CS 192

1
CS 192

• Lecture 9
• Winter 2003
• December 19, 2003
• Dr. Shafay Shamail

2
The for Loop

• Repetition
• construct to execute a section of code a
  specified number of times
• for (initialization test-condition increment)
• statement(s)
• (1) Set initial value of counter.
• (2) Perform test to see if loop should continue.
• (3) Execute statements.
• (4) Update counter.
• (5) Repeat from (2).
• include ltiostream.hgt
• int main()
• int count //loop control variable
• for (count1 countlt100 count)

3
The for Loop

• Output?
• for (count10 count lt 5 count)
• cout ltlt count
• A square root calculating program
• include ltiostream.hgt
• include ltcmathgt // for older compilers, use
  ltmath.hgt
• int main()
• int num
• for(num1 num lt 100 num)
• cout ltlt num ltlt " " ltlt sqrt((double) num) ltlt
  '\n'
• return 0

4
Variations of the for Loop

• int main()
• for(int i100 i gt -100 i i-5)
• cout ltlt i ltlt ' '
• return 0
• for(x0, y10 xlt10 x, --y)
• cout ltlt x ltlt ' ' ltlt y ltlt '\n'
• include ltiostream.hgt
• include ltconio.hgt
• int main()
• int i

5
Variations of the for Loop

• int x
• for(x0 x ! 123 )
• cout ltlt "Enter a number "
• cin gtgt x
• int x 0
• for( xlt10 )
• cout ltlt x ltlt ' '
• x
• for()
• body of loop

6
The while Loop

• Repetition construct
• its a for loop stripped of the initialization
  and update parts
• while (test-condition)
• statement(s)
• for ( test-condition ) statement(s)
• initialize counter
• while (test-condition)
• statement(s)
• update counter

7
The while Loop

• Might use it if do not know beforehand when to
  stop repeating
• int n 99 //why initialize?
• cout ltlt Guess my fav number
• while(n ! 0)
• cin gtgt n
• cout ltlt Yes, I was looking for 0 \n
• unsigned char ch
• ch 1
• while(ch)
• //when does it terminate?
• cout ltlt ch
• ch
• //displays what?

8
The do-while Loop

• Similar to while, except that tests condition
  after the body of the loop i.e. statements
  executed at least once
• do
• statement(s)
• while (test-condition)
• int num
• do
• cout ltlt "Enter a number (100 to stop) "
• cin gtgt num
• while (num ! 100)

9
Which Loop to Use?

• if (number of repetitions known)
• use for
• else if (body to be executed once at least)
• use do while
• else use while
• No hard rule all interchangeable mostly matter
  of style

10
The continue Statement

• continue, break and goto are jump statements.
• continue used in loops and causes program to skip
  rest of loops body and start the next iteration
  immediately. Actually takes you to the closing
  brace of the loop body
• long dividend, divisor
• char ch
• do
• cout ltlt "Enter dividend " cin gtgt dividend
• cout ltlt "Enter divisor " cin gtgt divisor
• if (divisor 0) //divide by zero error
• cout ltlt "Illegal divisor\n"
• continue
• cout ltlt "Quotient is " ltlt dividend/divisor
• cout ltlt " , remainder is " ltlt
  dividenddivisor
• cout ltlt "\nDo another? (y/n) "
• cin gtgt ch
• while (ch ! 'n')

11
The continue Statement

• Output of following?
• include ltiostream.hgt
• int main()
• int x
• for(x0 xlt100 x)
• if(x2)
• continue
• cout ltlt x ltlt ' '
• return 0

12
The break Statement

• Causes exit from loop or switch
• include ltiostream.hgt
• include ltcmathgt
• int main()
• double x
• while (1)
• cout ltlt "Enter number for square root "
• cin gtgt x
• if (x lt0.0)
• break //exit loop if x is -ve
• cout ltlt sqrt(x) ltlt endl
• // break jumps to here
• cout ltlt "Sorry, can't do that\n"

13
The break Statement

• int t
• for(t0 tlt100 t)
• if(t10)
• break
• cout ltlt t ltlt ' '
• for(i0 ilt1000 i)
• // do something
• if(kbhit())
• break
• // keypress stops execution
• break causes exit from the innermost loop or
  switch, and not from the enclosing loops or
  switches

14
The goto Statement

• Unconditional branch statement that jumps to a
  label (identifier)
• goto label
• label statement(s)
• int x 1
• loop1
• cout ltlt x ltlt endl
• x
• if(x lt 100)
• goto loop1
• goto end
• int i 10 //error goto skips initialization
• end

15
Example

• Read and understand the magic number program
  given at the end of the chapter 4

16
while

• include ltiostream.hgt
• int main()
• int count_down
• cout ltlt "How many greetings do you want? "
• cin gtgt count_down
• while (count_down gt 0)
• cout ltlt "Hello "
• count_down count_down - 1
• cout ltlt endl
• cout ltlt "That's all!\n"
• return 0

17
do .. while

• include ltiostream.hgt
• int main()
• char ans
• do
• cout ltlt "Hello\n"
• cout ltlt "Do you want another greeting?\n"
• ltlt "Press y for yes, n for no,\n"
• ltlt "and then press return "
• cin gtgt ans
• while (ans 'y' ans 'Y')
• cout ltlt "Good-Bye\n"
• return 0

18
Counting Space Characters(while)

• void countSpaces1(void)
• int ch 0
• int numofspaces 0
• printf("Enter a sentence ")
• ch getchar()
• while ( ch ! '\n' )
• if ( ' ' ch )
• numofspaces
• ch getchar()
• cout ltlt "\nThe number of spaces is " ltlt
  numofspaces ltlt endl

19
Counting Space Characters(do..while)

• include ltstdio.hgt // getchar()
• void countSpaces2(void)
• int ch 0
• int numofspaces 0
• printf("Enter a sentence ")
• do
• ch getchar()
• if ( ' ' ch )
• numofspaces
• while ( ch ! '\n' )
• cout ltlt "\nThe number of spaces is " ltlt
  numofspaces ltlt endl

20
Make Number from Digits

• include ltstdio.hgt // getchar()
• include ltctype.hgt // isdigit()
• void makeInteger(void)
• int num0
• int digit0
• printf("Enter a number ")
• digit getchar()
• for( isdigit(digit) digitgetchar() )
• num num 10
• num num (digit - '0')
• cout ltlt "The number is " ltlt num ltlt endl

21
Skip Spaces

• include ltctype.hgt // header file for isspace()
• for (int getchar() isspace( c ) c
  getchar() )
• ungetc( c, stdin ) // put the non-space char
• // back in the input buffer

22
Skip Spaces

• include ltstdio.hgt // getchar()
• include ltctype.hgt // isdigit(), isspace()
• void skipSpaces(void)
• int c 0
• int count 0
• cout ltlt "\nSkipping Spaces\n\n\n"
• cout.flush()
• for ( cgetchar() isspace(c) cgetchar() )
• count
• ungetc(c, stdin)
• cout ltlt count ltlt " spaces skipped\n"

• What is the behavior of this program
• What is the function of cout.flush()
• What will happen if it is removed
• What does ungetc() do