C

1
C
2
About the Crash Course

• Cover sufficient C for simple programs
• variables and statements
• control
• functions
• arrays and strings
• pointers
• Slides and captured lecture (video and sound) are
  available at
• www.cim.mcgill.ca/jer/C/

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About the Crash Course

• Next installments
• part II Wednesday, November 24, 1630-1800
• part III Thursday, November 25, 1800-1930
• Reference
• Kernighan Ritchie "The C programming
  language", 2nd edition. Prentice-Hall, 1988.

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Why C?

• C is not always a good choice ...
• Safety-critical systems
• Component-based programming / RAD
• ...
• ... but in many applications you do want C
• Legacy systems
• Speed, memory requirements
• Low-level programming (close to the machine)
• ...

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Your First C Program

• "The only way to learn a new programming language
  is by writing programs in it" KR
• / A simple program that prints something /
• include ltstdio.hgt
• main ()
• printf ("Hello, world!\n")

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Declarations

• Example
• float x
• double d 5
• int p, i, a100
• char s21
• Syntax
• type variable_name, ... value
• Rules
• declarations must precede executable statements
• int type may be modified by long, short,
  unsigned

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Changing Variable Values

• Example
• int x, y, z
• x 2
• x x 1
• Getting Fancy
• y z 4 5
• x 1
• x
• x
• y x--
• Note
• assignment statements return value, which may or
  may not be ignored same goes for increment
  statements

8
Formatted Output

• Example
• int i 10
• float f 2.5
• char s "hi"
• printf ("Jack\'s integer is d\n", i)
• printf ("Jill\0x27s float is f\n", f)
• printf ("My string s s\n", s)
• Syntax
• printf (string_with_formatting, var1, var2, ...)
• Formats d integer, f float, c character, s
  string,
• Note "escape sequences" \n newline, \' quote,
  \0x27, etc.
• include ltstdio.hgt is compulsory more about it
  later

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Formatted Input

• Example
• include ltstdio.hgt
• int i
• float f
• scanf ("d f\n", i, f)
• / inputs an integer and a float /
• Syntax
• scanf (string_with_formatting, var1, var2,...)
• Note
• The ampersand () is necessary because scanf
  modifies the values stored in the respective
  variables by comparison, printf only uses the
  values, without modifying them. More about this
  later

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I/O Example

• What does this print?
• include ltstdio.hgt
• main ()
• int n
• float x
• char mark
• scanf ("d f c", n, x, mark)
• printf ("Of d s,\nf got c\s\n",
• n, "students", x, mark)
• Type in the following input 86 85.999 A

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I/O Example

• include ltstdio.hgt
• main ()
• int n
• float x
• char mark
• scanf ("d f c", n, x, mark)
• printf ("Of d s,\nf got c\s\n",
• n, "students", x, mark)
• Input
• 86 85.999 A
• Output
• Of 86 students,
• 85.999001 got A's

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Conditional Statements

• Example
• if (age lt 0)
• printf ("warning negative age\n")
• age -age
• Syntax
• if (condition) statement
• if (condition) statement else statement
• Rules
• the condition is an int ! (no booleans)
• parentheses required around conditional
  expression
• use curly braces to make a compound statement

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More Conditionals

• Example
• if (x lt 0)
• printf ("x is less than 0\n")
• else if (x 0)
• printf ("x is equal to 0\n")
• else
• printf ("x is greater than 0\n")
• Whats wrong with this?
• if (x lt 0)
• if (y lt z)
• printf ("y is less than z\n")
• else
• printf ("x not less than 0\n")

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While Loops

• Example
• / print "hi" forever /
• while (1)
• printf ("hi")
• Syntax
• while (condition)
• statement
• Rules (again)
• the condition is an int ! (no booleans)
• parentheses required around conditional
  expression
• use curly braces to make a compound statement

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For Loops

• Example
• / print "hi" three times /
• int i / i continues to exist when loop ends /
• for (i 0 i lt 3 i)
• printf ("hi")
• Syntax
• for (statement1 condition statement2)
• statement3
• Equivalent to
• statement1
• while (condition)
• statement3
• statement2

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Loop Example

• / print squares up to 100 /
• main ( )
• int j, up 100
• for (j 0 j j lt up j)
• printf ("d \n", j j)
• Note
• cant do for (int j 0 ...
• waste of one multiplication per iteration
• can you make it more efficient?

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Example (contd)

• / print squares up to 100 /
• void main ( )
• int j, up 100, sq
• for (j 0 (sq j j) lt up j)
• printf ("d \n", sq)
• Note
• recall equivalence to a while loop condition is
  evaluated before the loop body

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Arrays

• int years45
• float temperatures 11
• void main ()
• years0 2000
• temperatures11 -45.67
• Rules
• indices start at zero
• maximum valid index is the size of the array
  minus 1
• but C lets you go beyond the declared boundaries
• temperatures11 is illegal

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Characters

• Characters
• char a, b, c1, c2
• a '0' b '\037' c1 'K' c2 c1 1
• Assigns values 48, 31, 75, 76
• The sequences '0',...,'9', 'a',...,'z',
  'A',...,'Z' contain characters numbered
  consecutively
• Casting
• printf ("c d\n", c1, (int) c1)
• Outputs K 75

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Strings

• Strings are '\0'-terminated arrays of char
• char s3 "hi" / invisible '\0' /
• char t3 'h', 'i', '\0'
• String operations
• include ltstring.hgt
• strlen ("there") / returns 5 /
• strcpy (s, t) / copy t to s /
• strcmp (s, t) / alphabetical comparison /

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Exercise Caesar's code

• A simple code used by Caesar in the Gallic wars.
• Input sequence of capital letters
• Output another sequence of capital letters
  obtained by shifting each letter in the original
  sequence three places in the alphabet. Note
  shifting wraps around.
• Example KENNEDY -gt NHQQHGB

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Functions

• / Increment takes an integer argument and
• returns the argument plus one.
• /
• int incr (int i)
• int j
• j i 1
• return j
• main ()
• int k, m 4
• k incr(m)
• printf ("k d, m d\n", k, m)
• output k 5, m 4

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More about Functions

• might have no return type, and no return
  statement
• void printhi ()
• printf ("hi\n")
• parameters are copied and can be modified
• int incr (int i)
• i
• return i
• default (unspecified) return type is int

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Variables within Functions

• But this does not work
• void no_incr (int i)
• i
• void main ()
• int x 5
• no_incr(x)
• printf ("d\n", x)
• beware that modifications are on internal copies
  of the parameters.
• note void main() since main does not return a
  value

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Exercise

• Write a function that checks whether a sentence
  has 'title case'
• Arguments sentence string
• Assume the string consists of letters and blanks
  only.
• Return true iff each word in the sentence starts
  with a capital letter and continues with
  lowercase letters.