Program doesn't calculate with input

My program is supposed to take input and use this to calculate the corresponding hydrogen. My code is this:

function Q = hydrogenusage(P)
P = 0:1:4000;
Q = 79.987*P+42.942;
plot(P,Q)
xlabel('Power (W)')
ylabel('Usage (l/minute)')
title('Hydrogen usage (no leak)')
axis([0 60 0 4000])
grid on 
if nargin == 0
Q = input('Enter the power to get the hydrogen usage: ');
%This needs a reference to the power of the program by Erik Rijckaert.
end

Entering 'hydrogenusage' gives me:

Enter the power to get the hydrogen usage

I enter for an example 40

If I fill that in my formula it gives me Q = 3242.422.

But when I enter 40 it gives me this:

ans =

      40

It looks wonderful and it very accurately repeats what I just entered but I'd like it to give me the answer of the function.

How can I make this happen?

[N/A]

To make clear: P is my input, my 'x' and Q is my output, my 'y'. I forgot to say I also tried this:

Q = input('Enter the power to get the hydrogen usage: ');

And I changed that to:

P = input('Enter the power to get the hydrogen usage: ');

That gave me a matrix as answer. It looks neat, but it's not what I need.

Answer 1

[Walter Roberson]

The first few lines of your code ignore any parameter passed in and assign a vector to P and assign a vector of values to Q based upon that vector. Then the bottom portion tests whether any parameters were passed (even though they were otherwise ignored) and if not then uses input() to overwrite the Q (or P?) that had been assigned before, but does not calculate any Q value.

I suggest,

function Q1 = hydrogenusage
P = 0:1:4000;
Q = 79.987*P+42.942;
plot(P,Q)
xlabel('Power (W)')
ylabel('Usage (l/minute)')
title('Hydrogen usage (no leak)')
axis([0 60 0 4000])
grid on 

Pt = input('Enter the power to get the hydrogen usage: ');
Q1 = interp1(P, Q, Pt);

Answer 2

[Amit]

I am not sure if why you defining P inside the function if you are taking it as a input. Moreover, I think you need to ask for input before doing any calculations. Something like this:

function Q = hydrogenusage(P)
%P = 0:1:4000;
if nargin == 0
P = input('Enter the power to get the hydrogen usage: ');
%This needs a reference to the power of the program by Erik Rijckaert.
end
Q = 79.987*P+42.942;
plot(P,Q)
xlabel('Power (W)')
ylabel('Usage (l/minute)')
title('Hydrogen usage (no leak)')
axis([0 60 0 4000])
grid on