Introduction to C Programming K Broerman Mentor, Team 868 TechHounds December, 2005

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Introduction to C Programming K
BroermanMentor, Team 868 TechHoundsDecember,
2005
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Disclaimer!

• This introduction to C will not make you a
  computer scientist! 0
• However, after this overview you should be able
  to read and understand some of the default FRC
  code. )

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Some Wisdom (no extra charge!)

• "Computer science is no more about computers than
  astronomy is about telescopes."
• E. W. Dijkstra

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A Very Simple C Program

• / This program will print out the message This
  is a C program /
• include ltstdio.hgt / find and use the stdio
  header file /
• main()
• printf ("This is a C program\n) / \n means
  add a newline (linefeed) /

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Files and what they do

• Header files (.h)
• Header files are used for defining macros a.k.a.
  aliases, and prototyping methods, and can also be
  used for defining structs. Macros are basically
  a way of referencing an unknown variable, in FRC
  programming these commonly reference inputs and
  outputs so that when a device is hooked up to a
  different port only 1 line needs to be changed
  instead of a very large number of lines. A
  method prototype is part of the C language and is
  used to let the compiler know what functions can
  be called in your C file. The function prototype
  requires one line of code, which states the name
  of the function, what the function returns and
  what must be passed into the function. Structs
  are sort of like data types but they can hold
  multiple variables when they are used.

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Files and what they do

• C Source Files (.c)
• C files contain the bulk of your code. This is
  where the heart and soul of the operations are
  housed. All the functions that were prototyped
  in your header files are used here.

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Some C Program Syntax (Nuts Bolts)

• Language Component Examples
• pre-processor statements include ltstdio.hgt
• define PI 3.14159
• keywords if, then, return, switch,
• built-in functions ,-,,/,!,,,,,
• variable definitions int motor_speed
• function definitions float square_root(float x)
• executable statements motor_speed 100
• conditional statements if (motor_speed gt 100)
  
• ANSI library functions printf, memcpy, malloc,
• comments / clip motor speed to prevent
  over-current condition /

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C Pre-Processor

• If first column contains , indicates
  pre-processor command
• Examples
• include ltstdio.hgt / find and use stdio.h /
• define PI 3.14159 / this is a MACRO defn /
• if 0 / conditional compile switch /
• //compiler will skip this code
• else
• //compiler will compile this code
• endif

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C Comments

• 2 forms
• / this is a multi-line comment /
• / now is the time for all good men to come
• to the aid of their country /
• // this is a one-line C style comment
• // you need double slashes on each line
• // for multi-line comments
• // like this

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C Syntax Rules (partial list)

• Every executable statement must end with a
  semicolon
• White space (tabs, etc) is ignored by compiler
• Keywords are reserved
• this will generate a compiler error int
  switch / front panel switch /
• C is case-sensitive!

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C Variable Types

• Variable names are arbitrary (with some
  compiler-defined maximum length, typically 32
  characters).
• C provides the following standard variable types
  
• int ? integer variable
• unsigned int ? integer variable
• short ? short integer
• long ? long integer
• float ? single precision real (floating point)
  variable
• char ? character variable
• unsigned char ? character variable

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Integer Data Type Sizes and Limits (MPLAB C18)

• Data Type Size (bits) Minimum
  Value Maximum Value
• char 8 -128 127
• unsigned char 8 0 255
• int 16 -32768 32767
• unsigned int 16 0 65535
• short 16 -32768 32767
• unsigned short 16 0 65535
• short long 24 -8,388,608 8,388,607
• unsigned short long 24 0 16,777,215
• long 32 -2,147,483,648 2,147,483,647
• unsigned long 32 0 4,294,967,295

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Fun with Math

• You must consider storage size and usage when
  selecting variable types!
• unsigned char counter 100 // uchar ranges from
  0.. 255
• counter counter 150 // counter now equals
  250
• counter counter 50 // counter now equals 44!
  
• int distance 32767 // int ranges from
  -32768.. 32767
• distance distance 1 // distance now equals
  -32768!

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Floating Point Data Type Sizes and Limits(MPLAB
C18)

• Type Size (bits) Minimum Value Maximum
  Value
• float 32 1.17549435e - 38 6.80564693e 38
• double data type ? same as float on C18 compiler

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So Why Not Use Floats for Everything?

