Codelab

1
Codelab

• A very useful adjunct to this course is Codelab,
  which is run by a group called Turingscraft.
• You are required to register with them and do
  assigned, online exercises.
• Codelab keeps track of which exercises you have
  done, the number correct, etc.
• It starts with very basic exercises and
  progresses from there.
• Here is what you need to do (as specified by the
  Codelab people)
• If you are a new user, after you log in for the
  first time, you should click PREFERENCES to
  change your password.
• PREFERENCES also will let you set a nickname to
  use for your username instead of your email
  address, and will let you set screen size
  dimensions for your CodeLab.

2
Codelab

• REGISTRATION FOR STUDENTS
• go to www.turingscraft.com
• click "register" in the upper right
  corner
• choose "I am a student in a course ..."
  and click CONTINUE
• enter the Section Access Code
• SOUFLO-8287-2420
• and click CONTINUE
• continue filling out the forms being
  careful to enter
• a VALID email address and first and
  last names
• (these will appear in the
  professor's roster)

3
Codelab

• Once you register, you will appear on the course
  roster and can log in, access the whole CodeLab
  and submit solutions to up to 10 exercises.
• To go beyond 10 you need to get full access as
  described below.
• These "freebies" make it possible for you to do
  CodeLab exercises on Day 1, even though it is
  still the "add/drop" period.
• LOGIN FOR EVERYONE
• go to www.turingscraft.com
• click "login" in the upper right corner
• the username is the email address given
  during registration
• the password is the password selected
  during registration
• GETTING FULL ACCESS
• log in to CodeLab
• click LOBBY
• click the button "Get Full Access"
• follow the directions

4
Odds and Ends

• How to write character constants using their
  ascii code
• \N, where N is an octal constant
• \xN where N is a hexadecimal constant

5
Odds and Ends

• Format specifiers for printf
• Code Format
• c chard signed decimal integeru unsigned
  decimal integero unsigned octalx unsigned
  hexadecimal, using lowercase charactersX unsigne
  d hexadecimal, using uppercase characters
• f decimal floating pointe scientific notation
  using lowercase eE scientific notation using
  uppercase Eg uses f or g, whichever is
  shorterG uses f or G, whichever is shorter
• s string
• the character
• Modifiers ld long decimal int hd short
  decimal int Lf long floating pt

6
Formatted Input

• The scanf function is used for formatted input
• This means that the function recognizes the type
  of the quantity to be input and automatically
  converts the appropriate character string to the
  binary representation of the value
• scanf uses format specifiers similar to printf.
• Examples
• To read an integer value into int variable i
  scanf("d",i)
• The use of the will be explained later. i
  effectively means the location assigned to the
  variable i
• The only time you will not use the notation is
  for strings
• char name10 scanf("s",name)
• The reason is that the compiler thinks of an
  array name as its location

7
Formatted Input

• Format specifiers for scanf
• Code Format
• c read a single characterd read a signed
  decimal integeru read an unsigned decimal
  integero read an octal numberx read a
  hexadecimal number
• f read a floating point number (type
  float)g read a floating point number (type
  float)lf read a floating point number (type
  double)lg read a floating point number (type
  double)Lf read a floating point number (type
  long double)Lg read a floating point number
  (type long double)
• s read a string (stops at any white-space
  character)
• Note the function gets() reads a string until a
  newline character is found
• Also scanf skips over blocks of white-space
  characters

the modifiers l (long) and h (short) may be used
here
8
Functions

• Functions are the primary means of structuring
  programs in C
• A function is a named subroutine
• It accepts a number of arguments, processes
  them, and (optionally) returns a result
• Functions also may have side effects, like I/O
  or changes to global data structures
• In C, any subroutine is called a function,
  whether it actually returns a result or is
  only called for its side effect
• Note A function hides its implementation
• To use a function, we only need to know its
  interface, i.e. its name, parameters, and
  return type
• We can improve the implementation of a function
  without affecting the rest of the program
• Functions can be reused in the same program or
  even different programs, allowing people to build
  on existing code

9
Function Definition

• A function definition consists of the following
  elements
• Return type (or void if the function does
  not return a value)
• Name of the function
• Parameter list
• Function body
• The name follows the same rules as variable names
• The parameter list is a list of coma-separated
  pairs of the form lttypegt ltnamegt
• The body is a sequence of statements included in
  curly braces
• Example
• int timesX( int number, int x) return
  xnumber

Function Prototype
10
Function Declaration

• A function declaration consists of the function
  prototype followed by a semi-colon
• A function may be declared in one place and
  defined later in the code
• A function declaration (or definition) must be
  declared before any call to the function is used.
• Of course, the function definition must be found
  somewhere either in the same file or in an
  external file (more later)
• Example
• int timesX( int number, int x) function
  declaration
• int main( )
• int timesX( int number, int x) return
  xnumber

Other code
Function definition
11
Function Calls

• A function is called from another part of the
  program by
• writing its name, followed by
• a comma separated list of arguments in
  parentheses
• Each argument
• is a variable or an expression such that
• the type of each argument matches that of the
  corresponding parameter of the function
• If a function is called, control passes to the
  code for the function
• The parameters are treated as local variables
  that have been initialized to the values of the
  arguments to the call
• Control returns from the function to the
  statement after the function call when
• the execution reaches the end of the function
  body or
• a return statement is executed
• A return statement may have a single argument of
  the same type as the return type of the function.
  
