Variables

1
Variables

• Names are made up of letters and digits
• First character must be a letter
• underscore _ counts as a letter
• Upper case and lower case letters are distinct,
  so a and A are two different variable names, so
  as Tom and tom
• Traditionally, lower case for variable names, and
  all upper case for symbolic constants
• You cannot use keywords as variable names
• Use meaningful variable names
• use count, instead of x for counting
• use year, instead of y for keeping track of
  the year

2
Variable names and keywords

• More examples on meaningful variable names
• Sum, Total_amount, radius, Room801, average,
  standard_dev, StandardDev, Std
• Examples of illegal variable names
• price, 1stclass, day of the year, int
• Keyword-reserved identifiers
• auto, break, case, char, const, continue,
  default, do, double, else, enum, extern, float,
  for, goto, if, int, long, register, return,
  short, signed, sizeof, static, struct, switch,
  typedef, union, unsigned, void, volatile, while

3
Data types and their limitations

• char a single byte, capable of holding one
  character, we usually refer to ASCII character
  set (127 char)
• int an integer. For our system, an integer uses
  4 bytes, therefore, it has a range of -2G, 2G-1
• float floating point (single-precision), uses 4
  bytes and therefore has a range of 10e-38 to
  10e38
• double floating point (double-precision) 8
  bytes from 10e-308 10e308
• qualifiers short, long, signed, unsigned
• For their usages, refer to any C Manual or
  References.
• In this class, we seldom use the above qualifiers.

4
Declarations

• All variables must be declared before use
• A declaration specifies a type
• int lower
• char c
• float total_cost
• int lower, upper
• A variable may also be initialized in its
  declaration
• int lower 0
• char esc \\
• float total_cost 2000.0
• For constant, the qualifier const can be
  applied to the declaration of any variable to
  specify that its value will not be changed
• const double PI 3.141592653548979
• const double e 2.71828182845905

5
Assignment statement

• Assigning a value or an expression to a variable
• a 5 / assign a value 5 to variable a /
• 5 a lt-- Wrong (syntax error), you cannot
  assign anything to a number
• Be careful about the R.H.S and the L.H.S
• int a 5 int a 5
• int b 10 int b 10
• a b b a
• The results are different!!
• Swapping the values between two variables
• int a 5 int a 5
• int b 10 int b 10
• int temp
• temp a a a b
• a b b a - b
• b temp a a - b

6
Arithmetic Operators - / (modulus)

• Integer division truncates any fractional part
• The operator cannot be applied to float or
  double
• int x 27 int x 28
• int y 4 int y 4
• int result int result
• result x / y gt 6 result x / y gt 7
• result x y gt 3 result x y gt 0
• Precedence (unary) , - ---gt , /, ---gt
  ,-
• Precedence and associativity of operators (as
  Table 2-1)
• (fahr - 32) 5 / 9 (fahr - 32) / 9 5
• 20 20
• -12 -12
• -60 -1
• -6 -5

7
Be Careful on Integer Division!

• 25 / 2 2 gt 24
• 25.0 / 2 2 gt 25.000000
• t t / 2 not equal to t t / 2.0 (if t is odd)
• Float to integer is by truncation, not by
  round-off
• int x 27 int x 27 int x 27
• int y 4 int y 4 float y 4.0
• int result float result float result
• result x / y result x / y result x / y
• result is 6 result is 6.00 result is 6.75

8

• Relational Operators gt, gt, lt, lt, , !
• Relational operators have lower precedence than
  arithmetic operators. For example,
• i lt lim - 1 lt---gt i lt (lim - 1)
• Logical Operators ,
• Expressions connected by or are evaluated
  left to right, and evaluation stops as soon as
  the truth or falsehood of the result is known
• The precedence of is higher than that of ,
  and both are lower than relational and equality
  operators
• By definition, the numeric value of a relational
  or logical expression is 1 if the relation is
  true, and 0 if false
• The unary negation operator ! converts a
  non-zero operand into 0, and a zero operand into 1

