Why can't my user defined function use a variable it calculates?

function[Impulse_Over_Burn_Time] = Impulse_Over_Burn_Time(I,T)

%UNTITLED Summary of this function goes here

%   The t= I/BT we use it to find q, which is used to find max velosity

I = input('What is the engines impulse?');

T = input('What is your engines burn time?');

t = I / T

end

Command Window

>> Impulse_Over_Burn_Time

What is the engines impulse?4

What is your engines burn time?8

t =

      0.5000

>> t+1

Undefined function or variable 't'.

>>

Answer 1

[Chad Greene]

The problem is your t=I/T line is printing the value of t to the command window, but it's not saving it to a variable. So change the top line to

function t = Impulse_Over_Burn_Time(I,T)

and suppress the printed version of t=I/T by placing a semicolon at the end of the line. Here's a rewrite of your function:

function t = Impulse_Over_Burn_Time(I,T)
% Impulse_Over_Burn_Time

%   The t= I/BT we use it to find q, which is used to find max velosity

if nargin==0
   I = input('What is the engines impulse?');
   T = input('What is your engines burn time?');
end

t = I ./ T; % <-ADD A SEMICOLON TO SUPPRESS OUTPUT

end

Note the other changes I made. Now you can use the function much as you were doing before but with an output argument:

>> t = Impulse_Over_Burn_Time
What is the engines impulse?4
What is your engines burn time?8
t =
          0.50
>> t+1
ans =
          1.50

Or you can simply enter your desired values into the function directly like this:

>> t = Impulse_Over_Burn_Time(4,8) 
t =
          0.50
>> t+1
ans =
          1.50

Or do it all in one line:

>> t = Impulse_Over_Burn_Time(4,8) + 1
t =
          1.50

[Stephen23]

@Chad Greene: I added the output argument to the example showing the function being called.

[Chad Greene]

Yes, perhaps that's more clear. Thanks Stephen.

Answer 2

[Jan]

Perhaps you want:

function t = Impulse_Over_Burn_Time
I = input('What is the engines impulse?');
T = input('What is your engines burn time?');
t = I / T
end

which replies "t", not "Impulse_Over_Burn_Time". In addition it does not use the inputs "I" and "T", because they are overwritten by the input() commands. Then call this like:

t = Impulse_Over_Burn_Time

It seems like the meaning of inputs and outputs of function is not clear yet. Read this: https://www.mathworks.com/help/matlab/ref/function.html .

[Guillaume]

It seems like the meaning of inputs and outputs of function is not clear yet

I think you hit the nail on the head. Gabriel needs to learn how to write functions and how to use functions.

Functions are not scripts, whichever variables are created by the functions are only available within the function, unless they're defined as output variables.

When you invoke a function, if you don't assign its output to something it is immediately lost, the same as if you did

sin(3.14)

even if sin creates a t variable internally, you don't have access to t after calling sin. However, if you do:

t = sin(3.14)

then you can do

t+1

Answer 3

[Walter Roberson]

This is correct. You create t as a local variable inside of your function, and it will be destroyed as soon as your function returns.