Storing the value of an Array from a For loop with an If statement

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Dunovan Mcguire 2021-2-13

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https://ww2.mathworks.cn/matlabcentral/answers/744212-storing-the-value-of-an-array-from-a-for-loop-with-an-if-statement

评论： Dunovan Mcguire 2021-2-14

Hello, I'm currently attmepting to use an if statemtn to store the info of my loop with certain criteria. the for loop cycles 200 times and if at any point if the cycle provides a value within 2% of the value 1, the code is to store it as value tss and set my flag as true. It only records the first time it happens in a series. if the loop ever than outputs something outside 2% of 1 I set flag false then keep doing till the system records a tss that never leaves within 2% of 1.

The problem is, tss isn't recording a value, I need some assistance. here is the code.

p=1;  %Flag 9p) set as 1(false)
prompt= 'Please input natural frequency'; %System asking for input of Natural frequency (Wn)
Wn=input(prompt);
prompt='please input a Zeta value between 0 and 1'; %System asking for input Zeta (Z)
    Z=input(prompt);
while Z<=0 || Z>=1                                      %While statement checks for Z value
    prompt='please input a Zeta value between 0 and 1'; %as a value outside 0 or 1 
    Z=input(prompt);                                    %works to correct if provide it
end
x=zeros(200,1);  %Sets array to 200 at steps of 1
t=zeros(200,1);  %Sets array to 200 at steps of 1
tmax=8/(Z*Wn);   %tmax based off of Zeta and Natual frequency
dt=tmax/200;     %dt as the intervals of the graph determined by max value and steps.  
tss=[];
for i = 1:200    %sets value for for statement to loop 200 times. 
    
    t(i)=dt*(i-1); %caluculates t for each value of the array
    x(i)=sec_ord(Wn,Z,t(i)); %calculates x for each value of the array
 
if 0.98<x&x<1.02  %if statement checks for 2% of steady-State 
    if p==1       %checks flage(p) for flase reading (1)
        p=0;      %sets p for 0(true)
        tss=t;      %records time enter steady state. 
        t=t+dt;   %increments t to the next value of the increment 
    elseif p==0   %checks for true flag
        t=t+dt;   %increments t to the next value of the increment
    else          %if other factors aren't satified the following occurs
        p=1;      %flag is set to false 
        t=t+dt;   %increments t to the next value of the increment
    end
   
end
end   

Any help would be great, the sooner the better.

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Walter Roberson 2021-2-13

在 MATLAB Online 中打开

tss(end+1) = t;

instead of tss=t ?

Dunovan Mcguire 2021-2-13

编辑：Dunovan Mcguire 2021-2-13

nothing. it's still not recording the tss value.

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Answer 1

Walter Roberson 2021-2-13

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https://ww2.mathworks.cn/matlabcentral/answers/744212-storing-the-value-of-an-array-from-a-for-loop-with-an-if-statement#answer_622507

在 MATLAB Online 中打开

if 0.98<x&x<1.02 %if statement checks for 2% of steady-State

x is a vector. That is a vector test, equivalent in MATLAB as-if you had tested

if all(0.98<x&x<1.02) %if statement checks for 2% of steady-State

You should only be testing x(i) at that point.

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Dunovan Mcguire 2021-2-14

在 MATLAB Online 中打开

function f=sec_ord(Wn, zeta, t)  %Function of Sec_ord with inputs provided
% Function Name: sec_ord
%sec_ord: calculates the value of x in a dampening System set to motion
%
%Syntax=x= 𝐴*{1 −(1/(1-Z^2)exp(-Z*Wn*T))*sin(Wn*sqrt(1-Z^2)*t+x0)
%
%Inputs:
%Wn=Natural Frequency 
%Z=the dampening Factor 
%t=time after beginning of movement
%
%Outputs:
%f: as Position of sytem at a time
%
%Example:
%Wn=1 Z=0.5 t=1
%f=x
%f=0.3403
%
% Other files needed: none
% 
% Author:Dunovan McGuire
% February 12,2021
% Revision: None
% --------------------------BEGIN CODE--------------------------------------
Z=zeta;                    %renames zeta as Z for further usage
A=1;                       %Steady state coded as 1
x0= acos(Z);               % equation of Delta expressed as x0 calculated. 
x2 = (Wn*sqrt(1-Z^2)*t+x0);   %x2 as the second half of the main equation.
x1=(1/sqrt(1-Z^2))*exp(-Z*Wn.*t); %x1 expressed as the first half of brackets on the original equation
f=A*(1-x1.*sin(x2)); % x(t) calculated with x1 and x2 solved for simple expression.
end

Walter Roberson 2021-2-14

在 MATLAB Online 中打开

p=1;  %Flag 9p) set as 1(false)
prompt= 'Please input natural frequency'; %System asking for input of Natural frequency (Wn)
Wn=input(prompt);
prompt='please input a Zeta value between 0 and 1'; %System asking for input Zeta (Z)
Z=input(prompt);
while Z<=0 || Z>=1                                      %While statement checks for Z value
    prompt='please input a Zeta value between 0 and 1'; %as a value outside 0 or 1
    Z=input(prompt);                                    %works to correct if provide it
end
x=zeros(200,1);  %Sets array to 200 at steps of 1
t=zeros(200,1);  %Sets array to 200 at steps of 1
tmax=8/(Z*Wn);   %tmax based off of Zeta and Natual frequency
dt=tmax/200;     %dt as the intervals of the graph determined by max value and steps.
tss=[];
for i = 1:200    %sets value for for statement to loop 200 times.
    
    t(i)=dt*(i-1); %caluculates t for each value of the array
    x(i)=sec_ord(Wn,Z,t(i)); %calculates x for each value of the array
    
    
    if 0.98<x(i)&&x(i)<1.02  %if statement checks for 2% of steady-State
        if p==1       %checks flage(p) for flase reading (1)
            p=0;      %sets p for 0(true)
            tss(end+1)=t(i);      %#ok<SAGROW> %records time enter steady state.
        end
    else
        %not in steady state
        p=1;      %flag is set to false
    end
    
end

Dunovan Mcguire 2021-2-14

I only need the last tss but close enough for my purposes Thak you.

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Answer 2

randerss simil 2021-2-13

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https://ww2.mathworks.cn/matlabcentral/answers/744212-storing-the-value-of-an-array-from-a-for-loop-with-an-if-statement#answer_622437

在 MATLAB Online 中打开

tss(i)=t; %records time enter steady state.

Use the index to record the time entered

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randerss simil 2021-2-13

编辑：randerss simil 2021-2-13

May be the if condition in x is not true. That's why it's not entering if loop and _tss_is not recorded. What is output of function

x(i)=sec_ord(Wn,Z,t(i))

Dunovan Mcguire 2021-2-13

编辑：Dunovan Mcguire 2021-2-13

lab3 graph.fig

This is the output for Wn=1 Zeta=0.5. As you can see, the condition is activated at several points as the x value approachs 1 and tss is supposed to record the time of when x enters 2% of the value and hold it. The system then continues to loop for the remainder and is supposed to the record the final time the system enters 2% of 1 and remains there.

To simplify. tss is first to record 2,1 then as it leaves steady state p=1(false) and the if statement is primed to record the next time it enters 2% with is roughly 5, then eventually 8 which is the correct answer for this graph. this system is failing to record ANY tss value and the system is not recognizing tss as a variable.

I added tss=[] to prime the system to look for this variable but the the system is unable to find tss=t within the if statement, for some reason.

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Storing the value of an Array from a For loop with an If statement

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