a larger string using occursin(substring, big_string).

Exercise 2.3.2: Write a for loop to print out all multiples of 3 from 0 to 100 that are not odd. The loop should stop when it hits upon a multiple of 31 that is not 0.

Exercise 2.3.3: Write a function cosines(start, stop) that returns an array of the cosine of each number in the range start:stop. Use a for loop to add items to the array.

Exercise 2.3.4: Rewrite your cosines implementation to allow for a different step size. In this implementation using dot syntax to broadcast the cosine function to each element of the range.