switch and do while

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Lecture 9

• switch and do while
• Reference Section 4.7, 4.8

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Review of C Structure

• All programs could be written in terms of THREE
  control structures
• Sequence structures Built into C. Programs are
  executed sequentially by default.
• Selection structures C has three types if,
  ifelse, and switch
• Repetition structures C has three types while,
  dowhile and for

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switch Multiple-Selection Statement

• switch
• Useful when a variable or expression is tested
  for all the values it can assume, and different
  actions are taken
• This is called multiple selection
• Format of switch statement
• Series of case labels and an optional default
  case
• It tests whether an expression matches one of a
  number of constant integer values. If a case
  matches, execution starts at that case.

• Remarks
• Each case is labeled by one or more
  integer-valued constants or constant expressions
• All case expressions must be different
• The case labeled default is executed if none of
  the other cases are satisfied
• Cases and the default clause can occur in any
  order

switch ( expression ) case const-expr
statements case const-expr
statements default statements
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Example
int choice printf(1 Beijing\n) printf(2
Shanghai\n) printf(3 Guangzhou\n) printf(E
nter your choice of destinations
\n) scanf(d, choice) switch( choice )
case 1 printf(The flight to Beijing
takes 3 hours.\n) break case 2
printf(The flight to Shanghai takes 2
hours.\n) break case 3
printf(The flight to Guangzhou takes 1
hour.\n) break default
printf(Wrong Destination!\n) break
The break statement breaks out of the switch
construct. E.g., if the user inputs 2, the switch
finds a match with case 2. Hence the execution
starts from the statement after label case 2.
Once the break statement has been executed, the
whole switch is finished.
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Example
int choice printf(1 Beijing\n) printf(2
Shanghai\n) printf(3 Guangzhou\n) printf(E
nter your choice of destinations
\n) scanf(d, choice) switch( choice )
case 1 printf(The flight to Beijing
takes 3 hours.\n) break case 2
printf(The flight to Shanghai takes 2
hours.\n) / break / case 3
printf(The flight to Guangzhou takes 1
hour.\n) break default
printf(Wrong Destination!\n) break
If the break in case 2 is removed ./a.out 1 -
Beijing 2 - Shanghai 3 - Guangzhou Enter your
choice of destinations 2 The flight to Shanghai
takes 2 hours. The flight to Guangzhou takes 1
hour.
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Character Type

• char data type
• Used to store a single character (slide 20 of
  Lecture 3)
• In C, the size of char is one byte, which is
  enough for ASCII code
• Character constant
• Enclosed by single quotation marks , such as
  A, b, , \t, 1
• String constant
• Enclosed by double quotation marks , such as
  Hello, OK, COMP1000, A

char ch1, ch2, ch3, ch4, ch5 ch1 a / lower
case letter / ch2 1 / digit / ch3 \n
/ escape sequence / scanf(c, ch4) ch5
getchar() printf(c, ch5)

• Remarks A is different from A
• A is a single character
• A is a string, which happened to have only one
  character

char ch A
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getchar() function

• getchar() is a C library function defined in
  stdio.h
• It is used to read one character from the
  keyboard
• Once your program is running getchar() function,
  and if the user doesnt input anything, your
  program will stop at the getchar() statement
• In C, characters can be stored in any integer
  data type
• printf( The character (c) has the value d.\n,
  a, a)
• The result is
• The character (a) has the value 97.
• 97 is the character as numerical representation
  in the computer.
• ASCII code (Slides 24, 25 of Lecture 3)

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Assignments have a value!

• while ( ( grade getchar() ) ! EOF )
• grade getchar() is an assignment
• getchar() will get a character from the user, and
  assign this character to variable grade
• The whole assignment ( grade getchar() ) has a
  value that is equal to the character returned by
  getchar()
• The condition in while loop in fact compares the
  value of grade to EOF
• EOF is a symbol standing for End of File, used as
  a sentinel
• It is an integer constant defined in stdio.h
• define EOF (-1)
• On Linux/UNIX system, the EOF indicator is
  entered by typing
• d
• On Microsoft Windows, the EOF indicator is enter
  by typing
• z

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break in switch ()
Controlling Expression

• switch ( grade )
• case A
• case a
• aCount
• break
• case B
• default

Falling through cases if there is no break
statement
The break statement causes program control to
continue with the first statement after the
switch statement.
If no match occurs, the default case is executed.
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switch multiple-selection statement with breaks.
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Good Programming Practices

• Provide a default case in switch statements.
  Cases not explicitly tested in a switch are
  ignored. The default case helps prevent this by
  focusing the programmer on the need to process
  exceptional conditions.
• Although the case clauses and the default case
  clause in a switch statement can occur in any
  order, it is considered good programming practice
  to place the default clause last.

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Common Programming Error

• Not processing newline characters in the input
  when reading characters one at a time can cause
  logic errors.
• Remember to provide processing capabilities for
  newline (and possibly other white-space)
  characters in the input when processing
  characters one at a time.

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dowhile Repetition Statement

• The dowhile repetition statement
• Similar to the while structure
• Condition for repetition only tested after the
  body of the loop is performed
• All actions are performed at least once
• Format
• do
• statements
• while ( condition )

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dowhile Repetition Statement

• Example (letting counter 1)
• do
• printf( "d ", counter )
• while (counter
• Prints the integers from 1 to 10
• Some programmers always include braces in a
  do...while statement even if the braces are not
  necessary. This helps eliminate ambiguity between
  the do...while statement containing one statement
  and the while statement.

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Flowcharting of the do...while repetition
statement
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Comparison of while and dowhile
Get input from user until y or n is input
/ method 2
using do while
/ char ch do
printf(Input y or n ) scanf(c, ch)
while (ch ! y ch ! n)
/ method 1 using
while / char
ch printf(Input y or n ) scanf(c,
ch) while (ch ! y ch ! n)
printf(Input y or n ) scanf(c, ch)
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Another Example of dowhile

• Problem Check for a palindrome. E.g., 12321,
  1342431 are palindromes.
• Solution reverse the integer and then compare!

int num, n, digit, rev0 scanf(d, num) n
num / make a copy of num / do digit n
10 rev rev 10 digit n / 10
while (n ! 0) if (num rev) printf(d is
a palindrom.\n, num) else printf(d is not
a palindrom.\n, num)
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Common Programming Error

• Infinite loops are caused when the
  loop-continuation condition in a while, for or
  do...while statement never becomes false.
• To prevent this
• make sure there is not a semicolon immediately
  after the header of a while or for statement.
• In a counter-controlled loop, make sure the
  control variable is incremented (or decremented)
  in the loop.
• In a sentinel-controlled loop, make sure the
  sentinel value is eventually input.

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Appendix C Keywords

• 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