Integrating a function and then plotting it's graph in Matlab

Question

I've been working on this code for some hours and it seems I'm doing something wrong which I just can't figure out. I have a function which I integrate and then plot it's values. But the graph I get looks strange. I expected some form of an exponential function but the one I get is far away from my anticipations, and when I try changing some of my constants, it does still behave in a strange manner. This is how the code looks like:

y = 0:0.5:D;
p = 65.5;
k = 1;
D =10:0.5:100; %Create a vector for D

M =zeros(1,181); %an array
for j = k:181 %For loop for all water values

  M(k) = integral(@(y) Func5(p,D,k,y),0,D(k));
  k+1;  
end

plot(D,M)
title('Hydrostatic pressure in bars');
xlabel('water level in fot');
ylabel('pressure i bars');

And this is the code for my function:

function FD = Func5(p,D,k,y)
 w = 40-20*exp(-(0.01*y))*2; %Width of the dam
 FD = p.*(D(k)-y).*w; %Claculate the pressure
end

Thanks in advance for any help.

Answer 1

I think the main problem you encounter is that your for-loop probably doesn't do what you expect.

k = 1;
M =zeros(1,181);
for j = k:181 
  M(k) = integral(@(y) Func5(p,D,k,y),0,D(k));
  k+1;  # does not increase k
end

The problem here is that the statement k+1 is useless and does not increase k which i think is what you are after. The problem with this is that j is still increased each iteration but the values of the integral are assigned to M(k) which remain M(1) rather than increase with the loop. Also the argument D(k) remains unchanged which i guess should instead change with the iteration aswell.

try this instead:

M =zeros(1,181);
for j = 1:181
  M(j) = integral(@(y) Func5(p,D,j,y),0,D(j));
end

this give me atleast a graph which resembles some sort of exponential function.

Some general advice about your code: try using comments which add useful information to improve readability. M =zeros(1,181); %an array is not a useful comment because i can already see this is 'an array', what i cant see is what M means; a better comment would be % preallocate the results vector , the same applies for D =10:0.5:100; %Create a vector for D; what is D? and for that matter all other variables? Often it is also better to use meaningful variable names like results or pressure rather than D and M. Furthermore, in a function like Func5 it is useful to include a larger comment block at the top to say what the function calculates (which equation) and what its in/outputs are.