What's an algorithm? - David J. Malan

Translator: Andrea McDonough Reviewer: Jessica Ruby

Prevoditelj: Neda Vrkic Recezent: Sanda L

What's an algorithm?

Što je algoritam?

In computer science,

U informatici,

an algorithm is a set of instructions

algoritam je skup uputa

for solving some problem, step-by-step.

za rješavanje nekog problema, korak-po-korak.

Typically, algorithms are executed by computers,

U pravilu, algoritme izvršavaju računala,

but we humans have algorithms as well.

ali i mi ljudi imamo algoritme.

For instance, how would you go about counting

Na primjer, kako biste izbrojali

the number of people in a room?

broj ljudi u prostoriji?

Well, if you're like me,

Pa, ako ste kao ja,

you probably point at each person,

vjerojatno biste pokazali na svaku osobu,

one at a time,

and count up from 0:

jednu po jednu,

i brojali od 0:

1, 2, 3, 4 and so forth.

1, 2, 3, 4 i tako dalje.

Well, that's an algorithm.

Dakle, to je algoritam.

In fact, let's try to express it

Zapravo, probajmo to izraziti

a bit more formally in pseudocode,

malo formalnije u pseudokodu,

sintaksi sličnoj engleskom

English-like syntax

that resembles a programming language.

koja nalikuje na programski jezik.

Let n equal 0.

Neka je n jednako 0.

For each person in room, set n = n + 1.

Za svaku osobu u prostoriji, skup n = n + 1.

How to interpret this pseudocode?

Kako protumačiti ovaj pseudokod?

Well, line 1 declares, so to speak,

Pa, redak 1 izjavljuje, tako reći,

a variable called n

varijablu nazvanu n

and initializes its value to zero.

i započinje svoju vrijednost na 0.

This just means that at the beginning of our algorithm,

Ovo samo znači da na početku našeg algoritma,

the thing with which we're counting

stvar s kojom računamo

has a value of zero.

ima vrijednost nula.

After all, before we start counting,

Nakon svega, prije nego počnemo brojati,

we haven't counted anything yet.

nismo još ništa izbrojali.

Calling this variable n is just a convention.

Nazivanje ove varijable n je samo dogovor.

I could have called it almost anything.

Mogao sam je nazvati bilo kako.

Now, line 2 demarks the start of loop,

Sada, redak 2 označava početak petlje,

a sequence of steps that will repeat some number of times.

slijed koraka koji će se ponoviti nekoliko puta.

So, in our example, the step we're taking

Pa, u našem primjeru, korak koji poduzimamo

is counting people in the room.

brojanje je ljudi u prostoriji.

Beneath line 2 is line 3,

Ispod retka 2 je redak 3,

which describes exactly how we'll go about counting.

što točno opisuje kako ćemo brojati.

The indentation implies that it's line 3

Uvlačenje podrazumijeva da će se redak 3

that will repeat.

ponavljati.

So, what the pseudocode is saying

Dakle, što pseudokod govori

is that after starting at zero,

je da nakon kretanja od nule,

for each person in the room,

za svaku osobu u prostoriji,

we'll increase n by 1.

n ćemo povećati za 1.

Now, is this algorithm correct?

Sada, je li ovaj algoritam ispravan?

Well, let's bang on it a bit.

Pa, ajmo malo to razbiti.

Does it work if there are 2 people in the room?

Funkcionira li to ako su dvije osobe u prostoriji?

Let's see.

Da vidimo.

In line 1, we initialize n to zero.

U retku 1, počinjemo s n od nule.

For each of these two people,

Za svaku od ove dvije osobe

we then increment n by 1.

povećamo onda n za 1.

So, in the first trip through the loop,

Dakle, u prvom krugu petlje

we update n from zero to 1,

ažuriramo n od nule na 1.

on the second trip through that same loop,

u drugom krugu kroz istu petlju,

we update n from 1 to 2.

ažuriramo n od 1 na 2.

And so, by this algorithm's end, n is 2,

I tako, do kraja ovog algoritma, n je 2,

which indeed matches the number of people in the room.

što doista odgovara broju ljudi u prostoriji.

So far, so good.

Zasada je dobro.

How about a corner case, though?

A što je onda s izuzetkom?

Suppose that there are zero people in the room,

Pretpostavimo da je nula osoba u prostoriji,

besides me, who's doing the counting.

osim mene, koji broji.

In line 1, we again initialize n to zero.

U retku 1 opet počinjemo s n od nule.

This time, though, line 3 doesn't execute at all

Ovaj put, međutim, redak 3 uopće se ne izvršava

since there isn't a person in the room,

budući da u prostoriji nema nikoga,

and so, n remains zero,

i tako, n ostaje nula,

which indeed matches the number of people in the room.

što doista odgovara broju osoba u prostoriji.

Pretty simple, right?

Prilično jednostavno, zar ne?

