Lecture 11:Control Structures II Repetitioncont.

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Lecture 11Control Structures II
(Repetition)(cont.)

• Introduction to Computer Science
• Spring 2006

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The while Loop

• The general form of the while statement is
• while(expression)
• statement1
• statement2

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The for Loop

• The general form of the for statement is
• for(initial statement loop condition update
  statement)
• statement
• for(initial statement loop condition update
  statement)
• statement1
• statement2
• The for loop executes as follows
• initial statement executes (initial statement
  initializes a variable)
• loop condition is evaluated
• If loop condition evaluates to true
• Execute for loop statement
• Execute update statement
• Repeat previous step until the loop condition
  evaluates to false

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Example
include using namespace std int
main() int i for (i0 ii) cout 6
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The for Loop (continued)

• initial statement in the for loop is the first to
  be executed and is executed only once
• If the loop condition is initially false, the
  loop body does not execute
• The update expression changes the value of the
  loop control variable which eventually sets the
  value of the loop condition to false
• The for loop executes indefinitely if the loop
  condition is always true
• A semicolon at the end of the for statement is a
  semantic error
• In this case, the action of the for loop is empty
  
• If the loop condition is omitted
• It is assumed to be true

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The for Loop (continued)

• In a for statement, all three statements (initial
  statement, loop condition, and update statement)
  can be omitted
• The following is a legal for loop
• for()
• cout

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The dowhile Loop

• The general form of a do...while statement is
• The procedure is
• statement executes first, and then the expression
  is evaluated
• If the expression evaluates to true, the
  statement executes again
• As long as the expression in a do...while
  statement is true, the statement executes

do statement1 statement2
While(expression)
do statement While(expression)
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Example for dowhile loop
include using namespace std int
main() int i0 do cout" ii5 while (irn(0)
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Break Continue Statements

• break and continue alter the flow of control
• break statement
• syntax break
• The break statement is used for two purposes
• In a repetition structure (e.g. while, for, and
  dowhile loop), it immediately exits from a loop
• In a switchcase structure, skip the remainder of
  the switch structure (an immediate exit)
• After the break statement executes, the program
  continues with the first statement after the
  structure
• The use of a break statement in a loop can
  eliminate the use of certain (flag) variables

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Example for break statement
include using namespace std int
main() int num, sum 0 cin
num while(cin) if (numgative number found!"umnum cinnum cout" 14
Break Continue Statements (continued)

• continue statement
• Syntax continue
• used in a loop (while, for, and do-while
  structures)
• Skips remaining statements and proceeds with the
  next iteration of the loop
• In a while and do-while structure
• Expression (loop-continue test) is evaluated
  immediately after the continue statement
• In a for structure, the update statement is
  executed after the continue statement
• Then the loop condition executes

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Example for continue statement
include using namespace std int
main() int num, sum 0 cin
num while(cin) if (numgative number found!"msumnum cinnum cout" 16
Nested Control Structures

• Suppose we want to create the following pattern
• In the first line, we want to print one star, in
  the second line two stars and so on
• Since five lines are to be printed, we start with
  the following for statement
• for(i 1 i
• The value of i in the first iteration is 1, in
  the second iteration it is 2, and so on

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Nested Control Structures (continued)

• The syntax is
• for(i 1 i
• for(j 1 j
• cout
• cout

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Nested Control Structures (continued)

• What pattern does the code produce if we replace
  the first for statement with the following?
• for (i 5 i 1 i--)
• Our program becomes

for(i 1 i j) cout
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• Answer

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End of lecture 11

• Thank you!