Add to a vector from loop

Team,

Just learning how to use MatLab here,

How do I insert the value n into a different variable so I can recall each number? Any other advice is much appreciated. I just started MATH240 so will need more help later ;)

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totaln=0;

n=1;

for n=1:1000000;

if abs(cos(n))<1/n;

totaln=totaln+1;

display(n) %this displays n but then updates it with the next iteration, I want to insert the value of n into a vector so I can see recall them later.

n=n+1;

else;

n=n+1;

end;

end;

[Stephen23]

Start here to learn the answer to your question and also fix several other bugs and features of your code:

https://www.mathworks.com/help/matlab/getting-started-with-matlab.html

[Matt Baron]

Thank you for pointing me toward the tutorial but which part of the tutorial has loop and vector information? I have already done the tutorial and I am going through some MATLAB exercises to learn. I have the correct answer I need using the code I pasted but I want to hone my skills. This is the first time I am using code and the amount of information is overwhelming. Although I have done the tutorial already, it is hard to go and find exactly what I am looking to do through the help command and tutorial every 5 seconds.

[Stephen23]

"which part of the tutorial has loop and vector information?"

https://www.mathworks.com/help/matlab/learn_matlab/scripts.html % FOR loop

https://www.mathworks.com/help/matlab/learn_matlab/matrices-and-arrays.html % vectors, etc.

But this might be more useful than those introductory pages:

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

Note that MATLAB usually works best when working with vectors/matrices/arrays. So rather than solving that task using a loop, the standard approach would be to generate a vector and use logical indexing to select the values that meet that condition:

https://www.mathworks.com/help/matlab/matlab_prog/vectorization.html

https://www.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html

"This is the first time I am using code and the amount of information is overwhelming."

I know that feeling! Read the documentation, experiment, read more documentation, don't worry about making mistakes :)

[Matt Baron]

Ok, thank you. I will take a look! I appreciate your time.

[Matt Baron]

Mr. Cobeldick,

Just wanted to share with you how far I have come since this question with you and others' help;

syms e imp r

A=zeros([],2);%create an empty 0 by 2 matrix for later

display("Hello! Welcome to Matt Baron's numerical solver. Press enter to continue.")

pause

step = input('What step size would you like?');

initcondx = input('What is the initial condition for x?');

initcondy = input('What is the initial condition for y?');

n=1;%start at 1

x=initcondx;%start at the initial condition

y=initcondy;%start at the initial condition

dfeq=input('What differential equation do you want approximated? i.e. .2 * x * y','s');

f=str2func(sprintf('@(x,y)%s',dfeq));%the function to be evaluated

est=input('What value do you want the equation estimated to?');

%actual=@(x) (exp(.1 .* (x .^2) - .1));%the solved function

choice = input('Press e for Euler, imp for Improved Euler, or r for RK4.');

if choice == e

while x <= est%what value do you want it estimated to

A(n,:)=[x,y];

y = (A(n,2) + ((step) * f(A(n,1),A(n,2))));

x = (initcondx) + ((n) * (step));

%abs = actual(x) - y;

%rel = (abs / actual(x)) * 100;

n=n+1;

end

else

if choice == imp

while x <= est

A(n,:)=[x,y];

A(n+1,2) = y + ((step) * f(A(n,1),A(n,2)));

A(n+1,1) = (initcondx) + ((n) * (step));

y = A(n,2) + ((step) * ((f(A(n,1),A(n,2)))+f(A(n+1,1),A(n+1,2))))/(2);

x = (initcondx) + ((n) * (step));

n=n+1;

end

A(n,:)=[];

end

end

A

Answer 1

[Stephen23]

The MATLAB way:

N = 1:1000000;
X = abs(cos(N)) < 1./N;
T = nnz(X) % total

T = 7

Z = N(X) % those which pass your condition

Z = 1×7

1 2 11 33 52174 260515 573204