Repetition in the programs

1
Repetition in the programs

• Example 1 Suppose we want to display the
  consecutive digits 0,1,2,.9. For doing that
  adding one to the previous number should be
  repeated 10 times.
• Example 2 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. The spacecraft
  would have needed to execute similar sets of
  instructions seven times, one set for each
  maneuver.

2
Repetition in the programs

• The questions that determined whether loops
  are required in the algorithm
• Are there any repeated steps in the algorithm?
• If repetition is required, how many times these
  steps should be repeated?
• If the number of repetition is unknown, how long
  the repeating the steps should be kept?
• The answer to question 1 indicates whether
  algorithm needs loop or not, and the answers to
  questions 2 and 3 determine which loop structure
  need to be used

3
Counting loop (counter-controlled loop)

• The repetition is managed by a loop control
  variable whose value represents a count
• Set loop control variable to an initial value (0)
• while loop control variable lt final value
• increase loop control variable by increment
  value (1)

4

• / Loop to Process Seven Spacecraft Maneuvers /
• 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")

5
count_mvr 0

Flowchart of While statement
count_mvr lt7
true
false
.. increase count_mvr
by 1
Exit loop
6
Counting loop and while statement

• Syntax of while statement
• while (loop repetition condition)
• statement
• Three steps for a loop with a control variable
  are
• Initialization of loop control variable (if
  missing loop may repeat 0 or a wrong number of
  times)
• Testing loop control variable (loop will be
  executed zero times if the condition is initially
  false)
• Updating loop control variable (if missing the
  loop will execute forever, which is referred to
  as infinite loop)

7
Computing a Sum in the Loop

• printf("Enter number of maneuversgt ")
• scanf("d", num_mvrs)
• total_time 0.0
• count_mvr 0
  
• while (count_mvr lt num_mvrs) loop
  can be repeated for any number
• 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\n",
• time_to_mvr)
• count_mvr count_mvr 1
• total_time total_time time_to_mvr
  accumulator increases with
• 
  each loop iteration
  
• printf("All maneuvers complete\n")
• printf("Total time spent in maneuver phase
  is 6.1f hours\n",
• total_time) printing
  the value of accumulator

accumulator variable
8
Computing a Product in a Loop

• The loop control variable holds the result of the
  computation that is performed in the loop
• product 1
• while (product lt 10000)
• printf(d\n, product) / Display
  product so far /
• printf(Enter next itemgt )
• scanf(d, item)
• product product item / Update
  product /

9
Compound Assignment Operators

• Instead of variable variable op expression
• variable op expression can be used for
• , -, , /, and operators.
• For example
• ct ct 1 ct 1
• time time 1 time - 1
• product product data product data
• n n (x 1) n x1

10
The for Statement

• Syntax
• for (initialization expression
• loop repetition condition
• update expression)
• statement
• Example
• for (count_star 0 count_star lt 20
• count_star1 )
• printf()

11
The for Statement

• Combines the three loop control steps of
  initialization, testing and update in one place
  (more complex than while)
• Provides a consistent location for each of loop
  initialization, testing and update (more popular
  that while among the C programmers in industry)
• Can be used to count up or down by any interval

12

• for (count_mvr 0 /
  initialization /
• count_mvr lt num_mvrs / loop
  condition /
• count_mvr 1) / update /
• 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\n",
• time_to_mvr)
• total_time time_to_mvr

13

• include ltstdio.hgt
• int
• main(void)
• int time, start
• printf("Enter starting time (an integer) in
  secondsgt ")
• scanf("d", start)
• printf("\nBegin countdown\n")
• for (time start
• time gt 0
• time - 1)
• printf("T - d\n", time)
• printf("Blast-off!\n")
• return (0)
• Enter starting time (an integer) in secondsgt 5
• Begin countdown
• T - 5

14

• / Conversion of Celsius to Fahrenheit
  temperatures /
• include ltstdio.hgt
• / Constant macros /
• define CBEGIN 10
• define CLIMIT -5
• define CSTEP 5
• int
• main(void)
• / Variable declarations /
• int celsius
• double fahrenheit
• / Display the table heading /
• printf(" Celsius Fahrenheit\n")

15

• / Display the table /
• for (celsius CBEGIN
• celsius gt CLIMIT
• celsius - CSTEP)
• fahrenheit 1.8 celsius 32.0
• printf(" 3d 9.2f\n",
  celsius, fahrenheit)
• return (0)
• Celsius Fahrenheit
• 10 50.00
• 5 41.00
• 0 32.00
• -5 23.00

