Introduction: C Pointers

1
Introduction C Pointers

• Day 2
• These Slides NOT From Text.

2
Constant Declarations

• Up to now we have used define to define a named
  constant.
• This depends on the pre-processor.
• Another way is to define a constant inside the
  regular code.
• e.g. const float PI 3.14159
• PI 3.0 is now illegal!

3
Constant Declaration (cont.)

• The keyword const indicates a variable that
  doesnt change.

4
const Restrictions

• Constants must be initialized when they are
  defined.
• Constant values can never be changed.
• I.E. They can never appear on the left of an
  assignment operator,

5
const vs. define

• C checks the syntax of const statements
  immediately. The define directive is not
  checked until the macro is used.
• const uses C syntax, while define has a syntax
  all its own.
• const follows normal C scope rules, while
  constants defined by a define directive continue
  on forever.

6
const vs. define (cont.)

• The define directive can only define simple
  constants. The const statement can define almost
  any type of C constant, including things like
  structures.
• The define directive is essential for things
  like conditional compilation and other
  specialized uses.

7
Conditional Compilation

• One use is to comment out a bunch of code.
  Suppose you had

i12 / This is a one line comment / J 15
/ This is a comment at the end of a line / K
-10 X sqrt (102.34) / Another one line
comment /
8
Conditional Compilation

• You would like to eliminate all this code
  temporarily. Comment before and after? No, that
  doesnt work.

/ i12 / This is a one line comment / J 15
/ This is a comment at the end of a line / K
-10 X sqrt (102.34) / Another one line
comment / /
Terminates the comment here.
9
Conditional Compilation

• You can use a pre-processor statement to do this
  easily.

ifdef _RAY_01 i12 / This is a one line
comment / J 15 / This is a comment at the
end of a line / K -10 X sqrt (102.34) /
Another one line comment / endif
None of this will be compiled into your program
because you havent ever defined a shell variable
called _RAY_01
10
Conditional Compilation

• Want to quickly turn on all the segments so
  marked? Simply define _RAY_O1.

define _RAY_01
ifdef _RAY_01 i12 / This is a one line
comment / J 15 / This is a comment at the
end of a line / K -10 X sqrt (102.34) /
Another one line comment / endif
All this will be part of your program. And, any
other section marked with _RAY_01.
11
Other Base Constants
Base 10 Base 8 Base 16
6 06 0x6
9 011 0x9
15 017 0xF
12
const Pointers

• const char answer_ptr Forty_Two
• Does NOT tell C that the variable answer_ptr is a
  constant! Instead, it tells C that the data
  pointed to by answer_ptr is a constant.
• The data cannot be changed, but the pointer can.

13
Pointer Is Constant

• If we put the const after the , we tell C that
  the pointer is constant.
• char const name_ptr Test
• The data can be changed, but the pointer cannot.

14
Or Both Unchangeable

• To make them both constants, put two const in.
• const char const title_ptr Title
• The data cannot be changed, and the pointer
  cannot be changed.

15
But WHY?

• I said newer programs like Ada and Java provide
  automatic protection for the programmer. Similar
  to guards and safety switches on a table saw.
• I also said C was like a spinning blade in space.
• Using const carefully can protect your code from
  inadvertent side effects.

16
Side Effects?

• One of the worst habits of people my age is being
  proud of their mastery of Cs side effects! Look
  how clever I am! However, no one can read the
  code!!!
• Read the enrichment puzzles for examples of
  these.
• But, we have looked at some, the increment and
  decrement operator.

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variable other_variable

• Main effect, assign the value of other_variable
  to variable.
• Side effect, increment other_variable.

18
if(variable expression) x0

• Main effect assign 0 to x if the value of the
  assignment is true (! 0).
• Side effect assign the value of expression to
  variable.
• Side effects are sort-of hidden actions taken by
  the language.
• Try to avoid them. If you use them, add comments!

19
Compiler Options

• Depend on the compiler, but many similar ones.
• Added on the command line, after the gcc command,
  before the file names.
• gcc options file_names
• Youve seen a couple
• -c compile only.
• -o define output run image file name.

20
Other Useful Compiler Options

• -g insert debugging statements.
• -Wall Print all warnings. Provides cleaner
  executing code.
• -E preprocess only and produce the results of
  all preprocessor directives.

21
Lab 5 Hints

• Use a separate file for each function.
• lab5.c
• get_problem.c
• get_rate_drop_factor.c
• get_kg_rate_conc.c
• get_units_conc.c
• fig_drops_min.c
• fig_ml_hr.c

• by_weight.c
• by_units.c
• proto.h
• These are small functions.
• Some have pass by reference (pointer) parameters.
  (book calls them output)

22
Lab5 Intermediate Makefiles

• Use Makefile variables

OBJSlab5.o get_problem.o get_rate_drop_factor.o
\ get_kg_rate_conc.o get_units_conc.o
fig_drops_min.o \ fig_ml_hr.o by_weight.o
by_units.o lab5 (OBJS) gcc o lab5
(OBJS) Lab5.o lab5.c proto.h gcc c
lab5.c get_problem.o get_problem.c proto.h gcc
c get_problem.c
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Lab5 Sample Input File
1 150 15 2 8 3 0.6 70 1 4 1000 25 5
24
Lab5 Sample Input File
1 150 15 2 8 3 0.6 70 1 4 1000 25 5
Problem 1
Problem 2
Problem 3
Problem 4
Sentinel -gt quit!
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Lab5 Sample output

• You test your lab by running it from the screen
  and keyboard.
• Then, when its working, redirect BOTH input and
  output.
• lab5 lt MyInput gt MyOutput
• MyOutput will NOT have the responses to the input
  prompts, just the prompts all run together.

26
Lab5 Matching Exactly!

• A little pain for you.
• Saves a big pain for us reading 65 submissions!
• See web page write-up, I added a sample output
  which you can easily count blanks.