1. Homework Reading Programming Assignments Finish K&R Chapter 1
Start K&R Chapter 2 for next time.
Programming Assignments
DON’T USE <string.h> and string library functions, e.g. strlen ( ) – Write your own!

2. Octal and Hex Numbers People normally deal in numbers base 10
Computers normally deal in numbers base 2
The problem:
Reading a long string of 1’s and 0’s not easy
Conversion between base 2 and base 10 not easy
The solution:
Convert binary digit strings to Octal or Hex
Easily done because 23 = 8 and 24 = 16

3. Octal and Hex Numbers Look at a long string of binary digits in groups
3 digits for Octal
4 digits for Hex
See the following examples:
Binary Digits
Grouped by threes
For Octal
Grouped by fours
For Hex
Don’t convert binary to/from Hex/Octal via decimal! … … …
The most significant bit is always zero in the ascii table …
0x xa 0xc …

4. ASCII Code
For computers to process our letters, digits, punctuation marks, etc, we need a binary code for each such “character”.
American Standard Code for Information Interchange (ASCII) provides these codes.
See ASCII code chart
Standard 8 bit bytes and 16 bit words are not integer multiples of 3 bits but are integer multiples of 4 bits – favoring use of Hex!

5. ASCII Codes in visitype.c
To convert a character’s (integer) value to a printable ASCII string for what it represents:
We need a table of 4 byte character strings each ending in a zero byte (created by using “\0”)
Arranged in the order of the character values used in the ASCII code to represent them
And to create the address of one of these strings to pass to function printf e.g. &asciiname[4*i]
Show visittype.c
You do not have to say the size of array if you are assigning the values right away.
Show the memory table again to show asciiname[]
Each one of them has 4 Bytes
Ampersand sign means the address and not the value
Printf needs the address for printing the arrays
You can use my visittype.c code and edit it in your hw

6. Analysis of visitype.c See separate program text “visitype.c”
ASCII code conversion array
char asciiname [] = …. ;
Initialization values for the array
128 strings of length 4 including the ‘\0’ …
asciiname[0]
…[1]
…[2]
…[3]
…[4]
…[5]
…[6]
‘N’
‘U’
‘L’
‘\0’
‘S’
‘O’
‘H’ …

7. Analysis of visitype.c
There are a few characters which can't be placed in the quoted initialization string directly, for example " and \.
"∆∆"\0" (where " is hex 22)
Problem is that " as a character will be interpreted as the end of string. We need to indicate that it is quoted, that it is to be taken literally by the compiler as an ASCII value, and the way to do that is precede it by a \. 
"∆∆\"\0" (where " is hex 22)
Now there seem to be five chars in that string, “∆∆\"\0”
But there are not, \" is a single character value, just as \0 is

8. Analysis of visitype.c
What other characters MAY need the same special treatment?
See K&R page 193.
Good page to mark for open book tests!
Do all of these require special treatment here?
No, which one other than \“ needs it?
Answer: (write in here) _________

9. Analysis of visitype.c Access the strings in the array (i = 0 to 127)
asciiname[4*i]
Array index 4*i indicates start of each string
However, printf needs a pointer to a string
&asciiname[4*i]
The “&” is the “address of” operator
Take on faith for now - more on pointers later
Part 4 of hw2 is histogram. Means to count every characters appearance. So you have a file and you will build a histogram for it

10. More on Debugging 2 ways to debug a program:
Use printfs
Insert printfs in multiple places in your program and print out intermediate values
Use gdb debugger
A professional programmer uses a debugger, rather than putting in lots of printf statements to track down a bug.
A quick gdb reference guide is posted here:
gdb-refcard.pdf
You don’t need to say the memory location for the variables, you should just use the name of them

11. Use of the gdb Debugger Start with the correct compiler options:
gcc -g vt.c -o vt
Type the following to run the program:
gdb vt
Gives message:
Ready to run -- not yet running.
creates an executable that has debugging info, e.g.
- data type for variables/functions
- correspondence between line # and addresses
You have the same executable file vt

12. Use of the gdb Debugger(cont’d)
Want to interact with running program, not letting it run free. To set a break point at main(), type:
b main
break at main()
program vt is not running yet
To run, type:
r <vt.in
run, taking stdin from vt.in
Will stop when encounters main() in program execution -- often lot of things get done first.
Now can single step through program, s or n (skip entering functions), put out values of variables.
You don’t need to edit run edit run…
Break point means the program will execute until that point. So it will stop at main and ask you what the next command is
Now you have some choices:
You can continue running
You can do single stepping. One statement at a time. It executes the next statement and then stop
Explain different of s and n
When you get to a func, when you type “s” it will go to the function and by each “s” it will execute one sentence in the function
But you have troubleshooted the function and you know it works well, type “n”, it will execute the entire function and return to the next statement

13. Use of gdb Examples of gdb commands: p i (print value of variable i)
p i=2 (set the variable i to 2 and print it)
p 3*i (print value of expression 3*i)
p/x i (print in hex format value of variable i)
set variable i=5 (set the variable i to 5 without printing)
i lo ("info" - give values of all local variables)
h (help -- pretty good messages -- lists topics)
h topic (help on named topic)
h p (help on command p for printf)
q TO QUIT (leave debugger)

14. Use of gdb (cont’d) More complex gdb commands in Reference Guide.
Setting breaks/conditional breaks at line numbers:
b 36
b fn.c:22 if i = = 3
Getting line numbers from "list" or "l" command:
l 22 print 10 lines around line 22 in main
l after listing some lines, then l means next 10 lines
i b to get info on breakpoints
d 3 to delete bkpt 3
c for continue after bkpt encountered
Show some demos