9
Relational and Logical Operators (examples)

• int i 10 int i 1, j 2, k 3
• int j int x
• j i gt 5 x i lt 0 j gt 2
• j i lt 10 x i gt 0 j gt 2
• j i lt 10 x i lt 0 j gt 2
• j i 1 lt 10 x i gt 0 j gt 2
• float p 10.0 x (i gt 0) (j gt 2)
• float q 5.0 x (i gt 0) (j gt 2) (k
  3)
• j p gt 0.0 x (i gt 0) (j gt 2) (k 3)
• j p lt 0.0 A B (AB)(AB)!A !(AB) !AB
• j 0.0 lt p - q T T T T F F F
• j i 10 T F F T F T F
• j i 1 ! 10 F T F T T T T
• j i ! 10 1 F F F F T T F

10
Type Conversions (Basic Conversion in C)

• main()
• float f1 123.456, f2
• int i1, i2 -150
• char c a
• i1 f1 / i1 123 /
• f1 i2 / f1 -150.000000 /
• f1 i2 / 100 / f1 -1.000000 /
• f2 i2 / 100.0 / f2 -1.500000 /
• i1 5
• i1 10
• printf(d, i1) / i1 10 /
• i1 i1 6
• printf(d, i1) / i1 16 /

11
Increment and Decrement Operators

• i i 1 can be written as i or i
• In both cases, the effect is to increment
  variable i
• i increments i after its value has been used
• i increments i before its valie is used
• Why and what is the difference?
• int i 5, x int i 5, x
• x i x i
• printf(xd id\n, x, i) printf(xd
  id\n, x, i)
• Program output
• x5 i6 x6 i6
• Short-cut of
• x i gt x i x i gt i i 1
• i i 1 x i

12
Assignment Operators and Expressions

• Assignment operators , -, , /, , ...
• expression1 (expression1) operator
  (expression2) can be written as expression1
  operator expression2
• i i 2 can be written as i 2
• stock - 100 is equivalent to
• stock stock - 100
• Be careful, x y 1 means x x (y 1)
  not x x y 1
• Why I dont like it
• sum count gt sum count gt sum sum
  count
• count count 1 count
  count 1
• How about sum count ?

13
Control Flow

• Statements and Blocks
• An expression such as x 0 or i or printf(...)
  becomes a statement when it is followed by a
  semi-colon
• In C, the semi-colon is a statement terminator
• Braces and are used to group declarations and
  statements together into a compound statement, or
  block, so that they are syntactically equivalent
  to a single statement
• No semi-colon after that ends a block
• Statement statement or
• 
  statement1
• 
  statement2 . . .
• 
  statementN
• 
  

14
The IF Statement

• if (expression) statement1
• if (expression) statement1 else statement2
• The expression is evaluated If it is true,
  statement1 is executed
• If the expression is false, and if there is an
  else part, statement2 is executed
• If the IF statement is simply testing the numeric
  value of an expression, you can simplify the
  syntax
• if (expression ! 0)
• becomes
• if (expression)
• IF statements can be nested That means there is
  an IF statement inside an IF statement

15
A Simple Program on IF statements

• include ltstdio.hgt if (tall)
• main() if (heavy)
• printf(football player)
• int sex, height, weight else
• int tall, heavy printf(basket ball
  player)
• ..... else
• if (sex 1) if (heavy)
• printf(TV watcher)
• printf(He can be a ) else
• if (height gt 175) printf(ice skater)
• tall 1
• else
• tall 0
• if (weight gt 200)
• heavy 1
• else
• heavy 0

16
More on IF statements

• if (!heavy) printf(ice skater)
• else printf(TV watcher)
• is the same as
• if (heavy) printf(TV watcher)
• else printf(ice skater)
• Ambiguity in an IF statement
• if (n gt 0) if (n gt 0)
• if (a gt b) if (a gt b)
• z a z a
• else else
• z b z b
• Associating the else with the closest previous
  else-less IF statement
• If that is not you want, braces must be used

