Using Inequality in a For loop

I am attempting to perform a sensitivity analysis on an inequality of two variables, of the form: 4x < y < 40x. I want to use a for loop to make an array of y values for each incremental change in x, and create a plot to visualize the values of y for each x. My biggest issue has been representing the inequality within the for loop, here is the code that I have so far:

%% Sensitivity Analysis

D_rs = 8; %Ratio of D_85(s) and D_15(s), for any value of each
i=0;

for D15_s = 0:0.1:2

4*D15_s < D15_f < (5*D_rs)*D15_s; % <<<issue with representing this inequality>>>

i=i+1; 

D15_sg(i)=D15_s;  % storing the varied input D15_s in an array 
D15_fg(i)=D15_f;  %  storing the output variable D15_f in an array
end

plot(D15_sg,D15_fg)
xlabel('-')
ylabel('-')
title('-')

Answer 1

[Les Beckham]

Walter has correctly shown how to express mutiple inequality conditions in Matlab (you must include the & (and) operator). However, even replacing that line of code with the correct syntax results in no effect on the rest of your code. The conditional will 'return' a logical value but you are doing nothing with the result.

Are you trying to apply upper and lower limits to D15_f?

If so, perhaps this is what you mean:

D15_f = max(D15_f, 4*D15_s); % apply the lower limit (max returns the larger of the two numbers)
D15_f = min(D15_f, (5*D_rs)*D15_s) % apply the upper limit (min returns the smaller of the two numbers)

If you replace your 'inequality' line by this code, you will be forcing D15_f to stay between these limits.

The rest of your code looks OK as far as I can tell.

[Ruben]

When I run the code with this change, I get this error: Undefined function or variable 'D15_f'. Also to clarify, I am attempting to plot the possible valves of "D15_f" within the specified range given by the inequality against each value of "D15_s" generated in the for loop.

%% Sensitivity Analysis

% input data 
D_rs = 8; %Ratio of D_85(s) and D_15(s), for any value of each

i=0; % a counter for the arrays 
for D15_s = 0:0.1:2

D15_f = max(D15_f, 4*D15_s); % apply the lower limit (max returns the larger of the two numbers)
D15_f = min(D15_f, (5*D_rs)*D15_s); % apply the upper limit (min returns the smaller of the two numbers)

i=i+1; % increasing the counter by 1

D15_sg(i)=D15_s;  % storing the varied input D15_s in an array 
D15_fg(i)=D15_f;  %  storing the output variable D15_f in an array
end

% Output data

%plot(D15_sg,D15_fg)
%xlabel('-')
%ylabel('-')
%title('-')

[Les Beckham]

I guess that I assumed that this was defined before your loop.

If you are just trying to find out and visualize what the limits are as a function of D15_s, this should do that:

%% Sensitivity Analysis
% input data
D_rs = 8; %Ratio of D_85(s) and D_15(s), for any value of each
D15_s = 0:0.1:2; % input

% Calculate min and max values
D15_f_min = 4*D15_s;
D15_f_max = (5*D_rs)*D15_s;

% Output data
plot(D15_s, D15_f_min, D15_s, D15_f_max)
xlabel('D15_s', 'Interpreter', 'None')
ylabel('D15_f', 'Interpreter', 'None')
legend('min', 'max', 'Location', 'best')

[Ruben]

This is more or less what I'm looking for, thank you for the assistance!

[Les Beckham]

You are welcome. Glad I could help.

Answer 2

[Walter Roberson]

4*D15_s < D15_f & D15_f < (5*D_rs)*D15_s