• If the statement is executed, the argument of
  return becomes the value returned to the caller

12
Example Printing Character Frequencies
13
Example Printing Character Frequencies (contd)

• Assume that the previous function definition is
  inserted into the frequency counting program just
  in front of the int main(void) line
• We can then modify main as follows
• ...
• for(i0 ilt128 i)
• if (isprint(i))
• print_freq(i, freq_counti)
• return EXIT_SUCCESS
• The program will then print frequency histograms
  instead of just numbers

14
Exercise

• Rewrite the Fahrenheit2Celsius Program to use a
  function for the actual conversion

15
Assignment

• A prime number is an integer greater than 1 that
  is evenly divisible only by 1 and itself
• 1. Write a function isprime() that determines if
  an integer number is prime.
• You can use the modulus operator (remainder
  operator on integers) or work with plain
  division. Use your function to implement a
  simple program primes that prints all primes
  between 2 and 10000.

16
Assignment

• 2. The Sieve of Erathostenes
• a more efficient (and ancient) algorithm for
  finding all primes up to a given number.
• It starts with a list of all numbers from 2 to
  the desired limit.
• It traverses this list, starting at two.
• Whenever it encounteres a new number, it strikes
  all multiples of it from the list.
• What remains at the end is a list of prime
  numbers.
• Example
• Initial list 2 3 4 5 6 7 8 9 10 11 12 13 14 15
  16
• Striking multiples of 2 2 3 5 7 9 11 13 15
• Striking multiples of 3 2 3 5 7 11 13
• (There are no multiples of any remaining number,
  so we skip to the end
• Use the Sieve algorithm in a second program,
  primes_sieve, that prints all primes between 2
  and 10000. Hint Use an array!

17
Example Reading Integers

• We want to write a function that reads a positive
  integer number from stdin, using only getchar()
• A number is defined as a sequence of decimal
  digits, i.e., characters from the range 0 to
  9
• We can use the function isdigit(c) from
  ctype.h to test if a character is a decimal
  digit
• The C standard guarantees that 0 to 9
  have consecutive ascii values.
• We can thus get the value of a single
  character c that represents a digit by the
  integer expression c -0
• Idea We read the most significant digits first.
  So whenever we read a new digit, the
  value of what we have read so far increases
  10-fold
• Read Value
• 1 1
• 13 101 3 13
• 137 1013 7 137
• 1375 10137 5 1375

18
Example read_int10(void)
19
Example Improved Function

• read_int10(void) works fine, but can only read
  number in decimal notation
• We want to have a function that can read numbers
  in any base between 2 and 10
• Examples
• 142 in base 8 has the value 182 481 280
  164 48 2 98
• 101010 in base two has the value
  125024123022121020 32 8 2 42
• 1873 is not a valid number in base 6! All
  digits have to be smaller than the base
• The principle for our modified function is the
  same as for read_int10(void), we just use a
  parameter base instead of the hardwired value 10!

20
Do We Have a Valid Digit?
21
Reading a Number in Any Base ? 10
22
Build General Functions!

• Good programs are build by breaking the task into
  many functions that are
• Small at most one screen page (in your
  favourite editor)
• Simple they only do one thing, and they do
  that well
• General so that they can be reused at
  other parts in the program
• Going from general to specific is (generally)
  easy

23
Recursive Functions

• As we stated above, functions can call other
  functions. They can also call themselves
  recursively
• A recursive function always has to handle at
  least two cases
• The base case handles a simple situation
  without further calls to the same function
• The recursive cases may do some work, and in
  between make recursive calls to the function
  for smaller (in some sense) subtasks
• Recursion is one of the most important
  programming principles!

24
Example Printing Integers

• We now want to print positive integer numbers to
  stdout, using only putchar()
• Consider a number in base 10 421 4210 1
• We can split the task into two subtasks
• Print everything but the last digit
  (recursively)
• Print the last digit
• Base case There are no digits to print any more
• Basic operations
• To get the last digit, we use the modulus
  operator
• To get rid of the last digit, we divide the
  number by the desired base
• remember integer division truncates

25
Example Decimal Representation of 421

• Lets do an example we want to print the number
  421 in base 10
• Step 1 42110 1 and 421/10 42.
• Hence the last digit to print is 1
• Before that, we need to print 42
• Step 2 4210 2 and 42/10 4. The second
  last digit is 2, the rest is 4
• Step 3 410 4 and 4/10 0. The next digit
  is 4
• Step 4 Since the upper part of the number is
  0, the recursion stops and there is
  nothing to do but printing the digits in the
  right order
• The same principle applies for other bases (just
  replace 10 by your base)

26
Writing a Number in Any Base ? 10
27
Putting Things Together a Simple Base Converter

• We now use the defined function to write a
  program that reads two integers number and base.
• number is considered to be a decimal number
• base should be a decimal number between 2 and
  10 (inclusive)
• number and base are separated by a single,
  arbitrary character
• The program terminates, if one of the numbers
  is invalid
• Otherwise, it prints the value of number in the
  indicated base

28
The Base Converter
29
Example Outputs for the Base Converter
30
Exercises

• 1. Extend the base converter to work with base
  16, using 0-9 and A-F as digits (allow both
  upper and lower case!)
• Extend the base converter to accept triplets
  input_base, value, output_base,where
• value is interpreted in input_base
• input_base is a single hexadecimal digit gt2 and
  
• output_base is a single hexadecimal digit gt2
• Add reasonably robust error handling!