17
Nested IF statements

• include ltstdio.hgt
• main()
• int score
• printf(Please type in your score)
• scanf(d, score)
• printf(Your grade is a )
• if (score gt 90) printfA)
• else
• if (score gt 80) printf(B)
• else
• if (score gt70) printf(C)
• else
• if (score gt50) printf(D)
• else
• if (score gt 40) printf(D)
• else
• printf(F)

18
Switch Statement

• switch (expression)
• case const-expr1 statements1
• case const-expr2 statements2
• .....
• case const-exprN statementsN
• default statements(N1)
• The switch statement is a multi-way decision that
  test whether an expression matches one of a
  number of constant integer values, and branches
  accordingly
• default is executed if there is no match
• default is optional
• The break statement causes an immediate exit from
  the switch statement

19
switch VS. if

• Try to write a program that print the name of the
  operation for that operator
• if (op ) printf(addition)
• else if (op -) printf(subtraction)
• else if (op ) printf(multiplicat
  ion)
• else if (op /)
  printf(division)
• else if (op )
  printf(mod...
• when there are more operators like, !, , you
  can run outof space, and have to type on the next
  line. But for the switch statement, we can have
• switch(op)
• case printf(addition) break
• case - printf(subtraction) break
• case printf(multiplication) break
• ...
• default break

20
The break Statement inside switch

• int num_count 0, num_char 0
• char ch
• while ((ch getchar()) ! EOF)
• switch (ch)
• case 1
• case 2
• case 3
• case 4
• ...
• case 9
• case 0 num_count
• default num_char
• break

21
Loops---while statement

• Statement structure
• while (expression)
• statements
• As mentioned in the previous lectures,
• statementsgt statement or
• statement1
• statement2
• ...
• statementN
• The expression is evaluated
• If the expression is true (non-zero), the
  statements are executed and expression is
  re-evaluated
• Until the expression return false (becomes zero),
  at this point, the execution get out of the while
  loop
• You can use the break statement to jump out of
  the loop
• In general, frequent use of the break statement
  is bad programming

22
Examples on while Statement

• int i 0
• while (i lt 9)
• printf(i d\n, i)
• i
• char ch
• ch getchar()
• while (ch ! EOF)
• if (ch 4) break
• ch getchar()
• if (ch 4)
• printf(Bad number\n)
• else
• printf(Good number\n)
• Beware of infinite loops!!!

23
Loops---for statement

• Statement structure
• for (expr1 expr2 expr3)
• statements
• equivalent to
• expr1
• while (expr2)
• statements
• expr3
• expr2 is evaluated
• If expr2 is true, the statements and expr3 are
  executed and expr2 is re-evaluated
• Until expr2 becomes false, then the execution
  will jump out of the for loop
• You can use the break statement to jump out of
  the loop
• Using a break statement to jump out from a for
  loop is a very bad practice in programming!

24
Examples on for statement

• int i int i
• i 0 for (i 0 i lt 9 i)
• while (i lt 9) printf(i d\n, i)
• printf(i d\n, i)
• i
• The for statements structure can be viewed as
• for (initialize-loop-index condition
  incremental-steps)
• statement-body
• initialize-loop-index and incremental-steps are
  usually assignments or function calls
• condition is usually a relational expression
• Any of the three parts can be omitted, but the
  semi-colons must remain
• If expr2 (condition) is omitted, it is
  permanently true
• The code for ( ) ... refers to an infinite
  loop! You will need a break statement to get out
  of it

25
Loops---do while statement

• Statement structure
• do
• statements
• while (expression)
• The do while statement is almost like a while
  statement
• while the while and for loops test the
  termination condition at the top, the do while
  statement test the condition at the bottom after
  executing the statements at least once
• Two examples keeping track of the number of
  char being read
• int n int n
• char ch char ch
• n 0 n 0
• while ((ch getchar()) ! ) do
• n ch getchar()
• n
• while (ch ! )

