Question

matlab help

Answer 1

How would I make a function for a Fibonacci sequence?

Answer 2

@phi

Answer 3

What is going on here? I am trying to understand the fib(1) = 1 and fib(2) = 2

Answer 4

what is the definition of fibonacci sequence ?

Answer 5

is equal to the sum of the preceding two numbers

Answer 6

so fib(1) and fib(2) are required as "startup values" , right ?

Answer 7

Yes, I would agree.

Answer 8

% asks user to enter a number
N = input('Pick a number: ');
fib = zeros(1, N);
fib(1) = 1;
fib(2) = 2;
k = 3;

while k <= N
fib(k)= fib(k-1) + fib(k-2);
k = k + 1;
end

fprintf('The Fibonacci sequence to %d term is \n', N);
fprintf('%g', fib);

Answer 9

often, people use recursion.
you define a function fib(n)
n= fix(n) % this makes sure you have an integer
if n<1 return 0; (this is not allowed, but you want the function to do something reasonable)
if the start values are fib(1) = 1 and fib(2)=2 , then the next statement could be
if (n<=2) return n; this would return 1 or 2 , depending on n
% otherwise, you recurse
return fib(n-1)+fib(n-2)

Answer 10

Okay how about this part?

fib(k)= fib(k-1) + fib(k-2)

Answer 11

that code is sequentially filling in the numbers
you start with k=3

the first time through the "while loop"
it calculates fib(3) = fib(2) + fib(1)
it then increments k to 4
and does fib(4) = fib(3) + fib(2)
and so on
until k reaches N

Answer 12

fib is an array, which starts out with 1 and 2 in the first two "slots"
and the code fills in the next entries, using the two previous values in the array

Answer 13

When you say fib(1) = 1 the 1 inside the para thesis is an index? Are we saying fib(1) = 1, so index 1 is assigned a number 1?

Answer 14

yes, that is assignment

you can create a small array
fib= zeros(1,10)
for example, and then do fib(1)=1
and see what it is doing

Answer 15

So for this little bit of code
Lets say N = 5 what happens? k = 3 which is less than 5 so does k = 3 run through the loop?

while k <= N
fib(k)= fib(k-1) + fib(k-2);
k = k + 1;
end

Answer 16

I am doing this out on paper.

Answer 17

if you define fib= zeros(1,5);
then you can cut and paste your code into matlab
if you leave off the semi-colon on the line
fib(k)= fib(k-1) + fib(k-2) <- no ;
it will print out what it's doing

if you put in a statement
pause,
then it will stop until you press a key

Answer 18

yes, for k=3 it executes the loop.
the logic is:

top: test if k is bigger than 5. if so, quit
otherwise:
do the statement fib(k)= fib(k-1) + fib(k-2)
incitement k
go to the top

Answer 19

*increment k

Answer 20

Is my logic correct?

Answer 21

almost. k=4 not 5 in your last iteration (fib(4) is 5, but fib is not k)

Answer 22

so fib(4) = 5 let me look at the math.

but does fib(k) = k ?

k = k + 1, the k on the right side is equal to k above

Answer 23

so k = 3 get throw into fib(k) after you enter the Nth number?

Answer 24

thrown*

Answer 25

beginning number for the loop. The loop doesn't start with k = 1 or 2

Answer 26

variables can be thought of as locations in memory, or mail boxes in a post office
when we say
fib(k) = fib(k-1) + fib(k-2)
we first go to location (or mailbox) k, find the value stuffed in there, and get it. we get a 4
we then do
fib(4) = fib(4-1) + fib(4-2)
fib(4-1) or fib(3) is a location: 3 entry in the fib array. get the number. it's is 3
ditto for fib(2), we get a 2
fib(4) = 3+2= 5
this means "put a 5 in location "4th entry in fib array"

next we do
k= k+1
this says " get value in k" ... we get a 4
4+1 is 5
k= 5
which means "put a 5 in location k"

Answer 27

so variables on the right-hand side mean "read or get the value in that location"
variables on the left side of the =, mean "set that location" to the value of the right-hand side

Answer 28

Okay I see. That is much better. Basically a k value isn't a number you will see on the output end. The k value is a place holder/and index.

Answer 29

an*

Answer 30

all variables are place holders (for values)
some variables you only need for "temporary use"
others you want to "return" to the calling procedure or print out on the screen

Answer 31

So k = k +1 increments the kth value which is an index and the fib(k) = fib(k-1) + fib(k-2) function find the fib numbers

Answer 32

yes. But to be clear, k is a variable, and we use it as an index (into the fib array)
we could also use k in calculations if we wanted or had to.

Answer 33

Okay that makes a lot more sense now. Thanks.