loops

• A loop is a repetition control structure.• body - statements to be repeated• control statement - decides whether another

repetition needs to be made • leading decision loop - control statement before

body• trailing decision loop - control statement after

body• Counted loop- for• Logical loop - WHILE..ENDWHILE

Loops

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Leading DecisionWHILE condition

body of loop – group of one or more statements

indent one level

ENDWHILE

loops

When the expression is tested and found to be false, the loop is exited and control passes to the statement which follows the loop body.

WHILE LOOP

FALSE

TRUE

bodystatement

Expression

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1-2-3-4 for While Loops

1.Initial condition2.Test - WHILE(…)3.Body4.Next - often the same as step 1

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Example

Display "Do you want to play?(y/n)" 1

Input ans

WHILE (ans = ‘y’) OR (ans = ‘Y’) 2

… //body 3

Display “Do you want to continue(y/n)?” 4

Input ans

ENDWHILE

Display “Thanks for playing!”

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Loops

Sentinel controlled

keep processing data until a special value is entered to indicate that processing should stop

Read blood pressures until a special value (like -1) selected by you is read.

Count Controlled

keep processing data for a specified number of times

Read 100 blood pressures.

End-of-file Controlled

keep processing data as long as there is more data in the file

Read all the blood pressures from a file no matter how many are there

Flag Controlled

keep processing data while a flag condition is true

Read blood pressures until a dangerously high BP (200 or more) is read.

loops

A Sentinel-controlled Loop

• Read numbers until -1, 999

• Not always easy to determine sentinel value

• requires a “priming read”

• “priming read” means you read one set of data before the WHILE

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1-2-3-4 Sentinel Value

1. Initial condition • Get first value

2. Test - WHILE(…) • While (val != sentinel)

3. Body4. Next - often the same as step 1

• Get next value

loops

// Sentinel controlled loop

total = 0;

Display "Enter a blood pressure (-1 to stop ) " 1

Input thisBP;

WHILE (thisBP != -1) // WHILE not sentinel 2

total = total + thisBP; 3

Display “Enter a blood pressure (-1 to stop ) ” 4

Input thisBP

ENDWHILE

Display total

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Example

Input number

WHILE (number < 0)

Display “Enter positive values only!”

Input number

ENDWHILE

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Reading in records with sentinel value

• Trailer record

Read fahrTemp 1

WHILE fahrTemp <> 999 2

cels_temp = (5 * (fahrTemp – 32))/9 3

Print fahrTemp, celsTemp

Read fahrTemp 4

ENDWHILE

loops

End-of-File Controlled Loop

• depends on fact that a file goes into fail state when you try to read a data value beyond the end of the file

• No trailer recordWHILE (there is a record)WHILE (not end of file)WHILE (records exist)

• Computer indicates there are no more records by sending a signal to the program

• Must read record before entering loop – there may be no records

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1-2-3-4 Reading in Records

1. Initial condition • Read first record

2. Test - WHILE(…) – • While (not eof)

3. Body4. Next - often the same as step 1

• Read next record

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Example

Read fahrTemp 1

WHILE (there is a record) 2

cels_temp = (5 * (fahrTemp – 32))/9 3

Print fahrTemp, celsTemp

Read fahrTemp 4

ENDWHILE

loops

Open file

total = 0;

Read thisBP; // priming read 1

WHILE (there is a record) 2

total = total + thisBP 3

Read thisBP // read another 4

END WHILE

Display total;

// End-of-file controlled loop

loops

• Do something a set number of times

• Need counter– initialize– increment

• iteration counter - incremented during each iteration of the loop

• event counter - incremented each time a particular event occurs

Count-controlled loop

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1-2-3-4 Count

1. Initial condition • Initialize counter

2. Test - WHILE(…) • While (counter > 0)

3. Body4. Next –

• Increment Counter

loops

//Print Hello 10 times

Declare integer count

count = 0; 1

WHILE (count < 10) 2

Display "Hello" 3

count = count + 1 4

ENDWHILE

Known Count

loops

//Print Hello 10 times

Declare integer count

Display "How many times should we print Hello?"

Input count 1

WHILE (count > 0) 2

Display "Hello" 3

count = count -1 4

ENDWHILE

Count is variable

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Accumulators and Counters• To find the average of a group of numbers-need running

total and how many numbers• Counter – storage area in which we count

– increment– counter = counter + 1– paychecks = paychecks + 1

• Accumulator – storage area for keeping cumulative or running totals– accumulator = accumulator + number– total_wages_paid = total_wages_paid + net_pay– Total = total + num

• You must initialize the values of accumulators and counters

loops

Declare integer thisBP, total, count

Open filecount = 0 // initialize 1total = 0Read thisBP WHILE ( count < 100 AND there is a record) 2 total = total + thisBP ; 3

count= count + 1 Read thisBP; 4END WHILEDisplay “The total = “ total Display "The average is " total/count

21

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Infinite Loop

index = 1

WHILE (index < 5)

Print “Good Morning!”

ENDWHILE

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Never executed

WHILE ans = “yes”

….

Print “Do you want to add another number?”

Input answer

ENDWHILE

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Don't forget to prime the loop!

• Initialize initial condition by reading in or setting value

• Input ansWHILE (ans = 'y')

• index = 0WHILE (index < 10)

• Read name, ssNum,phone WHILE (there is a record)

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Declare integer countDeclare real total, avg, numtotal = 0count = 0Input numWHILE not end-of-file

total = total + numcount = count + 1Input num

ENDWHILEIF count = 0 THEN

Print “No numbers entered”ELSE

avg = total/countPrint “The average is “,avg

ENDIF

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Trailing Decision Loop

REPEAT

Body

UNTIL condition

Test at the bottom

Statements are executed at least once

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Trailing decision loop

Body

conditionFALSE

TRUE

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Example

REPEAT

Display “Enter two numbers”

Input num1,num2

Display num1, “ + “, num2, “ = “, num1+num2

Display “Do you want to enter two numbers again?”

Input ans

UNTIL ans = “no”

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Counted loop

• Fixed number of iterations• Use a variable as a counter which starts at a

specified number and increments the variable each time the loop is processed

• Repeats until the counter is greater than an ending number

• Beginning value, ending value, increment value

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Counted LoopDO counter = initial_value to end_value

…ENDDOAutomatic:1. When the computer executes the DO instruction, it sets

the counter equal to the initial value2. When the loop executes the ENDDO, it increments the

counter3. When the counter is less than or equal to the ending

number, the processing continues to the body. When the counter is greater than the ending number, the processing continues at the instruction that follows the ENDDO instruction.

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Examples• DO count = 1 to 10

Display "Hello"ENDDO

• DO count = 1 to 100Display count

ENDDO• DO num = 10 to 0 step - 1

Display numENDDODisplay "Blast off"