Introduction to Computing

1
Introduction to Computing

• Programming is an activity that is carried out
  through some programming language. Here the
  programming language we use is called C.
  Programming is instructing the computer how to
  solve a computational problem. In a
  computational problem, there will be some input
  , and for each input there is some desired
  output . A program instructs how to obtain
  the desiredoutput from the given input.

2
Notion of a program

• A program is a sequence of instructions to the
  computer.
• For example, a program for adding two numbers may
  be that sequence of instructions which tells the
  computer how to carry out the algorithm for
  addition we had learnt in school.
• Given any two numbers to be added, the computer
  executes the program to find the result.
• Here the input is the two numbers, and the
  output is their sum.

3
Other examples

• A more complex example of a computational problis
  the monthly pay-bill computation.
• Here, the input is various data about the
  ployees of a firm--- their attendance record,
  overtime record, salary, overtime rate etc.
• The output is the amount to be paid to each
  ployee.
• The program bodies the method of computing
  monthly salary.

4

• A program is a series of easy to follow steps
• A computer knows how to carry out some simple
  instructions.
• Let us explain this idea by an example.
• Suppose we have a computer that knows how to
  carry out the instruction
• drawline (x1, y1, x2, y2)
• (x1, y1) and (x2, y2) are co-ordinates of any two
  points on the screen.
• This instruction makes the computer draw on
  the screen a straight line from point (x1, y1)
  to point (x2, y2).

5

• Complex tasks from simple tasks
• The four instructions below
• drawline ( 0, 0, 0,10)
• drawline ( 0,10,10,10)
• drawline (10,10,10, 0)
• drawline (10, 0, 0, 0)
• when executed, will draw a square on the screen.
• Thus by performing a series of simple
  instructions, each drawing a straight line, we
  have managed to get a more complex task, drawing
  a square, done.

6
Programming language C

• The programming language C provides a set of
  instructions.
• A program in C is a sequence of such
  instructions.
• C is complete if a computational probl does
• have a solution, then it is possible to write a C
  program for the probl.
• Examples of other programming languages Fortran,
  Basic, Cobol, Pascal, Prolog,
• Lisp, C etc.
• Moreover, it is easy to pick up many other
  languages once you know C.

7
Basic Notions

• Now we introduce certain basic notions
  likethe notion of variables. We also discuss
  some basic kinds of instructions-- you will see
  their parallels in C shortly.
• Using these instructions, we shall write a
  programand discuss how the program works. This
  is not a C program but we hope it will prepare
  you for your first C program that you will see
  shortly.

8
Variables

• A fundamental notion in languages like C is the
  notion of variables. A non-trivial program will
  have a number of variables. Each variable has a
  name, like x, tp, etc. as given by the
  programmer. At any time a variable has a
  value, and the value of the variable, in general,
  changes as the program executes. Recall that
  we said that a program is a sequence of
  instructions. It is these instructions which
  change, update, and/or test values of variables.

9
Expressions, assignment

• Suppose a program has the variables x, y,
  z, and tp.Let us say that these variables have
  values 5, 3, -9, and 50.Consider
• 2 ( x y ) tp This is an
  expression, and it evaluates to a value, in this
  case, its value is 66. A fundamental
  instruction is assignment which assigns the value
  of an expression to a variable.The general form
  of assignment instruction is
• variable expressionwhich tells the computer
  to first evaluate expression to get a value,
  and then assign this value to the variable.

10
Sequencing of instructions is important

• Remember that executing a program is
  executing a sequence of instructions, which is
  what the program is.
• A program starts its execution by first executing
  the first instruction, then the second
  instruction, and so on.
• Execution of the program ends when there are no
  more instructions left to be executed.
• The order of sequencing of instructions is
  important. For example, the effect of two
  sequences.
• Sequence 1
• y x y x x 1

11
Sequencing is important cont'd

• and Sequence 2
• x x 1
• y x y
• are different, even though both have the same
  two assignments. To see this, suppose we start
  with the case x has 3, y has 5.
• Then effect of sequence 1 isx has 4, y has
  8.But the effect of sequence 2 will result in
  x has 4, y has 9.

12
Other kinds of instructions

• There are instructions which do something if some
  condition holds.
• For example, if ( x y ) z z 1 The
  effect of this instruction is to increment z by 1
  only if x and y both have the same value,
  otherwise no value gets changed. An example of
  another form of such a conditional instruction is
  if ( x y ) z z 1 else z z
  - 1 This instruction increments z by 1 if x and
  y have same value, else it decrements z by 1.