• Floating point arithmetic is 100x slower than
  integer arithmetic! (no FPU on PIC 18F8520 uC)
• Your FRC code will not run once the total code
  execution time exceeds the FRC main loop timer
  (26 mS)!
• FRC controller contains only 2 kB of data RAM

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Array Variables

• Arrays are defined using square brackets as
  follows
• define NUM_EMPLOYEES 10
• define MAX_NAME_LENGTH 256
• int employee_idNUM_EMPLOYEES
• int employee_salaryNUM_EMPLOYEES
• char employee_nameNUM_EMPLOYEESMAX_NAME_LENGTH
  //2-D array
• memcpy (employee_name2, "Ronald McDonald", 15)
• employee_id2 10023
• employee_salary2 85000 / ooops! /

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Conditionals

• Conditionals are used within the if and while
  constructs
• if (conditional_1)
• ...block of statements executed if
  conditional_1 is true...
• else if (conditional_2)
• ...block of statements executed if
  conditional_2 is true...
• else
• ...block of statements executed otherwise...

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Conditionals

• Conditionals are logical operations involving
  comparison of quantities (of the same type) using
  the conditional operators
• lt less than
• lt less than or equal to
• equal to
• ! not equal to
• gt greater than or equal to
• gt greater than

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Boolean (True/False) Operators

• There are 3 boolean operators
• logical AND
• logical OR
• ! logical NOT (inversion)
• Example usage
• fun_weekend !homework !chores
• in_trouble broke_window spilled_milk
• good_student (homework_done !daydreaming)
  have_apple

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Functions

• Functions allow large programs to be partitioned
  into smaller blocks, each of which is easier to
  write, read, and maintain.
• A function has the following layout
• return-type function-name ( argument-list-if-neces
  sary )
• ...local-declarations...
• ...statements
• return return-value

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Functions

• Functions as stated earlier must be prototypes
  in a header file and can have a number of
  different attributes to them so here is your
  explanation of what these functions do and what
  they are used for. First here is an example of a
  method with a breakdown of what every part does
  
• void My_Function(unsigned char x1, unsigned
  char x2)
• pwm03 x1 x2
• return pwm03
• My_Function(1, pwm02)
• The first word void is what the function
  returns, this can be a data type or it can be
  void which means nothing. The second word is the
  name of the function and is what the user will
  need when they call the function. Inside the
  parenthesis are the arguments, which are
  variables that the function needs in order to
  execute and the code for the function is inside
  the brackets. You can have as many arguments
  passed into a method as you want but it is not
  recommended that you have a function that
  required more than three, for the sake of code
  readability.

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Symbolic Constants

• // You can define constants of any type by
  using the define //
  compiler directive.
• define SPEED_MIN -100 // max reverse speed
  is -127
• define SPEED_MAX 100 // max forward speed
  is 127
• int rate_limit( int requested_speed )
• if (requested_speed lt SPEED_MIN)
• requested_speed SPEED_MIN
• else if (requested_speed gt SPEED_MAX)
• requested_speed SPEED_MAX
• return requested_speed

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Loops

• Most real programs contain some construct that
  loops within the program, performing repetitive
  actions on a stream of data or a region of
  memory. There are several ways to loop in C.
• Two of the most common are the while loop and the
  for loop
• while (expression-is-true)
• ...block of statements to execute...
• for (expression_1 expression_2 expression_3)
• ...block of statements to execute...

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Looping while

• The while loop repeats a statement until the test
  at the top proves false.

count 0 while(count lt 7) // do something
useful count count 1
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Looping while

• The while loop can also be used for waiting
• while( !rc_dig_in01 ) //limit switch 1
  closed?
• / energize motor! /

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Looping for

• The for loop can execute a block of code N times
  
• for (i0 i lt FILTER_SIZE-1 i)
• filter_valuei 0 / init array /
• for (month1 month lt 12 month)
• / calc days in month /

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Some C Syntax Shortcuts

• Example 1
• Standard x x 1
• Shortcut x 1
• Even Better x
• Example 2
• Standard x x 0xF
• Shortcut x 0xF

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Almost Done!

