Problem using a text file

Good evening. I have a matlab program (I'm not the author but I must modify it). In the original source (See attachment) files are opened by the code

load 'dia_221_2_0_2.txt'; %spettro diamante in diffrazione 
load 'hopg_220_4_0_2.txt'; %spettro grafite in diffrazione campione 2 ex 'hopg_220_4_0_2.txt'
load 'c60_217_0_0_2.txt'; 
load 'piro_216_0_0_2.txt'; % spettro PIROVALVOLA in diffrazione 

later the data files are used in:

ck(j) =piro_216_0_0_2(j);% sample 9 ora 7 (il più grafitico) 
ckdiadiff(j) =dia_221_2_0_2(j);%% spettro del diamante in diffrazione

      ckgradiff(j) = hopg_220_4_0_2(j);%% spettro della grafite in diffrazione

      c60diff(j)= c60_217_0_0_2(j); %%% spettro c60 in diffrazione

Now I want to change the program so that it the user can write the data file name. I tried to do it, but I couldn't! I just tried for one file (for example the dia_221_2_0_2.txt file) I first defined the variable:

global diamdiff

later I wrote

    diamdiff=input('Diamond spectrum name= ');
  [fid msg]=fopen(diamdiff,'r');

instead of

load 'dia_221_2_0_2.txt'; %spettro diamante in diffrazione

and lastly I wrote

ckdiadiff(j) =diamdiff(j);%% spettro del diamante in diffrazione

instead of

ckdiadiff(j) =dia_221_2_0_2(j);%% spettro del diamante in diffrazione

Unfortunately, running the program Matlab gives this error:

Index exceeds matrix dimensions.

Error in sp2sp3diff_Titantah (line 69)
     ckdiadiff(j) =diamdiff(j);%% spettro del diamante in diffrazione

Please, can you help me?

I think the reason is that in the original program, in the line

ckdiadiff(j) =dia_221_2_0_2(j);%% spettro del diamante in diffrazione

there is just the name of the file WITHOUT the extensions! In my code

    ckdiadiff(j) =dia_221_2_0_2(j);%% spettro del diamante in diffrazione

the variable dia_221_2_0_2 maybe is read not just like the name, but like name and extension of file! Maybe I can resolve assigning the name of the file (Without extension) to a new variable and putting this variable in this line

    ckdiadiff(j) =variable(j);%% spettro del diamante in diffrazione

But what script can I use to assign the name of the file (without extension) to a variable?

Thank you