13
Expressions evaluating to true/false

• In the above
• ( x y )
• is also an expression its value is, however,
• not a number but true or false.Such expressions
  are called boolean expressions

14
Iterative Instructions

• In a program, some action might need to be
  repeated till some condition is met. Iterative
  instructions help us in achieving this. One
  important instruction in this class has the form
  while( B ) S
• where B is a boolean expression, and S is a
  sequence of one or more instructions. On reaching
  such a statement, first B is tested, if false
  nothing gets done. But if B is true, S is done,
  and then B is tested again. If false, the work is
  over, otherwise S is done again, and so on. S
  gets done repeatedly till B becomes false.

15
Example

• Here is a program', (not in C) using
  instructions
• that we have seen.
• a x b y while( a ! b )
  if( a gt b ) a a - b else b b - a
  How do we understand what this program is
  doing? We can see that there are four variables,
  x, y, a, and b, and the instructions modify a and
  b.To understand what the program does, we need
  to observe how the values of the variables
  change.

16
When to observe

• To see changes, we would like to observe the
  values of variables just before and just after
  each instruction.But after an instruction,
  usually, another instruction follows.Therefore,
  we observe the variables just before an
  instruction is about to be executed.

17
Example cont'd
Observation time Where in program x y a b
1 Before 1st instn. 4 6
2 Before 2nd instn 4 6 4
3 About to test a!b 4 6 4 6
4 About to test agtb 4 6 4 6
5 About to test a!b 4 6 4 2
6 About to test a gt b 4 6 4 2
7 About to test a!b 4 6 4 2
18
Example Contd

• When both a, b have the same value, (here it was
  2), the test in while instruction evaluates to
  false. That ends the while instruction. After
  this, our example program had no other
  instructions to execute, therefore, the execution
  of the program ends.What we have done is to
  hand-execute the program. The final value of a in
  this case is 2.Had we started with x as 12 and y
  as 8, the final value of a would have been 4.In
  general, the final value of a will be the gcd of
  x and y, provided both are greater than
  0.Therefore, the program is doing something
  useful! To compute gcd of x and y, we can execute
  the program, and print the final value of a as
  the result.

19
Programming Language C

• Now, we shall discuss a problem which is to be
  solved using the computers.
• We shall design a method to solve the problem,
  and then implement the method using a C program.

20
A problem

• Arrange a list of n numbers a1, a2, a3, ..., an
  in ascending order.
• A solution Using Insertion sort.
• What is Insertion sort?
• Insertion sort works as follows we look at the
  numbers one by one, maintaining the numbers
  already seen in sorted order. Each new number is
  inserted into its proper place in this sorted
  list so that the sorted order is preserved.

21
A solution cont'd.

• Suppose the given list of numbers is 5, 7, -3,
  2, 20.
• Following the idea above, we obtain the
  following sorted lists increntally5 ( after
  inserting 5 into its proper place in an pty list
  !) 5, 7 (after inserting 7 ...)-3, 5, 7 (after
  inserting -3 ...)-3, 2, 5, 7 (after inserting
  2...)-3, 2, 5, 7, 20 (after inserting
  20....)Finally, we obtain a sorted list of all
  the numbers.

22
A program for insertion sort

• We address the problem of designing a program
  for insertion sort. The approach that we shall
  adopt is one of handling complexity by adding
  details incrementally. This is popularly known
  as the top-down or stepwise refinement
  approach to programming.
• A first cut in a top-down approachStep1
  Read listStep2 Insert-Sort listStep3
  Print list

23
A program for insertion sort cont'd.

• Top-down approach A second cut
• .
• .
• Step 2 for i 1 to n do insert ai
  into sorted(a1, a2, ..., ai-1),
  by linear search from the high-end .
• .

