Julia: Tutorial & Code-Collection

Source: https://github.com/markomlikota/CodingSoftware

MIT License, © Marko Mlikota, https://markomlikota.github.io

Conditionals & Loops

Conditionals let a code do different things depending on the input it receives.

Loops let us execute a chunk of code repeatedly, possibly with slight modifications. Changes to the code are then made much more easily than when the code is manually written out several times (with slight modifications).

---

Conditional

# A conditional executes a certain script only if the stated condition is true:

a = 1

if a < 2
    b = 1
else
    b = 3
end

if a < 2
    b = 1
elseif a == 2
    b = 2
else
    b = 3
end

1

# For such short scripts, these can be written more compactly as

a < 2 ? b = 1 : b = 2  # if-else
a < 2 ? b = 1 : (a == 2 ? b = 2 : b = 3) # if-elseif-else

1

# If in one case nothing is done,
# we do not write anything in the longer version of the conditional,
# and we write "nothing" in the compact version:

if a < 2
    b = 1
else
end

a < 2 ? b = 1 : nothing

1

# In fact, if in one case nothing is done,
# then an even more efficient conditional is available:
# the so-called "short-circuit evaluation":
(a < 2) && (b = 1)

# This works since " statement1 && statement2 " 
# evaluates statement2 only if statement1 is true.

# Analogously, we could write
(a < 2) || (b = 2)
# which sets b=2 if the condition is false,
# but we could also write
!(a < 2) && (b = 2)
# or
(a >= 2) && (b = 2)

false

# The compact conditional is useful for compactly defining functions
# that apply a conditional:

f1(x) = isodd(x) ? 2x : x
println("f1(2) = ", f1(2), " f1(3) = ", f1(3))

# This is particularly advantageous when passing a non-anonymous function
# fo a command like "map" or "replace":
vb = [2,3]
map(x -> isodd(x) ? 2x : x, vb)

f1(2) = 2 f1(3) = 6

2-element Vector{Int64}:
 2
 6

---

For-Loop

# A for-loop executes a script for each value of a so-called indexing variable,
# denoted in the following by "ii":

for ii = 1:3
    b = ii^2
    println("b = ", b)
end

# This is called an iteration; we iterate over ii going from 1 to 3;
# we iteratively execute a script for ii = 1:3.

b = 1
b = 4
b = 9

# We can use "break" to stop the iterations once some condition is satisfied:

for ii = 1:3
    b = ii^2
    b > 5 ? break : nothing
    println("b = ", b)
end

# We can use "continue" to skip (the rest of) an iteration if some condition is satisfied:
for ii = 1:3
    b = ii^2
    b == 4 ? continue : nothing
    println("b = ", b)
end

b = 1
b = 4
b = 1
b = 9

# We can have nested for-loops:

for ii = 1:3
    for jj = 1:2
        b = ii^2
        c = jj^2
        println("b = ", b, ", c = ", c)
    end
end

# More compactly:

for ii = 1:3, jj = 1:2
    b = ii^2
    c = jj^2
    println("b = ", b, ", c = ", c)
end

b = 1, c = 1
b = 1, c = 4
b = 4, c = 1
b = 4, c = 4
b = 9, c = 1
b = 9, c = 4
b = 1, c = 1
b = 1, c = 4
b = 4, c = 1
b = 4, c = 4
b = 9, c = 1
b = 9, c = 4

# A compact for-loop (a so-called comprehension)
# is useful for assigning values to vectors/matrices:

va = [exp(ii) for ii in 1:5]

ma = [jj*exp(ii) for ii in 1:5, jj=1:3]

ma = ones(5,2)
[ma[ii,2] = exp(ii) for ii in 1:5]

5-element Vector{Float64}:
   2.718281828459045
   7.38905609893065
  20.085536923187668
  54.598150033144236
 148.4131591025766

# Also useful:

collect(ii for ii in 1:10)

collect(ii for ii in 1:10 if isodd(ii))

5-element Vector{Int64}:
 1
 3
 5
 7
 9

# Just like we looped above over vectors of integers,
# we can also loop over vectors of floats, complex and rational numbers.
# We can also loop over vectors of strings:

for ss = ["CHE","AUT","AUS"]

    println("country = ",ss)

end

# We can also loop over vectors:

for options = [[1,34],[0,56]]

    option1, option2 = options

    println(" NEW ITERATION ")
    println(" option1 in this iteration = ", option1)
    println(" option2 in this iteration = ", option2)
    println(" ")

end

# This contrasts with a nested for loop where 
# option1 takes on values 1 and 0 and option2 takes on values 34 and 56, 
# since not all 4 combinations of these values may be needed.

country = CHE
country = AUT
country = AUS
 NEW ITERATION 
 option1 in this iteration = 1
 option2 in this iteration = 34
 
 NEW ITERATION 
 option1 in this iteration = 0
 option2 in this iteration = 56
 

---

While-Loop

# A while-loop executed some script while a condition is true:

ii = 1

while ii < 5
    println(" ii = ", ii)
    ii += 1
end

# In this particular case, the same output can be achieved with a combination of
# a for-loop and the command "break" ...

for ii = 1:10
    ii < 5 ? nothing : break
    println(" ii = ", ii)
    ii += 1
end

# ... but with more complex code we don't know ex-ante how long the condition will be true.

 ii = 1
 ii = 2
 ii = 3
 ii = 4
 ii = 1
 ii = 2
 ii = 3
 ii = 4

Try Exercises 6.a.i - 6.a.iii