how do i choose material in command window?

I am trying to write a program where the material can be chosen and a graph will be plotted for stress against strain. i am having difficulty choosing the material in the command window however. Can anyone point out where my mistake is?

 select_material=input('Choose a plate material,Steel (st),Aluminium Alloy (al)')
 if select_material==st
    load('material_data_strain_st.txt');
    load('material_data_stress_st.txt');
    p=polyfit(material_data_strain_st.txt,material_data_stress_st.txt,1)
 elseif select_material==Al
    load('material_data_strain_al.txt');
    load('material_data_stress_al.txt');
    p=polyfit(material_data_strain_al.txt,material_data_stress_al.txt,1)
 else 
    fprintf('invalid entry')
 end

Cheers,

Answer 1

[Geoff Hayes]

Mike - if you are requesting that the user enter a string, then you need to include the 's' option so that the input is not evaluated and is treated as a string. If you run your code as is, you are probably observing the

 Error using input
 Undefined function or variable 'st'.

when you type in st. Instead change the input call to

 select_material=input('Choose a plate material,Steel (st),Aluminium Alloy (al): ','s');

Now, select_material will be a character array (or string) and you can compare the selection to the strings st and al. In your code, you are doing this as

 if select_material==st

But since st is undefined and select_material is a string then this evaluation will fail. Instead use strcmpi to compare strings

 if strcmpi(select_material,'st')
    % do stuff
 elseif strcmpi(select_material,'al')
    % do other stuff
 else
    fprintf('invalid material %s\n',select_material);
 end

Try the above and see what happens!

[mike lynch]

Thanks Geoff, I think that worked! I am now getting a message saying "attempt to reference field of non structure array" but I think that has to do with the files I'm trying to load. Thanks for the help, Mike

[Geoff Hayes]

Mike - you must have some code that is trying to access data within an object using the period. i.e. something like

 x = 42;
 x.myData

The above generates the same error. Just look in your code for where you are doing something similar.