24
A program for insertion sort cont'd.

• Top-down approach A third cut
• To insert ai we first claim space for it to
  the right of ai-1 and propagate it left if
  necessary.
• Step 2 for i 2 to n do set ai-1 as
  current element while (position not found) do
  if no more elements insert ai and
  report position found

25
Full-fledged C-program

• include ltstdio.hgtmain() int n / number
  of elements to sort / int i, j / loop
  counters / int key / element to insert /
  int a15 / storage for list / / read
  in the list /

26
Full-fledged C program cont'd.

• scanf("dn", n)
• /n is the number of integers to be sorted /
• for (i0 iltn i) scanf("d", ai)
  /read in the numbers // find the correct
  position of key, that is aj, /
• /in the sorted list a0,a1, ... ,aj-1
  /
• for (j1 jltn j) / initialize while-loop
  variables / key aj ij-1

27
Full-fledged C program cont'd.

• while (i gt 0 ai gt key) ai1
  ai i i - 1
• / insert key in the correct
  position / ai1 key / print the
  sorted list / for (i0 iltn i)
  printf("d\n", ai)

28
Variables and constants

• Data that can be changed by the actions in a
  program are called variables.
• Variables have names and locations in mory
  corresponding to these names, that store the
  associated values.
• Data that cannot be changed by the actions in a
  program are called constants.
• There may or may not be (symbolic) names
  corresponding to constants.

29
Names in C

• These are a combination of letters, digits and
  underscore, beginning with a letter.
• At most 31 characters long
• Case-sensitive
• Variable names in lower case
• Names of symbolic constants in upper case
• Keywords like else, if... cannot be used
• Examples of valid names abc, a_bc, a12,
  a_1_b, ABc, A_Bc, PI, ....
• Some invalid ones are a-bc, 1ab, A b, ....

30
Basic types in C

• The type of a variable indicates the values that
  it can assume
• Among other things, we need different types of
  variables to be able to handle different types
  of data
• In C, we have variables of the following basic
  types
• char The type char corresponds to the
  set of all possible 8-bit (that is a byte)
  values.
• int The type int denotes the set of
  values that can be stored in a number of bits
  equal to the host machine's word size.

31
Basic types in C cont'd.

• float The type float corresponds to the set
  of floating-point numbers that can be represented
  in the host machine.
• double The type double corresponds to the set
  of double-precision floating-point numbers that
  can be represented in the host machine.

32
Qualified types

• We get more types by qualifying the basic types
• signed, unsigned, long, short int
• give us eight different ways of prefixing
  the type int.
• For example, signed long int, short int
  etc.
• signed, unsigned char
• give us two different ways of prefixing the
  type char.
• For example, signed char
• long double
• We can prefix the type double by long

33
Constants

• Like variables, we also have different types of
  constants.
• Numerical constants are of the types
• integers For example, 123, -12354 etc.
• integers suffixed with u, U l, L 123457L or
  1234567l is of type long int
• 1234567uL is of type unsigned long
• integers in octal and hex representation
• An octal integer constant is prefixed with 0
• a hexadecimal integer constant with 0x or 0X
• 077 is the octal equivalent of 63, while
  0xAF is the hexadecimal equivalent of 175. These
  can also be suffixed with u, U and l, L.
• For example, 077777U is an unsigned octal
  constant.

34
Constants cont'd

• String constants These are a sequence of 0 or
  more characters put inside double quotes. For
  example, "I am going.
• Enumeration constants We can define our own
  type, by associating a name with this type and
  explicitly enumerating the associated set.
• A constant of the enumerated type is one of a
  list of values of such an explicitly enumerated
  type. For example,
• enum hotmonths MAY, JUNE, JULY, AUGUST
• is an enumerated type. MAY, JUNE, JULY,
  AUGUST are enumerated (symbolic) constants in
  this list whose values are 0, 1, 2, 3
  respectively.

35
Declarations

• What are declarations?
• A C program is made up of declarations,
  followed
• by statements. In the declarations part we
  describe
• the data, that is, choose suitable names and
  types for the data.
• For example
• int lower, upper, step
• char c, line100

36
Declarations Contd

• Why declarations ?
• The most important reason is that declarations
  enable a compiler to check that the actions that
  are being performed on this data are indeed
  permissible. The technical term for this is
  type-checking.
• It also enables a programmer to control the
  life-time (or scope) of any data.

37
Operators, Expressions, and Statements

• Operators allow us to manipulate the data
  represented by constants and variables
• Arithmetic operators , , /, -, .
• The first four are the familiar plus, product,
  division, subtraction while the last is
  themodulus operator - a b gives the remainder
  in the division of a by b.

38
Precedence of operators

• Operators are used with variables and constants
  to build up expressions. For example, a b - 2
  is an expression. In the presence of several
  operators in any expression, the rules that tell
  us the order in which to do the operations are
  called precedence rules.
• The precedence of binary , - are the same so
  are those of , /, the unary and - also
  have the same precedence. The first group has
  lower precedence than the second which in turn
  have lower precedence than the third.
• In cases of equal precedence, arithmetic
  operators associate from left to right. With
  these rules, the expression 2 3 - 4evaluates
  to 2 instead of -2.