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Data Structures

• A data structure is a user-defined abstract data
  type.
• Used to group related data as a single unit
  (object).
• Programs containing data structures are
  conceptually easier to write, maintain, and
  understand.
• Algorithms (methods) are then written to
  perform operations on the abstract data type.

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Data Structure Example

• struct database
• int id_number
• int age
• float salary
• int main()
• struct database employee / There is an
  employee variable with modifiable
  variables inside it. /
• employee.age 22
• employee.id_number 1
• employee.salary 17500.21
• promote( employee, 5000.)

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C Coding Style

• For readability, use block indentation
• Use descriptive variable names
• avoid keiths_var 2.71828
• One executable statement per line
• avoid motor100 if (switch_off) goto end
  wait(ONE_SEC)
• Avoid highly nested function calls
• avoid sqrt(1(tan(1-hypot(PIangle/180) ltlt
  2 length).
• Provide useful comments
• avoid my_var my_var 1 //increment
  my_var

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Bad Style! Q What does this code do???

• include ltstdio.hgt
• main(t,_,a)
• char a
• /printf("main(d,d,\"s\")\n",t,_,a)/
• return!
• 0ltt?
• (
• ( tlt3? main(-79,-13,a main(-87,1-_,
  main(-86, 0, a1 ) a)) 1 ),
• ( tlt_? main(t1, _, a ) 3 ),
• ( main ( -94, -27t, a ) t 2 ?
• ( (_lt13) ? main ( 2, _1, "s d
  d\n" ) 9 )
• 16
• )
• )
• ( tlt0?
• ( tlt-72?
• main( _, t,

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Answer 12 Days of Christmas!

• On the first day of Christmas my true love gave
  to me
• a partridge in a pear tree.
• On the second day of Christmas my true love gave
  to me
• two turtle doves
• and a partridge in a pear tree.
• On the third day of Christmas my true love gave
  to me
• three french hens, two turtle doves
• and a partridge in a pear tree.
• On the fourth day of Christmas my true love gave
  to me
• four callaing birds, three french hens, two
  turtle doves
• and a partridge in a pear tree.
• On the fifth day of Christmas my true love gave
  to me
• five gold rings
• four callaing birds, three french hens, two
  turtle doves
• and a partridge in a pear tree.

On the eighth day of Christmas my true love gave
to me eight maids a-milking, sevean swans
a-swimming, six geese a-laying, five gold
rings four callaing birds, three french hens,
two turtle doves and a partridge in a pear
tree. On the ninth day of Christmas my true love
gave to me nine laadies dancing, eight maids
a-milking, sevean swans a-swimming, six geese
a-laying, five gold rings four callaing birds,
three french hens, two turtle doves and a
partridge in a pear tree. On the tenth day of
Christmas my true love gave to me ten lords
a-leaping, nine laadies dancing, eight maids
a-milking, sevean swans a-swimming, six geese
a-laying, five gold rings four callaing birds,
three french hens, two turtle doves and a
partridge in a pear tree. On the eleventh day of
Christmas my true love gave to me eleven pipers
piping, ten lords a-leaping, nine laadies
dancing, eight maids a-milking, sevean swans
a-swimming, six geese a-laying, five gold
rings four callaing birds, three french hens,
two turtle doves and a partridge in a pear
tree. On the twelfth day of Christmas my true
love gave to me twelave drummers drumming, eleven
pipers piping, ten lords a-leaping, nine laadies
dancing, eight maids a-milking, sevean swans
a-swimming, six geese a-laying, five gold
rings four callaing birds, three french hens,
two turtle doves and a partridge in a pear tree.
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Common C Gotchas

• Forgetting during equivalence checking
• if (test_score 93) / WRONG! sets test_score
  to 93 /
• printf (Hooray\n)
• if (test_score 93) / correctly compares
  test_score / printf (Hooray\n)
• Best Method of Prevention
• if (93 test_score)
• printf (Hooray\n)
• C is case sensitive!
• unsigned char wheel_count / NOT the same as
• unsigned char Wheel_count /

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The best way to learn C is to write a program
that does something useful!Happy Programming!