CPS 125: Digital Computation and Programming

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CPS 125 Digital Computation and Programming

• Repetition and Loop Statements

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Outline

• Using Loops to Solve Problems
• Counting Loop
• Conditional Loop
• Nested Control Structure
• Case Study Computing Radiation Level
• Debug and Test
• Common Programming Errors

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Using Loops to Solve Problems

• Problem 1 Mars Observer spacecrafts mission
  plan called for seven orbit adjustment maneuvers
  in order to move from initial orbit to the
  desired orbit for mapping Mars.
• Problem 2 Process a data file of Celsius
  temperatures. Count how many are above 0ºC.
• The number of temperatures 100
• Dont know the number, terminating on End
• Dont know the number, count till the end of file

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Using Loops to Solve Problems

• Three questions
• Any steps repeated?
• Know in advance how many times to repeat?
• If answer to second question is no, how long to
  keep repeating the steps?
• Solutions with loop statements
• A set of statements are executed multiple times
• Two basic types of repetition
• Counting loop
• Conditional loop

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Counting Loops

• A counter variable to control loops
• An initial value and an end value
• An increment amount
• Steps
• Set counter to an initial value
• Loop when (counter lt end value)
• increase counter by increment amount

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The while Statement

• Syntax
• while (loop repetition condition)
• statement
• Execution
• 1. Test loop repetition condition
• 2. If condition is true, execute statement and go
  back to 1
• 3. If condition is false, exit while loop
• Executed 0 or more times

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Flowchart of while
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• count_mvr 0 / no maneuvers
  done yet /
• while (count_mvr lt 7) / test value of
  count_mvr /
• printf("Number of orbits to next maneuvergt
  ")
• scanf("d", orbits)
• printf("Orbit period, in hours and
  fraction of hoursgt ")
• scanf("lf", period)
• time_to_mvr orbits period
• printf("Time to next maneuver is 6.1f
  hours\n", time_to_mvr)
• count_mvr count_mvr 1 /
  increment count_mvr /
• printf("All maneuvers complete\n")

Example Loop to process seven spacecraft
maneuvers
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• int count
• while (count lt 10)
• count count 1
• int count 0
• while (count lt 10)
• printf(d, count)
• Three critical steps initialization, testing,
  and updating
• Infinite loop

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Exercises of while
What is displayed if input is 10 scanf(d,
n) ev0 while (ev lt n) printf(3d\n,
ev) ev 2 printf(\n)
Predict the output i 0 while ( i lt 5 )
printf(3d3d\n, i, 10-i) i i 1
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Computing a Sum in a Loop

• double grade, total 0.0
• int count 0
• while (count lt 4)
• printf(Enter a grade -gt )
• scanf(lf, grade)
• total total grade
• count count 1
• printf(The total of the grades is lf, total)

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Computing a Product in a Loop
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Exercise

• printf (Enter an integergt)
• scanf(d, x)
• product x
• count 0
• while (count lt 4)
• printf(d\n, product)
• product x
• count
• Output values if a data value 5? 6? 7?

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Compound Assignment Operator

• Convert variable variable op expression
• to variable op expression
• op could be , -, , /, and
• ct ct 1 gt ct 1
• time time 1 gt time - 1
• product product data gt
• n n (x 1) gt

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Increment/Decrement Operators

• Increment operator
• Prefix 1. increment variable 2. evaluate
  expression
• Postfix 1. evaluate expression 2. increment
  variable
• e.g. i 2
• j i j i
• Decrement operator --
• Prefix and postfix
• e.g. n 4
• printf(3d, --n) printf(3d, n--)
• printf(3d, n) printf(3d, n)
• y i x i (if x 5 and i 2)

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Exercises

• r r / 10
• z z x 1
• q q r m
• m m (n p)
• i 3
• j 9
• n i --j
• m i j--
• p i j

int count 5 while (count--)
printf(count d\n, count)
int count 5 while (--count)
printf(count d\n, count)
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The for Statement

• Syntax
• for (initialization expression
• loop repetition condition
• update expression)
• statement
• Execution
• 1. Execute initialization expression
• 2. Test loop repetition condition
• 3. If it is true, statements are executed, update
  expression is evaluated, go back to 2
• 4. If it is false, the for loop is exited

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for vs. while

• Logic of for is like that of while

Compare the code
int ctr for (ctr 1 ctr lt 10
ctr) printf(ctr d\n, ctr)
int ctr ctr 1 while (ctr lt 10)
printf(ctr d\n, ctr) ctr
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Exercise

• How many times does the body loop?
• for (count 0 count ltn count)
• sum count
• if execute the loop body n times, how to change
  code?
• How many times does the body loop?
• sum0 k1
• for (i -n i lt n-k i)
• sum ii
• for ( ) infinite loop

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Exercise

• Trace the loop for n8. Show the values of odd
  and sum after the update of the loop counter for
  each iteration?
• sum0
• for (odd 1 odd lt n odd 2)
• sum odd
• printf(Sum of positive odd numbers less than d
  is d.\n, n, sum)

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Exercise

• Trace the following program segment
• j10
• for (i1 ilt5 i)
• printf(d d \n, i, j)
• j -2
• Rewrite it so that it produced the same output
  but uses 0 as the initial value of i ?

