Problem with storing for-loop values in array

Hi!

I know the question of storing for-loop values in an array has been asked before.

I am trying to compare the time it takes to perform factorial calculations with two different methods. I will just post one of the methods since it is the only one I am having trouble with.

My code is this:

 clear all, close all, clc
 n=150;                      %factorial to calculate
 T2=zeros(1,n);              %empty (1 x n)-vector
 B=zeros(1,n);               %empty (1 x n)-vector
 p=n;

    tic;
for i=1:n
    while n>1
    n=n-1;                   %calculates n!                 
    p=p*n;                   %calculates n!   
    B(1,n)=p                 %store values of factorial in (1 x n)-vector
    end;
    T2(i)=toc;               %times the function
end;
plot(T2,B)                   %plots the function

The problem I am having is that, instead of storing each value of p in one of the columns of the vector B and thus being able to track the growth of the factorial, all of the columns of the vector B, except for the last one which is just showing the final value. All of the values of p seem to be accurate.

How can I write this to instead store each value of the factorial in its own column in the vector B?

I hope I made it possible to understand my question!

Thanks in advance.

Answer 1

[Jos (10584)]

After the first iteration of the for-loop n has become 1. So for the next iteration, the while-loop is never entered. so T(k) will be almost the same as T(k-1), and B(k) will be zero, for k > 1.

[Alexander Engman]

I see it now, thanks. Do you have any suggestions on how to write it to make it work?

I could remove the while statement, but that makes the last element of B(1,150)=0 (because n is 0).

[Jos (10584)]

I am not sure what you want to do. There are several easy ways to find all factorials from 1 unto N

N = 10 ;

F1 = cumprod(1:n)

F2 = ones(N,1) ;
for k=2:N,
  F2(k) = F2(k-1)*k ;
end

[Alexander Engman]

Basically, I want to calculate any factorial but a requirement is that the factorial is recursive. But I suppose that the code you wrote would count as a recursive formula since it is using F2(k-1)? From what I understand that is the whole point of a recursive formula.

Then I want to be able to plot the time it takes for Matlab to calculate the factorial and plot it in a value/time diagram. But I think I will manage to figure it out now thanks to your help.

Many thanks!!

[Jos (10584)]

for and while loops do not truly represent recursion. The function below does:

function Y = recurF(X)
% recurF - calculates the factorial of X using recursion
if X < 2
    Y = 1 ;
else
    Y = X * recurF(X-1) ;
end