Index exceeds matrix dimensions

Hi, the script i am working on is:

a = {'not follow', 'we are not ready', 'dont call',...
  'footpush not pressed', 'proximity selected', 'clear hoist',...
  'stop loading', 'no sequence start'};
b = {'follow', 'we are ready', 'call', 'footpush pressed',...
  'call DA selected', 'load hoist', 'load bag', 'sequence start'};
x = dec2bin(hex2dec('7F'));    
for i = 0:7  
if x(7 - i + 1) == '0'
  disp(a{i+1})
else
disp(b{i+1});
end

end

My code works fine when the hex value entered is between '80' and 'FF' however when i enter hex values between '00' and '7F' i get an error saying:

Index exceeds matrix dimensions.

Error in script (line 8)

if x(7 - i + 1) == '0'

Any help is much appreciated.

Answer 1

[Cedric]

When your hex code is below 0x80, the length of the string that you get with DEC2BIN is smaller than 8 chars.

Try with that:

 x = hex2dec('7F') ;
 for k = 1 : 8  
    if bitand(x, 2^(8-k))
       disp(b{k}) ;
    else
       disp(a{k}) ;
    end
 end

Cheers,

Cedric

[Darren]

That returns the following error:

Undefined function 'bitand' for input arguments of type 'char'.

[Cedric]

You certainly forgot to remove the call to DEC2BIN.

[Darren]

Thanks very much Cedric! However, i don't understand how the code can work without doing a dec2bin conversion...can you explain please?

[Cedric]

You're most welcome! DEC2BIN actually returns a string that is a representation of the binary code that corresponds to its argument; it is just for display purpose (or string processing) as its elements are chars '0' and '1' (which is what appears in your test). Now what I test is the value of each bit (within the first 8 bits) of the binary code that represents the decimal number outputted by HEX2DEC. I do this by performing an AND binary operation with 2^p. The latter generates 2^0=1d=00000001b, 2^1=2d=00000010b, 2^2=4d=00000100d, .., 2^7=128d=10000000b, for p =0,..,7 (in your case, p=8-k). You can see that bits with a 0 value will mask any bit value of the other operand in the binary AND operation, so the output of BITAND will be different from 0 only when bit number p is 1. Try the mechanism "by hand" to see how it works, e.g with decimal 9 whose 8 bits binary representation (as unsigned int) is 00001001:

 >> x = 9 ;
 >> bitand(x, 2^0)              % Least significant bit (LSB).
 ans =
     1
 >> bitand(x, 2^1)
 ans =
     0
 >> bitand(x, 2^2)
 ans =
     0
 >> bitand(x, 2^3)
 ans =
     8

So testing whether the output of BITAND is 0 or not provides you with the information about each bit value.

 >> bitand(x, 2.^(7:-1:0)) ~= 0
 ans =

     0     0     0     0     1     0     0     1

Hope it helps!

[Darren]

Thanks very much, appreciated!