CISC181 010-012 09/12/05 05F

If you notice that a file on the web site comes
up with the message "permission denied",
please send Prof. Conrad an email and he will
fix it.


(0) Auto-save files and tilde files in emacs

In class, I demonstrated that while editing
the file 09.12.txt, the tilde file is always
the previous version of what I saved.

The 09.12.txt file is the current saved version.

If if go in and edit, and have unsaved changes,
periodically, emacs will save an intermediate
version for me in a file that starts and ends
with the pound sign (#).

> ls
09.12.txt   09.12.txt~

[right here, I went into emacs, made
a change, and waited a couple of minutes.
then typed ls again...]

> ls
#09.12.txt#  09.12.txt    09.12.txt~
> 

Notice the #09.12.txt# file.  If "red x"
out of ssh, that is, if I exit improperly,
the #09.12.txt# file will stick around.

If on the other hand, instead of exiting
emacs with the red x, I save my file
properly, then the #09.12.txt# auto save
file will go away by itself.  In fact,
everytime you save your unsaved changes,
the auto save file automatically goes away.


Attendance:
 

(1) Calendar is online.

(2) Conversion from decimal to binary by hand

271 in decimal is what in binary?

271 - 256 = 15
15  - 8 = 7
7   - 4 = 3
3   - 2 = 1
1   - 1 = 0

So the bits are 256, 8, 4, 2, 1 as shown here:

512 256 128  64  32  16   8   4   2   1
 0   1   0   0   0   0   1   1   1   1


(3) Meaning of % and / with integers in
    C, C++, Java

% is remainder after division.

(4) A technique to convert from decimal to 
    binary using %  and /

    271 % 2 => 1   271 / 2 => 135
    135 % 2 => 1   135 / 2 =>  67
     67 % 2 => 1    67 / 2 =>  33
     33 % 2 => 1    33 / 2 =>  16
     16 % 2 => 0    16 / 2 =>   8
      8 % 2 => 0     8 / 2 =>   4
      4 % 2 => 0     4 / 2 =>   2
      2 % 2 => 0     2 / 2 =>   1
      1 % 2 => 1     1 / 2 =>   0

What we get from the results of the %
operations is the bits of the number
we started with, in reverse order.

(From least significant to most significant).

512 256 128  64  32  16   8   4   2   1
 0   1   0   0   0   0   1   1   1   1

Can we make this a function?

Use a C string as our target:

char binaryAnswer[9];

Limitations:

min 0 minimum value for decimal input
max 255 maximum value for decimal input
number of bits in binary answer 8

represent the binary answer as a C string

char binaryAnswer[9];

Note the role of \0

'0' in binary is 0010 1000
'1' in binary is 0010 1001
'\0' in binar is 0000 0000 [null terminator]

void computeBinary (int decVal, char binAns[9])
{

  for (int i = 0; i<8, i++)
   binAns[i] = '0';
  binAns[8] = '\0';

  @@@ FILL IN DETAILS HERE
  
 return;
}