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Structure
Conditionals

Perform an action if a condition is true (IF .. THEN)

Given a variable name, if the value is "Bob", display the string "Hello, Bob!". Perform no action if the name is not equal.
python
if name == 'Bob':
print 'Hello, Bob!'
clojure
(def person "Bob")
(if (= person "Bob")
(println "Hello, Bob!"))
fsharp
if name = "Bob" then printfn "Hello, %s!" name
name = "Bob" && begin printfn "Hello, %s!" name ; true end
fantom
if (name=="Bob") echo("Hello, Bob!")
java
if (name.equals("Bob")) {
System.out.println("Hello, Bob!");
}

Perform different actions depending on a boolean condition (IF .. THEN .. ELSE)

Given a variable age, if the value is greater than 42 display "You are old", otherwise display "You are young"
python
if age > 42:
print 'You are old'
else:
print 'You are young'
print age > 42 and 'You are old' or 'You are young'
clojure
(def age 41)
(if (> age 42) "You are old" "You are young")
fsharp
if age > 42 then printfn "You are old" else printfn "You are young"
let message = if age > 42 then "old" else "young"
printfn "You are %s" message
fantom
if (age > 42)
echo("You are old")
else
echo("You are young")
echo((age > 42) ? "You are old" : "You are young")
java
if (age > 42) {
System.out.println("You are old");
} else {
System.out.println("You are young");
}
System.out.println("You are " + ((age>42)?"old":"young"));

Perform different actions depending on several boolean conditions (IF .. THEN .. ELSIF .. ELSE)

python
if age > 84:
print 'You are really ancient'
elif age > 30:
print 'You are middle-aged'
else:
print 'You are young'
clojure
(println
(condp <= age
84 "You are really ancient"
30 "You are middle aged"
"You are young"))
fsharp
if age > 84 then printfn "You are really ancient"
elif age > 30 then printfn "You are middle-aged"
else printfn "You are young"
let message = match age with
| _ when age > 84 -> "really ancient"
| _ when age > 30 -> "middle-aged"
| _ -> "young"
printfn "You are %s" message
fantom
if (age > 84)
echo("You are really ancient")
else if (age > 30)
echo("You are middle-aged")
else
echo("You are young")
java
if (age > 84) System.out.println("You are really ancient");
else if (age > 30) System.out.println("You are middle-aged");
else System.out.println("You are young");

Replacing a conditional with many branches with a switch/case statement

Many languages support more compact forms of branching than just if ... then ... else such as switch or case or match. Use such a form to add an appropriate placing suffix to the numbers 1..40, e.g. 1st, 2nd, 3rd, 4th, ..., 11th, 12th, ... 39th, 40th
python
def affix(num):
num = num == 1 and str(num) + 'st' or num == 2 and str(num) + 'nd' or \
num == 3 and str(num) +'rd' or str(num) + 'th'
return num

print [affix(x) for x in xrange(1,41)]

clojure
(def n 112)

(println (str n
(let [rem (mod n 100)]
(if (and (>= rem 11) (<= rem 13))
"th"
(condp = (mod n 10)
1 "st"
2 "nd"
3 "rd"
"th")))))
fsharp
let suffix = function
| n when n > 10 && n < 20 -> "th"
| n when n % 10 = 1 -> "st"
| n when n % 10 = 2 -> "nd"
| n when n % 10 = 3 -> "rd"
| _ -> "th"

seq { 1 .. 40 }
|> Seq.iter (fun n -> printfn "%i%s" n (suffix n))
fantom
suffix := |Int n -> Str|
{
if ((4..20).contains(n % 100))
return "th"

switch((n.toStr)[-1])
{
case '1': return "st"
case '2': return "nd"
case '3': return "rd"
default: return "th"
}
}

(1..40).each { echo("$it${suffix(it)}") }
java
String[] array = new String[40];
for(int n = 1; n <= array.length; n++)
array[n-1] = Integer.toString(n);
for(int n = 0; n < array.length; n++)
{
int y = Integer.parseInt(array[n]);
if(array[n].length() > 1)
y = Integer.parseInt(array[n].substring(1));
switch(y)
{
case 1: {array[n] += "st"; break;}
case 2: {array[n] += "nd"; break;}
case 3: {array[n] += "rd"; break;}
default: array[n] += "th";
}
}
Loops