26
Nested Loops

• Nested loops means loops inside a loop
• int i, j int i, j
• for (i 1 i lt 10 i) i 1
• printf(3d, i) while (i lt 10)
• for (j 1 j lt 10 j) printf(3d,
  i)
• printf(3d, ij) j 1
• while (j lt 10)
• printf(\n) printf(3d,ij)
• j
• printf(\n)
• i
• Program output
• 1 1 2 3 4 5 6 7 8 9
• 2 2 4 6 8 10 12 14 16 18
• 3 3 6 9 12 15 18 21 24 27
• 4 8 12 ....................

27
A program called dice.c
include ltstdio.hgt main() int count 0 int
overall 0 int i, j, k, n n 7 for (i
1 i lt 6 i) for (j 1 j lt 6 j)
for (k 1 k lt 6 k) if (i
j k n) count
printf("3d3d3d\n", i, j, k)
overall
printf("\nCount d", count) printf("\nPercentag
e f\n", count 100.0 / overall)
1 1 5 1 2 4 1 3 3 1 4 2 1 5
1 2 1 4 2 2 3 2 3 2 2 4 1 3 1
3 3 2 2 3 3 1 4 1 2 4 2 1 5 1
1 Count 15 Percentage 6.944444
28
A Word on the break Statement

• The break statement provides an early exit from
  for, while , do-while and switch statements
• A break causes the innermost enclosing loop or
  switch to be exited immediately
• Commands and Sections not recommended to be used
• The continue statement. (section 3.7) page 64-65
• Not a natural way to test for a condition
• The goto and labels. (section 3.8) page 65-66
• Produced spaghetti codes.

29
Arrays

• The declaration int a10 defines an array a of
  size 10
• a10 represents a block of 10 consecutive object
  named a0, a1, a2, ... a9
• a0 a1 a2 .....
• a10 declares a to be an array of 10 integers
• Array subscripts always starts at zero in C
• A subscript can be any integer expression, which
  includes integer variables like i, and integer
  constants
• The notation ai refers to the i-th element of
  the array
• Each element is to be treated as a variable of
  the defined type ai is an integer variable

30
A Program on Counting Digits

• include ltstdio.hgt
• main()
• char c
• int i, num_digit10
• for (i 0 i lt 10 i)
• num_digiti 0
• while ((c getchar()) ! EOF)
• switch (c)
• case 0 case 1 case 2 case 3
  case 4
• case 5 case 6 case 7 case 8
  case 9
• num_digitc - 0
• break
• default break
• printf(Distribution among the digits \n)
• for (i 0 i lt 10 i)
• printf(num_digitd 3d\n, i,
  num_digiti)

31
A program on Binary Search

• include ltstdio.hgt
• main()
• int x, n, a100, low, high, mid, found
• / find x in array a a0lta1lt...lta99
• assume we filled array a with values /
• n 100 / number of elements /
• x 37 / the number to be found /
• low 0
• high n - 1
• found 0
• while ((low lt high) (!found)) while (low
  lt high)
• mid (lowhigh) / 2 mid
  (lowhigh) / 2
• if ( x lt amid) if (x lt amid)
• high mid -1 high mid -
  1
• else if (x gt amid) else if (x gt
  amid)
• low mid 1 low
  mid 1
• else else
• found 1 found
  1

32
Multi-dimensional Arrays

• The declaration int A520 declares a
  two-dimensional array A with 5 rows by 20 columns
• The variable Axy represents the element in
  the x-th row and y-th column in the array A
• Two-dimensional arrays can represents
• Your appointment schedule --- int
  schedule247
• Two-dimensional matrix --- int matrix_A33
• When you declare an array, like a variable, you
  have to initialize it
• You cannot assume any value when you declare an
  array, you have to initialize it.
• int a100, i
• for (i 0 i lt 100 i) ai 0
• You can define the array of any type, not just
  integer, but floating point or character or
  others
• An array of char is also called string