But counting people one a time is pretty inefficient, too, no?

Ali brojanje ljudi jedan po jedan je također prilično neučinkovito, zar ne?

Surely, we can do better!

Sigurno možemo bolje!

Why not count two people at a time?

Zašto ne brojati dvije osobe odjednom?

Instead of counting 1, 2, 3, 4, 5, 6, 7, 8, and so forth,

Umjesto brojanja 1, 2, 3, 4, 5, 6, 7, 8 i tako dalje,

why not count

zašto ne brojati

2, 4, 6, 8, and so on?

2, 4, 6, 8 i tako dalje?

It even sounds faster, and it surely is.

Čak i zvuči brže, i sigurno jest.

Let's express this optimization in pseudocode.

Izrazimo ovu optimizaciju u pseudokodu.

Let n equal zero.

Neka je n jednako nula.

For each pair of people in room,

Za svaki par osoba u prostoriji,

set n = n + 2.

skup n = n + 2.

Pretty simple change, right?

Prilično jednostavna promjena, zar ne?

Rather than count people one at a time,

Umjesto da brojimo osobe jednu po jednu,

we instead count them two at a time.

brojimo ih po dvije odjednom.

This algorithm's thus twice as fast as the last.

Ovaj algoritam je stoga duplo brži od prošlog.

But is it correct?

Ali je li točan?

Let's see.

Da vidimo.

Does it work if there are 2 people in the room?

Funkcionira li ako su u prostoriji dvije osobe?

In line 1, we initialize n to zero.

U retku 1, počinjemo s n od nule.

For that one pair of people, we then increment n by 2.

Za taj jedan par osoba, povećamo tada n za 2.

And so, by this algorithm's end, n is 2,

I tako, do kraja algoritma, n je 2,

which indeed matches the number of people in the room.

što doista odgovara broju osoba u prostoriji.

Suppose next that there are zero people in the room.

Pretpostavimo sljedeće, da je nula osoba u prostoriji.

In line 1, we initialize n to zero.

U retku 1, počinjemo s n od nule.

As before, line 3 doesn't execute at all

Kao prije, redak 3 se uopće ne izvršava

since there aren't any pairs of people in the room,

s obzirom da u prostoriji nema parova osoba,

and so, n remains zero,

i tako, n ostaje nula,

which indeed matches the number of people in the room.

što doista odgovara broju osoba u prostoriji.

But what if there are 3 people in the room?

Ali što ako su tri osobe u prostoriji?

How does this algorithm fair?

Kako se ovaj algoritam slaže?

Let's see.

Da vidimo.

In line 1, we initialize n to zero.

U retku 1, počinjemo s n od nule.

For a pair of those people,

Za par tih osoba

we then increment n by 2,

tada povećamo n za 2,

but then what?

ali što onda?

There isn't another full pair of people in the room,

U prostoriji nema još jedan puni par osoba

so line 2 no longer applies.

pa se redak 2 više ne primjenjuje.

And so, by this algorithm's end,

I tako, do kraja algoritma,

n is still 2, which isn't correct.

n je još 2, što nije ispravno.

Indeed this algorithm is said to be buggy

Doista se za ovaj algoritam kaže da je neispravan

because it has a mistake.

jer ima grešku.

Let's redress with some new pseudocode.

Ispravimo se nekim novim pseudokodom.

Let n equal zero.

Neka je n jednak nuli.

For each pair of people in room,

Za svaki par osoba u prostoriji,

set n = n + 2.

skup n = n + 2.

If 1 person remains unpaired,

Ako jedna osoba ostane bez para,

set n = n + 1.

skup n = n + 1.

To solve this particular problem,

Da bismo riješili ovaj poseban problem,

we've introduced in line 4 a condition,

u redak 4 smo uveli uvjet,

otherwise known as a branch,

inače poznat kao grana,

that only executes if there is one person

koja se izvršava samo ako postoji jedna osoba

we could not pair with another.

koju ne možemo spariti s drugom.

So now, whether there's 1 or 3

Pa sad, bilo da postoji 1 ili 3

or any odd number of people in the room,

ili bilo koji neparan broj osoba u prostoriji,

this algorithm will now count them.

ovaj će ih algoritam sada brojati.

Can we do even better?

Možemo li još bolje?

Well, we could count in 3's or 4's or even 5's and 10's,

Pa, mogli bismo brojati po 3 ili 4, ili čak 5 i 10,

but beyond that it's going to get

ali osim toga to će biti

a little bit difficult to point.

malo teško za naznačiti.

At the end of the day,

Na kraju dana,

whether executed by computers or humans,

bilo da ih izvršavaju računala ili ljudi,

algorithms are just a set of instructions

algoritmi su samo skup uputa

with which to solve problems.

kojima se rješavaju problemi.

These were just three.

Ovo su bila samo tri.

What problem would you solve with an algorithm?

Koji biste vi problem riješili algoritmom?