Perform an action multiple times based on a boolean condition, checked before the first action (WHILE .. DO)

Starting with a variable x=1, Print the sequence "1,2,4,8,16,32,64,128," by doubling x and checking that x is less than 150.
python
x = 1
while x < 150:
print '%s, ' % x,
x *= 2
clojure
(take-while #(< % 150) (iterate #(* 2 %) 1))
fsharp
let mutable x = 1
while x < 150 do printf "%d, " x ; (x <- x * 2) done
// The problem is clearly geared towards imperative languages ;-)
// No need to mutate any variable, here's how to do it loop-free functional:
let rec powers2 i = seq { if i < 150 then yield i; yield! powers2 (i*2) }
powers2 1 |> Seq.iter (fun i -> printf "%i, " i)
fantom
x := 1
while (x < 150) {
Env.cur.out.print("$x,")
x *= 2
}
echo
java
int x = 1;
while (x < 150) {
System.out.println(x+",");
x*=2;
}

Perform an action multiple times based on a boolean condition, checked after the first action (DO .. WHILE)

Simulate rolling a die until you get a six. Produce random numbers, printing them until a six is rolled. An example output might be "4,2,1,2,6"
python
import random, itertools

def dice():
while True:
yield random.randint(1,6)

print ", ".join(str(d) for d in itertools.takewhile(lambda x: x < 6, dice()))
clojure
(loop [r (rand-int 6)]
(if (= r 5)
nil
(do
(println r)
(recur (rand-int 6)))))
fsharp
open System
let rand = Random()

Seq.initInfinite (fun _ -> rand.Next(1, 7))
|> Seq.takeWhile (fun x -> x < 6)
|> fun items -> String.Join(",", items)
|> function s when s = "" -> printfn "6" | s -> printfn "%s,6" s
fantom
rnd := 0
while(rnd != 6) {
rnd = Int.random(1..6)
Env.cur.out.print(rnd)
if (rnd != 6)
Env.cur.out.print(",")
}
echo
java
int rnd;
do {
rnd = (int)(Math.random()*6)+1;
System.out.print(rnd);
if (rnd!=6) {
System.out.print(",");
}
} while(rnd!=6);

Perform an action a fixed number of times (FOR)

Display the string "Hello" five times like "HelloHelloHelloHelloHello"
python
print "Hello" * 5
for i in range(5):
print "Hello"
clojure
(dotimes [_ 5]
(print "Hello"))
fsharp
for i = 1 to 5 do printf "Hello" done
dotimes 5 (fun () -> printf "Hello")
// Repetition via ranging over a List type(index ignored)
for _ in list do printf "Hello" done
// Repetition via ranging over a Sequence type(index ignored)
for _ in sequence do printf "Hello" done
// Repetition via ranging over an Array type(index ignored)
for _ in array do printf "Hello" done
fantom
5.times { Env.cur.out.print("Hello") }
for (i := 0; i < 5; i++)
Env.cur.out.print("Hello")
(1..5).each { Env.cur.out.print("Hello") }
java
for(int i=0;i<5;i++) {
System.out.print("Hello");
}

Perform an action a fixed number of times with a counter

Display the string "10 .. 9 .. 8 .. 7 .. 6 .. 5 .. 4 .. 3 .. 2 .. 1 .. Liftoff!"
python
print " .. ".join(str(i) for i in range(10, 0, -1)), ".. liftoff!"
clojure
(dotimes [i 10]
(print (str (- 10 i) " .. ")))

(println "Liftoff!")
fsharp
for i = 10 downto 1 do printf "%d .. " i done
printfn "Liftoff!"
// Repetition via ranging over a Sequence type
for i in {10 .. -1 .. 1} do printf "%d .. " i done ; printfn "Liftoff!"
fantom
(10..1).each { Env.cur.out.print("$it .. ") }
Env.cur.out.print("Liftoff!")
for (i := 10; i >= 1; i--)
Env.cur.out.print("$i .. ")
Env.cur.out.print("Liftoff!")
java
for(int i=10; i>=1; i--) {
System.out.print(i + " .. ");
}
System.out.print("Liftoff!");