39
Relational Equality Operators

• Relational lt, lt, gt
• Equality , !
• Equality operators have higher precedence than
  relational but lower than arithmetic operators.
• Evaluation of the expression
• 2 lt 3 ! 5
• gives us 0 (which is the value of false in C)

40
Logical operators

• The relational operators give us relational
  expressions which are combined using the
  logical operators and (), or (), not (!)
• For example, in the expression i gt 0 ai
  gt keyprecedence order is gt, gt,
• The precedence of is lower than that of
  while both have lower precedence than the
  equality operators.
• The unary operator ! converts a non-zero
  operand true into false ( 0), and a 0 operand
  into 1.

41
Operators cont'd

• Increment operator
• it is a unary operator which can be prefixed or
  suffixed to its operand. The effect is to
  increase the value of itsoperand by 1. Thus the
  effect of both n and n is to increase n
  by 1.
• Decrement operator - -
• it is a unary operator which can be prefixed or
  suffixed to its operand. The effect is to
  decrease the value of itsoperand by 1. Thus the
  effect of both --n and n-- is to decrease n
  by 1.

42
Expressions

• An expression is made up of variables, constants
  and operators.
• An arithmetic expression evaluates to a numerical
  value. A boolean expression is either a
  relational expression or a logical expression
  that evaluates to true or false.
• For example, if a, b, c are all of type int
  then
• a b - c is an arithmetic expression, a
  ! b is a relational one and
• (a lt b) (b gt c) is a logical expression.

43
Expressions cont'd.

• An assignment expression has the form
• variable expressionThe value of such an
  expression is the value of its left-operand,
  obtained by evaluating the expression on the
  right. Thus as a result of the assignment x
  5 2 x has the value 7.

44
statements

• A statement causes an action to be carried out.
• In C, statements are either expression
  statements, compound statements or control
  statements.
• Expression statement
• This is an expression, terminated by a semicolon.
  For examplea 3 c a b n

45
statements cont'd.

• Compound statements
• These consist of several statements enclosed with
  a pair of braces For example while (i gt 0
  ai gt key)
• ai1 ai i i - 1
• Control statements
• We will discuss these later.

46
Operators Once More

• Assignment operator This has the form op.
  The assignment expression n n 2 can be
  written as n 2.As such n and 2 become the
  operands of the operator .In general, if op
  is a binary operator, we have expr1 expr1
  op expr2,which can be written as expr1 op
  expr2so that expr1 and expr2 are the operands
  of the operator op which we call the
  assignment operator.

47
The sizeof operator

• The sizeof operator operates on either a variable
  name, or a type name.
• It gives the number of bytes occupied by the
  givenvariable, or any variable of the given type
  respectively.
• For examplesizeof(int)would have the value 4.

48
Precedence among operators

• Operators Associativity
• ( ) left to right !
  -- - type sizeof right to left /
  left to right - left to right lt
  lt gt gt left to right ! left to
  right left to right left to
  right ? right to left - /
  left to right
• The precedence decreases from the top to
  thebottom of the table.

49
Type conversions

• The constituent variables and constants in an
  expression can be of different types. Then the
  question that naturally arises is what is the
  type of the value of an expression.
• For example, if in the expression a b, a
  is of type int and b is of type float, then
  what is the type of the sum?
• Type conversion rules tell us what should be the
  type of the sum. We address these questions in
  the next couple of slides.

50
Type conversions cont'd.

• Implicit type conversionsSuch conversions in C
  take place in several contexts.
• In arithmetic operations
• When operands of an arithmetic operator have
  different types.For example, if a is of type
  int and b is of type char then the type of the
  result of a b is int.
• The general rule here is that 'a lower' type is
  elevated to 'a higher' type.

51
Type conversions cont'd.

• Across assignments
• In an assignment expression the right-hand side
  is first evaluated the returned value is the
  value of the left-hand side and also the value
  of the assignment expression. The type of the
  left-hand side and that of the assignment
  expression is the type of the right hand side.
• For example,if x is of type int and i is of
  char then as a result of the assignment x i,
  x becomes of type char.

