Incorrect subtraction value in Matlab GUI

I am working with COM port and I am getting a value (e.g. temperature value) from COM. Now when I am trying to subtract a specific value (e.g. 8) from this value it is not showing desired value. Here is the code:

function pushbutton1_Callback(hObject, eventdata, handles)
  instrreset; %resets the serial devices
  clc;
  s = serial ('COM6');
  RxText3 = fscanf(handles.serConn);
  key = 8;
  P = minus(RxText3,key);
  set(handles.tempserial,'string',P);
  fclose(s);

What I am expecting: Suppose I am getting a value as 32 and my key value is 8. Then I am expecting to get 24.

What I am getting: I am getting two values as 42 and 45.

Additional problem:

How can I set the value so that it would show 24 Celcius? I tried this : set(handles.tempserial,'string',P, 'Celcius') in which case nothing is shown.

Answer 1

[Azzi Abdelmalek]

Maybe you need to convert from string to double

P = minus(str2num(RxText3),key);