MATLAB Trapezium Rule - Input Error

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M P
M P 2016-12-9
编辑： Sanaz Kb 2018-4-17
Hi there,
I have written a mathematical code to calculate the trapezium rule for a set function. For my entire program, I have attempted to make it as 'user-friendly' as possible; whereby all variables are set upon running the code. This way, the script does not need to be editted in any way - see my code if you're confused in any way.
The issue I have is that when I input the actual mathematical 'function' into the program, it keeps being called multiple times, when I only want it to be called once. For example, if the function I needed evaluating was: 2x^2, I would input ' 2*x.^2 ', but would have to do this several times instead of once.
I think that the number of times the program calls upon this input is related to the number of strips to calculate - try this and see if you think I'm right? Therefore, I believe the loop inside the script keeps calling the mathematical 'function' user input continuously - I don't know how to stop this from happening?
I am a beginner to MATLAB and whilst it appears that I'm overcomplicating everything, I'm only trying to make everything as 'user-friendly' as possible...
My script: >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.
disp('Trapezium Rule Calculator');
disp('Truth and Absolute Values');
disp('-------------------------');
disp(' ');
% F. Variables
a = input('Enter the value of the lower bound:');
b = input('Enter the value of the upper bound:');
n = input('Enter the number of strips required:');
disp(' ');
h = ((b - a)/ (n));
% Summation of First and Last Tra.
sum = 0.5*( f(a) + f(b) );
% Loop for Middle Tra.s
for i = 1 : n-1;
sum = sum + f(a+i*h);
end
% Truth - Integral Calculation
g = @f;
truth = integral(g,a,b);
disp(' ');
disp('Actual Value of Integral:');
disp(truth);
% Final Value of Tra.
Answer = h*sum;
ModAnswer = abs(Answer);
disp('Value of Trapezium Area Calculated:');
disp(ModAnswer);
% Difference (Accuracy)
ModTruth = abs(truth);
diff = abs( ModTruth - ModAnswer);
disp('Difference in Calculation:');
disp(diff);
% Percentage Accuracy
format short
PInaccuracy = ( (diff) / (ModTruth) )*100;
PA = 100 - PInaccuracy;
disp('Percentage Accuracy (%):');
disp(PA);
% Functions Formulae
function y = f(x)
y = input('Enter function for processing:');
end
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Please help me on how to solve this issue please?
Thank you!

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采纳的回答

M P
M P 2016-12-9
Solved it!
The best way is to NOT use the function script at the bottom of the file...
Instead, assign a variable and use an input function along with it - see code below!
Here, adding >> @(x) << before whatever you input stores it as a function, and the following code will do its job as normal!
Thanks for your help anyways!
disp('Trapezium Rule Calculator');
disp('Truth and Absolute Values');
disp('-------------------------');
disp(' ');
% F. Variables
a = input('Enter the value of the lower bound: ');
b = input('Enter the value of the upper bound: ');
n = input('Enter the number of strips required: ');
f = input('Enter function: [@(x)...]: ');
disp(' ');
h = ((b - a)/ (n));
% Summation of First and Last Tra.
sum = 0.5*( f(a) + f(b) );
% Loop for Middle Tra.s
for i = 1 : n-1;
sum = sum + f(a+i*h);
end
% Final Value of Tra.
Answer = h*sum;
ModAnswer = abs(Answer);
disp('Value of Trapezium Area Calculated:');
disp(ModAnswer);
% Truth - Integral Calculation
truth = integral(f,a,b);
disp('Actual Value of Integral:');
disp(truth);
% Difference (Accuracy)
ModTruth = abs(truth);
diff = abs( ModTruth - ModAnswer);
disp('Difference in Calculation:');
disp(diff);
% Percentage Accuracy
format short
PInaccuracy = ( (diff) / (ModTruth) )*100;
PA = 100 - PInaccuracy;
disp('Percentage Accuracy (%):');
disp(PA);
For example:
Trapezium Rule Calculator
Truth and Absolute Values
-------------------------
Enter the value of the lower bound: 1
Enter the value of the upper bound: 2
Enter the number of strips required: 16
Enter function: [@(x)...]: @(x) 1./x
Value of Trapezium Area Calculated:
0.6934
Actual Value of Integral:
0.6931
Difference in Calculation:
2.4402e-04
Percentage Accuracy (%):
99.9648

更多回答（2 个）

Jan
Jan 2016-12-9
The problem is that "f" is called n times, such that the input command appears so often.
Use a function instead of a script. Then provide the function as anonymous function as usual in Matlab (see ode45 or fzero).
Avoid "sum" as name of a variable, because this is an important Matlab function and shadowing it causes unexpected behavior frequently.
  1 个评论
M P
M P 2016-12-9
Hi Jan,
Thank you for your prompt reply!
I somewhat undrestand where you're coming from in terms of 'n' times the input is called.
I'm using R2016b and so I've placed the function within the script at the bottom - is this any different from creating a 'm' (function) file, since I'll still want the function to be 'user-friendly' and be open to the user's choice?
Is there by any chance a way to set a mathematical function once, then call upon the 'function' function and then let that run through the program?
I am a beginner but I've heard of prompts and strings - could this be a solution, i.e. string in some user prompt and set that as a function? If you understand what Im trying to say?
I only used 'sum' because my engineering computation book told me to do so in such a code - I will change the name of this variable, so thank you!
Thank you for your help!

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Sanaz Kb
Sanaz Kb 2018-4-17
编辑：Sanaz Kb 2018-4-17
Hello,
I have a question about the accepted code. When I've implemented your code, in the line which is related to the "truth = integral(f,a,b);", I've faced with this error:
Error using integral Too many input arguments.
Error in Untitled12 (line 24) truth = integral(f,a,b);
and also I have another questioin, How can I solve the integral if the upper limit be the vector not scalar? Could you please help me?
Thanks

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