16
Increment and Decrement operators

• Suppose i 2 and j ?
• Prefix form j i
• increments i and then use it gt i3,j3
• Postfix form j i
• uses i and then increment it gt i3, j2
• Example for n4
• printf ( d,--n) gt 3
• printf ( d,n--) gt 4
• Self-Check 5.4.4 for i 3 and j 9 what are
  the results of
• n i --j
• m i j--
• p i j

17
Conditional loops with for statement

• printf("Number of barrels currently in
  tankgt ")
• scanf("lf", start_supply)
• for (current start_supply
• current gt min_supply
• current - remov_brls)
• printf(".2f barrels are
  available.\n\n", current)
• printf("Enter number of gallons
  removedgt ")
• scanf("lf", remov_gals)
• remov_brls remov_gals /
  GALS_PER_BRL
• printf("After removal of .2f gallons
  (.2f
• barrels),\n",
• remov_gals, remov_brls)

18
Sentinel-Controlled Loop

• Correct Sentinel loop Incorrect Sentinel loop
• 1. sum0 sum0
• 2. get score while
  (score!SENTINEL)
• 3. while (score!
• SENTINEL) get score
• 4 . sumscore sumscore
• 5. Get score

19
Sentinel-Controlled Loop

• / Compute the sum of a list of exam scores. /
• include ltstdio.hgt
• define SENTINEL -99
• int
• main(void)
• 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) What is the
  result if score entered as 7o?
• 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)

20

• / Batch Version of Sum of Exam Scores Program /
• include ltstdio.hgt / defines fopen,
  fclose, fscanf,
• 
  fprintf, and EOF /
• int
• main(void)
• FILE inp / input file
  pointer /
• int sum 0, / sum of scores input
  so far /
• score, / current score /
• input_status / status value
  returned by
• fscanf /
• inp fopen("scores.dat", "r")
• printf("Scores\n")
• for (input_status fscanf(inp, "d",
  score)
• input_status ! EOF
• input_status fscanf(inp, "d",
  score))
• printf("5d\n", score)
• sum score

21
Avoiding Infinite Loops on Faulty Data

• To avoid infinite loop when processing faulty
  data the loop in the Sum exam score program can
  be modified as the follows
• for (input_status fscanf(inp, "d", score)
• input_status 1
• input_status fscanf(inp, "d",
  score))
• printf("5d\n", score)
• sum score
• if (input_status EOF)
• printf(Sum of scores is d\n, sum)
• else
• printf(Error in input, bad character)
• Note that in case of reaching EOF input_status is
  negative and in case of faulty data input_status
  is zero otherwise it is always 1.

22

• / Finds the product of nonzero data. /
• include ltstdio.hgt
• define SENT -9999.0
• int
• main(void)
• double product 1.0 / product so far
  of nonzero data /
• int status / status of input
  operation /
• double data_item / current data
  value /
• printf("Enter a data item (.1f to quit)gt
  ", SENT)
• for (status scanf("lf", data_item)
• status 1 data_item ! SENT
• status scanf("lf", data_item))
  
• if (data_item ! 0) /
  if statement is nested within the loop /
• product data_item
• printf("Enter next data item (.1f to
  quit)gt ", SENT)
• if (status ! 1)
• printf("Error in data list\n")

23
Nested Loops

• include ltstdio.hgt
• int
• main(void)
• int i, j / loop control variables /
• printf(" I J\n") /
  prints column labels / Program output
• for (i 1 i lt 4 i) / outer
  for loop /
• printf("Outer 6d\n", i)
• for (j 0 j lt i j) /
  inner loop /
• printf(" Inner9d\n", j)
• / end of inner loop /
• / end of outer loop /
• return (0)

I J Outer 1 Inner
0 Outer 2 Inner 0 Inner
1 Outer 3 Inner 0 Inner 1
Inner 2
24
The do-while Statement

• The first iteration of the loop body occurs
  unconditionally (number of repetition is 1 or
  more instead of 0 or more in the while statement)
• All the statements in the loop are processed
  identically
• Can be used efficiently for checking the valid
  input data

25
The do-while Statement

• SYNTAX
• do
• statement
• while ( loop repetition condition)
• Example
• do / Finding the first even number input
  /
• status scanf(d num)
• while (status gt0 num 2 !0)