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Multiple Control Variables in for
expression 2
expression 3
expression 1
for ( i 1, k 1, j 1 i lt 10 j lt 3 k
i, j 2, k 2 i ) printf(i d, j
d, k d\n, i, j, k)

• initialize variables i, j, and k each to 1
• test to end loop when i gt 10 or j gt 3 k
• increment i by 1, j by 2, and k by 2 i
• print i, j, and k in the body of the loop

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The do-while Statement

• Syntax
• do
• statement
• while (loop repetition condition)
• Execution
• 1. Execute the statement
• 2. Test the repetition condition
• 3. If it is true, go back to 1
• 4. If it is false, exit the do-while loop

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Flowchart of do-while Loop
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while vs. do-while
int x 10, y 1 while ( x lt 10 ) if( x
10 ) y x x printf(x d\n,
x) printf(y d\n, y)
int x 10, y 1 do if( x 10 ) y
x x while ( x lt 10 ) printf(x d\n,
x) printf(y d\n, y)
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• How to generate number from 1 to 10 Using
  different loops
• for while
  do-while
•  
• for (n1 nlt10 n) n1
  n1
• while
  (nlt10) do
• ------
  
• ------ ------
  ------
• ------ ------
  ------
• 
  nn1 nn1
• 
  while (nlt10)
•  

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Conditional Loops

• The number of repetition is unknown before loop
  execution begins
• Looping continues until some event occurs or some
  value is encountered
• Steps
• Initialization
• Loop when condition is tested to be true
• update statement

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Example Using for Statement

• printf(Enter number of observed valuesgt )
• for (scanf(d, num_obs)
• num_obs lt 0
• scanf( d, num_obs))
• printf(Negative number invalid try againgt
  )
• Purpose of this code is to control the validity
  of input data.

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Loop Design

• Condition Number of times loop must be executed
  is unknown
• Data too large to count
• Data depends on how computation proceeds
• Solution sentinel-controlled loops
• User enters a unique value to mark the end of
  data
• The program terminates when input value is equal
  to the sentinel value

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Form of Sentinel-Controlled Loop

• 1. Get some data
• 2. While the sentinel value has not been
  encountered
• a. Process the data
• b. Get more data
• c. Go back to 2

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• / Compute the sum of a list of exam scores. /
• include ltstdio.hgt
• define SENTINEL -99
• int main()
• int sum 0, / sum of scores input so
  far /
• score / current score /
• printf("Enter first score (or d to quit)gt
  ", SENTINEL)
• for (scanf("d", score)
• score ! SENTINEL
• scanf("d", score))
• sum score
• printf("Enter next score (d to
  quit)gt ", SENTINEL)
• printf("\nSum of exam scores is d\n",
  sum)
• return (0)

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• int sum 0, / sum of scores input so far /
• score / current score /
• while (score ! SENTINEL)
• printf("Enter the score (d to quit)gt ",
  SENTINEL)
• scanf("d", score)
• sum score
• Errors?

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• Correct program segment

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End-of-File-Controlled Loops

• double grade 0.0, total 0.0
• printf(Enter a grade -gt )
• while (scanf(lf, grade) ! EOF)
• total grade
• printf(Enter a grade -gt )
• printf(The total of the grades is lf, total)

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EOF

• EOF character (end-of-file) a negative integer
• Z in DOS
• D in UNIX
• Operating system reads Z or D and converts to
  EOF
• In file operation functions, fscanf will return
  EOF when it reads the end of the file

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Return Value of scanf
while(( vals scanf( lfclf, op1, action,
op2)) 3 )
scanf statement
result stored here optional
this value compared to
If the inputs are 35.6c9.3, 34.7bb, and Z, what
are output for vals scanf( lfclf, op1,
action, op2)
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Example Using fscanf

• / open the file /
• in_file fopen("scores.dat", "r")
• / begin reading from the file /
• for (input_status fscanf(in_file, "d",
  score)
• input_status ! EOF
• input_status fscanf(in_file, "d",
  score))
• / process the value read from the file /
• printf("5d\n", score)
• sum score
• Input 7o instead of 70, what will happen?

