# I want to make a recursive formula and execute two statements with the same variables at the same time

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Jelle Bosschaart on 11 Oct 2020
Commented: Rena Berman on 13 Oct 2020
clc
clear all
a=1
b=0
while irrelevant
b = [a] % The answer I want from this statement is b = [1]
a = [a b] % The answer I want from this statement is [1 0] , I dont want to use calculated b (from the statement before) but b=0 ('the first b').
end
% I dont want to really want to make b0,b1,b2,b3 because this needs to become recursive with an 'while' and 'for' statement.
% So I really want to execute b = [a] and a = [a b] at the same time; but I cannot figure out how to fix this.
Although it doesn't really work for our problem.
We'd like the following to be recursive:
... etc ...
clc
clear all
a1=1
b1=0
b2=[a1]
a2=[a1 b1]
b3=[a2]
a3=[a2 b2]
b4=[a3]
a4=[a3 b3]
... etc ...

Stephen Cobeldick on 12 Oct 2020
Rik on 12 Oct 2020
Rena Berman on 13 Oct 2020

Walter Roberson on 11 Oct 2020
NO.
You cannot do what you want in MATLAB.
At the moment, I cannot think of any programming language that supports what you are asking for.

Alan Stevens on 11 Oct 2020
What about a temporary variable for b:
...
bt = b;
b = [a];
a = [a bt];
...

#### 1 Comment

Walter Roberson on 11 Oct 2020

Steven Lord on 12 Oct 2020
Use a cell array (or two cell arrays, though b is a copy of most of a.)
a = {1};
b = {0};
cellToFill = 2;
while cellToFill < 20
b{cellToFill} = a{cellToFill-1};
a{cellToFill} = [a{cellToFill-1}, b{cellToFill-1}];
cellToFill = cellToFill + 1;
end
Instead of referring to numbered variables, refer to cells in a and b.
a{4}
b{5}

#### 1 Comment

Steven Lord on 12 Oct 2020
So you need all permutations of 8 ones and 4 zeros? That's a very different question than the one you asked.
If that's not the full question please state exactly what you're trying to do. In particular, what restrictions are there (if any) about where the 1's can be placed.

Alan Stevens on 12 Oct 2020
I'm obviously not understanding something here (not unusual!),. Doesn't the following do what you want:
a = 1;
b= 0;
n = 5;
[a, b] = recur(a,b,n);
disp('Recursive')
disp(a)
disp(b)
% Compare with
a1 = 1; b1 = 0;
b2 = a1; a2 = [a1 b1];
b3 = a2; a3 = [a2 b2];
b4 = a3; a4 = [a3 b3];
b5 = a4; a5 = [a4 b4];
b6 = a5; a6 = [a5 b5];
disp('Incremental')
disp(a6)
disp(b6)
function [a, b] = recur(a,b,n)
if n>1
bt = b;
b = a;
a = [a bt];
n = n-1;
[a, b] = recur(a,b,n);
else
bt = b;
b = a;
a = [a bt];
end
end
This produces the following comparison:
Recursive
1 0 1 1 0 1 0 1 1 0 1 1 0
1 0 1 1 0 1 0 1
Incremental
1 0 1 1 0 1 0 1 1 0 1 1 0
1 0 1 1 0 1 0 1

Bruno Luong on 12 Oct 2020
Edited: Bruno Luong on 12 Oct 2020
I'm surpised nobody proposes yet a very MATLABish solution
[a,b] = deal([a b],a]