26
Flowchart of do-while when using for input
validation
prompt user to get a value for checking
valid input

• value not in desired range

true
false
27
do-while When Using for Input Validation

• / Gives an error message on input of an invalid
  data type /
• printf("Enter minimum and maximum valid valuesgt
  ")
• scanf("dd", n_min, n_max)
• do
• printf("Enter an integer in the range
  from d to d inclusivegt ",
• n_min, n_max)
• status scanf("d", inval)
• if (status 1)
• error 0
• else
• error 1
• scanf("c", skip_ch)
• printf("\nInvalid charactergtgtcgtgt
  Skipping rest of line.\n",
• skip_ch)
• do
• scanf("c", skip_ch)
• while (skip_ch ! '\n')
• / Exit
  the loop only if the entry is valid /
• while (error inval lt n_min inval gt
  n_max)

28
Flag_conrolled Validation Loop

• do
• error 0 / No errors detected yet
  /
• / Get a fraction from the user /
• printf("Enter a common fraction as two
  integers separated by ")
• printf("a slash\nand press ltentergt\ngt
  ")
• status scanf("dcd", num, slash,
  den)
• / Validate the fraction /
• if (status lt 3)
• error 1
• printf("Input invalid-please read
  directions carefully\n")
• else if (slash ! '/')
• error 1
• printf("Input invalid-separate
  numerator and denominator")
• printf(" by a slash (/)\n")
• else if (den lt 0)
• error 1
• printf("Input invalid-denominator
  must be positive\n")
• do / Discard extra input characters /

29
Case Study Computing Radiation Level

• Problem ..Determine how long does it take before
  the radiation is down to a safe level of 0.466
  millirem a day. Use a chart to display radiation
  level for every three days with the message
  unsafe and safe after every line. The program
  should stop just before the radiation level is
  one-tenth of the safe level.
• Analysis data requirements
• Constants (SAFE_RAD, SAFETY_FACT),
• Input (init_radiation), Output (day,
  radiation_lev)

30
Case study Computing Radiation Level

• Design Algorithm
• Initialize day to zero.
• Compute the stopping level
• Prompt user to enter the initial radiation level
• Compute day number and radiation level every 3
  days for the levels that exceed the stopping
  radiation level
• 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

Can we Change it to the for loop?
31
Case study Computing Radiation Level

• Implementation
• ............................
• ./ Prompts user to enter initial radiation level
  /
• printf("Enter the radiation level (in
  millirems)gt ")
• scanf("lf", init_radiation)
• / Displays table
  /
• printf("\n Day Radiation Status\n
  (millirems)\n")
• for (radiation_lev init_radiation
• radiation_lev gt min_radiation
• radiation_lev / 2.0)
• if (radiation_lev gt SAFE_RAD)
• printf(" 3d 9.4f
  Unsafe\n", day, radiation_lev)
• else
• printf(" 3d 9.4f Safe\n",
  day, radiation_lev)
• day 3
• return (0)
• ..

32
Case study Computing Radiation Level

• Testing Program result
• Enter the radiation level (in millirems)gt 150.0
• Day Radiation Status
• (millirems)
• 0 150.0000 Unsafe Initial value
• 3 75.0000 Unsafe 75 is 150 / 2,
  it shows calculation is ok
• 6 37.5000 Unsafe
• 9 18.7500 Unsafe The number of
  days increases by 3
• 12 9.3750 Unsafe
• 15 4.6875 Unsafe
• 18 2.3438 Unsafe
• 21 1.1719 Unsafe
• 24 0.5859 Unsafe
• 27 0.2930 Safe
• 30 0.1465 Safe Ending value,
  shows the loop exited
• 
  correctly

33
How to Debug and Test Programs

• Using debugger programs Quincy debugger
• Debugging without a Debugger
• Diagnostic call to printf
• define DEBUG 1
• if (DEBUG)
• printf(score is d, sum is d\n,score,sum)
  
• fflush(stdout) / copying the contents of buffer
  to associated output device /

34
Debugging without a Debugger

• 2- Off-by-One loop errors
• Checking the loop repetition. For example
• for (count 0 count ltn count) sumcount
  /executes n1 times/
• for (count 0 count ltn count) sumcount
  /executes n times /
• Checking the boundaries initial value, end value
  and tracing loop with a small loop control
  variable

35
Common Programming Errors

• Confusion between if , while and for
• Forgetting to put braces for more than two
  statements in the loop body
• Making mistake when using tests for inequality to
  control loop repetition
• while (balance !0.0) instead of
• while (balancegt0.0)
• The first while, repeats for negative balance

36
Common Programming Errors

• Using instead of in the loop repetition
  condition produces infinite loops
• do .
• while (again1)
• do-while can be used when there is no possibility
  of zero loop
• Confusion in compound assignment operators
• a b c is a a (b c) it is not a a b
  c
• Confusion in increment and decrement operators by
  ignoring operators side effect of changing the
  value of the operand