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F# For Dummys - Day 10 Loop

Published at

5/21/2024

What is loop

loop is used to repeat a block of code multiple times
we need to set when to start, how many times it repeats or condition to end loops
a loop never ends is called infinite loop

Three ways to loop

for loop Iterates over a specified range of values. The loop variable takes each value in the range one by one

// Forward for loop
for i = 1 to 5 do
    printfn "Jim has run %d laps" i

// Reverse for loop
for i = 5 downto 1 do
    printfn "Days until birthday: %d days left" (i - 1)

while loop Repeatedly executes the block of code as long as the condition remains true

let mutable counter = 1
while counter <= 5 do
    printfn "Jim has run %d laps" counter
    counter <- counter + 1

recursive function Calls itself with updated parameters

syntax: let rec funcName

let rec recursiveLoop n =
    if n > 0 then
        printfn "Jim run 1 lap: %d laps left" (n - 1)
        recursiveLoop (n - 1)

recursiveLoop 5

Understanding recursive function:

Imagine you have a set of Russian dolls, where each doll contains a smaller doll inside it

To find the smallest doll, you would:

Open the current doll
Check if there is another smaller doll inside
If there is, repeat the process with the smaller doll

// Pseudocode
let findSmallestDoll doll =
    if doll contains no smaller doll then
        return doll
    else
        let innerDoll = open(doll) // smaller doll inside
        findSmallestDoll(innerDoll)

How to choose

for loop is simple and easy to understand when iterating over a known range of values
while loop is useful when the number of iterations is not known beforehand
recursive function making code more expressive and often easier to reason about

exercises:

1 + 2 + 3 + ... + 99 + 100 = ?
1 + 3 + 5 + ... + 97 + 99 = ?

answers:
question 1

let mutable sum = 0
for i = 1 to 100 do
    sum <- sum + i
printfn "Sum using for loop: %d" sum

while

let mutable sum = 0
let mutable counter = 1
while counter <= 100 do
    sum <- sum + counter
    counter <- counter + 1
printfn "Sum using while loop: %d" sum

recursive function

let rec sumRecursive n acc =
    if n = 0 then acc
    else sumRecursive (n - 1) (acc + n)

printfn "Sum using recursive function: %d" (sumRecursive 100 0)

question 2

let mutable sum = 0
for i = 1 to 100 do
    if i % 2 <> 0 then
        sum <- sum + i
printfn "Sum using for loop: %d" sum

while

let mutable sum = 0
let mutable counter = 1
while counter <= 100 do
    if counter % 2 <> 0 then
        sum <- sum + counter
    counter <- counter + 1
printfn "Sum using while loop: %d" sum

recursive function

let rec sumOddRecursive n acc =
    if n = 0 then acc
    else 
        let newAcc = if n % 2 <> 0 then acc + n else acc
        sumOddRecursive (n - 1) newAcc

printfn "Sum using recursive function: %d" (sumOddRecursive 100 0)

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