52
Type conversions cont'd

• Explicit type conversions This is done by using
  the cast operator. The syntax is (type-name)
  expressionThe meaning is that the type of the
  expression is changed to the type specified
  within round-brackets. For example,
• sin((float) n)
• converts the value of n to type float
  before passing it on to sin.

53
Input and Output

• Basic Input Output Functions
• printf
• scanf
• getchar
• putchar

54
printf

• It is a function in the standard library
  ltstdio.hgt
• It converts internal values to charactersThe
  syntax is printf( format string, arg1,
  arg2,...)
• The format string contains, within double
  quotes, formatting information for the arguments
  to be printed viz., arg1, arg2,..
• There is one group of characters for each
  argument.
• Each such group contains a percentage sign ,
  followed by a conversion character

55
printf cont'd.

• For example, in the insertion sort program, we
  had
• printf("d\n", ai)In this case, we had only
  one argument ai, and its formatting
  information is given by d, indicating that
  ai is a decimal integer. The \n indicates that
  after printing ai we should move to a newline.
  

56
printf examples

• include ltstdio.hgtmain()
• char line20 ramanujam" int x
  123 float y 123.45printf("d f s", x, y,
  line)
• Output
• 123 123.449997 ramanujam

57
printf examples

• include ltstdio.hgtmain()
• int i 12345, j -13579, k -24680long
  ix 123456789short sx -2222unsigned ux
  5555float a 2.2, b -6.2
• printf("d d d ld d u\n", i, j, k, ix, sx,
  ux)printf(" i 3d j 3d 3d \n", i, j,
  k)printf(" 4.2f 7.1f \n", a, b) Output
• 12345 -13579 -24680 123456789 -2222 5555
• i 12345 j -13579 -24680 2.20
  -6.2

58
printf cont'd.

• We can summarise the formatting specifications
  as follows
• Each conversion specification begins with a
  and ends with a conversion character. Between
  the and the conversion character there may be,
  in order A minus sign, which specifies left
  adjustment of the converted argument A number
  that specifies the minimum field width A period,
  which separates the field width from the
  precision A number that specifies the precision
  An h if the integer is to be printed as short
  or l if as long

59
scanf

• This is a function in the standard library
  ltstdio.hgt.
• It reads characters from standard input The
  syntax is scanf( format string, arg1,
  arg2,...)
• The format string contains, within double
  quotes, formatting information for the way the
  arguments are to be stored
• It controls how arguments are stored
• There is one group of characters for each
  argument
• Each such group contains a percentage sign ,
  followed by a conversion character

60
scanf cont'd.

• Note that each argument is an address, indicating
  where in the mory the corresponding argument is
  to be stored
• For example, in the insertion sort program we
  had scanf("d\n", n)the count of numbers to
  be read in.
• In this case we had only one argument n, and
  itsformatting information is given by d,
  indicating that n is to be stored as a decimal
  integer. Once again the \n indicates that
  after reading n we should move to a newline.

61
scanf examples

• include ltstdio.hgtmain() char line20
  int x float y scanf("d f s", x, y,
  line)

62
scanf examples cont'd

• The following data it can be entered during
  execution 12345 0.05 ramanujamor even one
  per line as123450.05ramanujam
• Note that we have a different conversion
  character for each of the three different
  argument types

63
scanf examples cont'd.

• If we input the following data from the
  terminal 10 256.875 TAfter executing the
  following program include ltstdio.hgtmain()
• char c int i float x scanf("3d 5f
  c", i, x, c)
• 10 will be assigned to i, 256.8 to x and
  the character 7 to c.

64
Functions getchar and putchar

• When we need to read or write data character by
  character, functions getchar and putchar are
  very handy.
• To read a single character in variable c, we
  give
• c getchar() (equivalent to scanf("c", c)
• To write the character in variable c, we give
• putchar(c)(equivalent to printf("c", c)

65
The execution flow

• The execution of any C program starts from the
  first line of the program and proceeds
  sequentially downwards.
• For example, if a program has ten assignment
  statements, these would be executed in order of
  their appearance.
• However, in most of the programs that we would
  like to write,we would want to execute some
  statements only if some condition holds.
• For example, a program that takes as input marks
  between 0 and 100, and outputs FAIL if the marks
  are less than 40.

66
control statement

• We use the if control statement to achieve
  this.The syntax of an if statement is if (
  condition) statement
• The statement is executed only if the
  conditional expression condition is true,
  otherwise the control goes to the statement
  following the if statement.
• Note that the statement may actually be a
  compoundstatement, or a control statement, or a
  combination.