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Correct Form for EOF loop

• input_status 1
• After loop,
• if (input_status EOF)
• printf(Sum of exam scores is d\n, sum)
• else
• fscanf(input, c, bad_char)
• printf( Error in input c \n,
  bad_char)

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Flag-Controlled Loop for Input Validation

• int id_number
• int error 0
• do
• printf("Please enter your identification
  number -gt ")
• scanf("d", id_number)
• if(id_number lt MINNO id_number gt MAXNO)
• printf("\nYour id number is
  out-of-range\n")
• error 1
• while (error)

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Nested Control Structure

• Create a 10x10 multiplication?
• for (row 1 row lt10 row)
• for (col1 collt 10 col)
• printf(4d, rowcol)
• printf(\n)
• Countdown from 10 to 0
• for (t 10 t gt0 t-- )
• for ( j 0 j lt t j )
• printf(2d\n, t)

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Exercise

• Which is better to implement a sentinel-controlled
  loop? Why?
• for (scanf(d, num) do
• num ! SENT scanf(d, num)
• scanf(d, num)) if (num ! SENT)
• /process num / /process num/
• while (num ! SENT)

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Case Study

• Problem In a certain lab, some yttrium-90 has
  leaked into a coffee room. The leak would
  currently expose 150 millirems of radiation a
  day. The radiation level is only half of what it
  was three days ago. The analysts want to know how
  long it will be before the radiation is down to a
  safe level of 0.466 millirem a day. They want a
  chart to display radiation level for every three
  days with the message unsafe and safe after every
  line. It should stop just before the radiation
  level is one-tenth of the safe level.

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Case Study

• Analysis - data requirements
• Constants (SAFE_RAD, SAFETY_FACT), Input
  (init_radiation), Output (day, radiation_lev)
• Design
• Initial algorithm
• 1. Initialize day to 0
• 2. Compute the stopping radiation level
• 3. Prompt user the enter the initial radiation
  level
• 4. Compute and display the day and radiation
  level when radiation level exceeds stopping
  level, indicating (un)safe

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Case Study

• Algorithm refinement
• Additional variable (min_radiation)
• Step 4 refinement
• 4.1 Initialize radiation_lev to init_radiation
• 4.2 while radiation_lev exceeds min_radiation
• 4.3 Display the value of day, radiation_lev,
  and the string Unsafe or Safe
• 4.4 Add 3 to the value of day
• 4.5 Compute radiation_lev for the next period

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Debug and Test

• Debugger program
• Single-step execution to trace
• Choose key variables to watch
• Set breakpoints to locate the error in a segment,
  then to further trace the error
• Trace unit coarse -gt fine

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Debug without Debugger

• Insert diagnostic printf to display critical
  variables in critical points
• define DEBUG 1 to turn on diagnostics and
  define DEBUG 0 to turn off
• Use
• fflush(stdout)
• to display every diagnostic message immediately

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Debug Example

• for (scanf("d", score)
• score ! SENTINEL
• scanf("d", score))
• sum score
• if (DEBUG)
• printf( score is d, sum is d\n,
  score, sum)
• fflush(stdout)
• printf("Enter next score (d to quit)gt ",
  SENTINEL)

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Common Programming Errors

• for (scanf(dlf, code, amount),
• balance ! 0.0,
• scanf(dlf, code, amount)
• while (x gt xbig)
• x - 2
• xbig

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• printf(Experiment successful? (Y/N)gt )
• scanf( c, ans)
• if (ans Y)
• printf(Enter one number per line (d to
  quit)\ngt , SENT)
• scanf(d, data)
• while (data ! SENT)
• sum data
• printf(gt )
• scanf(d, data)
• else
• printf(Try it again tomorrow.\n)
• printf(Now follow correct shutdown
  procedure.\n)

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• scanf(dlf, code, amount)
• while (balance ! 0.0)
• scanf(dlf, code, amount)
• do
• printf(One more time? (1 continue/0 quit)gt )
• scanf(d, again)
• while (again 1)

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break and continue statement

• break statement
• Used in loops (e.g., for, while, and do-while
  statements) and in the switch statement
• Causes immediate exit from innermost loop (exits
  switch in switch statement)
• continue statement
• Used in loops only (e.g., for, while, and
  do-while, NOT in the switch statement Why?)
• Causes immediate exit from BODY of innermost loop
  AND loop condition is reevaluated for loop
  continuation

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break Statement
while ( expr1 ) statement1 while ( expr2 )
statement2 statement3 if ( expr3
) break statement4 statement5
when expr3 is true break exits loop
and the next stmt is done
even if expr2 is still true!
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continue Statement
while ( expr1 ) statement1 while ( expr2 )
statement2 statement3 if ( expr3
) continue statement4 statement5
when expr3 is true continue exits loop BODY
and goes BACK to see if expr2 is still true!
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continue Statement
do statement1 do statement2 statement3 if
( expr3 ) continue statement4 while (expr2
) statement5 while (expr1 )
when expr3 is true continue exits loop BODY
and tests to see if expr2 is still true! If it
is, control continues in the loop
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continue Statement
Does same thing in a for() statement
for ( expr1 expr2 expr3 ) statement1 state
ment2 if( expr4 ) continue statement3
jumps to end of loop
which causes expr3 to be executed and expr2 to
be evaluated for continuing the loop
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• int ctr 0
• while (ctr lt 6)
• if (ctr lt 3)
• ctr 2
• printf(d\n, ctr)
• continue
• else
• printf(d\n, ctr)
• break
• printf(Bottom of loop\n)
• What is printed?