67
Example program
main() int marks printf("input marks
") scanf("d", marks) if (marks lt 40) /
fail / printf("FAIL\n")
68
The if-else statement

• When, depending on a condition, we want to
  execute onestatement block or other, we use
  if-else statement. Its syntax is if (
  condition) statement1 else statement2
  statement1 is executed if the condition is
  satisfied otherwise statement2 is executed.

69
Example

• main() int marks printf("input marks
  ") scanf("d", marks) if ((marks lt 0)
  (marks gt 100)) printf("Out of range!\n")
  else if (marks lt 40) / fail /
  printf("FAIL\n") else / pass /
  printf("PASS\n")

70
Example

• main() int a, b printf("Give two
  numbers ") scanf("d d", a, b)
  printf("\nLarger number is ") if (a gt b)
  printf("d\n", a) else
  printf("d\n", b)

71
if-else ladder

• main()
• int marks
• printf("input marks ") scanf("d",
  marks) if ((marks lt 0) (marks gt 100))
  printf("Out of range!\n") else if (marks lt 40)
  / fail / printf("Grade F\n")
  else if (marks lt 70)
  printf("Grade B\n")
  else printf("Grade A\n")

72
The ? operator

• Suppose we want to assign different values to a
  variable depending upon whether some condition is
  true or not.One way to do it is, of course,
  using an if-else statement.There is another,
  more concise, way of doing this via the ?
  operator.The syntax of the operator is
• condition ? expn-1 expn-2
• The above expression returns the value of
  expression expn-1 if the conditional expression
  condition evaluates to true, otherwise returns
  the value of expn-2.

73
An example

• Suppose that there were only two rates of taxes
• No tax for income less than or equal to 150,000
  and a flat rate of 10 above it.
• Then the tax calculation program ismain()
  int income float tax printf("Enter the
  taxable income ") scanf("d", income)
• tax (income lt 150000)? 0.0
• (income - 150000)0.1 printf
  ("tax f\n", tax)

74
Correct but not advised example

• main() int income float tax
  printf("Enter the taxable income ")
  scanf("d", income) tax (income lt 150000)
  ? 0.0 ((income lt 300000) ?
  (income - 150000) 0.10
  ((income lt 500000) ? 15000
  (income - 300000) 0.2
  55000 (income - 500000) 0.3))) printf
  ("tax f\n", tax)

75
Iterations Loops

• The computational power of a machine is exploited
  at the program level by means of constructs that
  repeat a set of actions again and again until
  some conditions are satisfied.
• For example, in the Insertion-sort program we
  repeatedly take a new element from the unsorted
  list, and insert it into its proper place among
  the elements already seen and kept in sorted
  order.
• Or consider evaluating the sine of a quantity x
  by using the power series expansion of sin x.
• C has several repetitive constructs, like the
  for- loop, the while- loop etc.

76
The while - loop

• Syntax while ( expression)
  statementwhile is a key word, expression and
  statement are programmer-defined.
• Meaning of the constructThe statement is
  executed as long as expression evaluates to
  true (non-zero value)

77
while loop Example

• . . while (i gt
  0 ai gt key) ai1 ai
  i i - 1
• . . This while- loop
  had been used in the Insertion-sort program
  discussed earlier.

78
while - loop example program

• include ltstdio.hgtinclude ltmath.hgtmain()
  
• int sum, n scanf("d", n) sum
  0 while (n ! 0) sum n 10
• n / 10 printf("The sum of the
  digits is d", sum)

79
The for - loop

• The syntax for ( expr1 expr2 expr3)
  statement
• Meaning of the construct The initialization of
  the loop-control variable is done by means of the
  expression expr1 the second expression expr2
  controls the number of times the statement that
  follows is executed the third expression expr3
  updates the loop control variable after each
  execution of the statement.

80
for-loop program first n positive numbers

• include ltstdio.hgtmain()
• int n, i long sum /sum is a long
  integer/ printf("How many numbers to sum ? ")
  scanf("d", n) / input how many to/
  /add / sum 0 for(i 1 i
  lt n i) sum i printf("The sum is
  ld\n", sum)

81
while versus for

• A for- loop as a while- loopexpr1while (
  expr2) statement expr3Centralized loop
  control of for is an advantage in some
  situations.Otherwise the choice between the two
  is largely a matter of personal preference.