HIGHER CS STANDARD ALGORITHMS Part 2- Techniques to help with learning about standard algorithms.
Standard Algorithms. 4 Standard Algorithms Input Validation Finding the Maximum / Minimum Counting...
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Transcript of Standard Algorithms. 4 Standard Algorithms Input Validation Finding the Maximum / Minimum Counting...
4 Standard Algorithms
• Input Validation
• Finding the Maximum / Minimum
• Counting Occurrences
• Linear Search
Standard Algorithms
• Other than input validation the other 3 standard algorithms are ways of extracting different pieces of information from an array
Input Validation
Input validation can be achieved in two ways:
• Restrict the data the user can input by only presenting them with a limited set of possibilities such as a drop box or pull down menu,
• Accept the input but reject any which does not meet the restrictions imposed by the software and ask them to input it again.
This second approach is the one we are going to use.
Input Validation
PROCEDURE GetValidInput()
RECEIVE userInput FROM (INTEGER) KEYBOARDWHILE userInput < lowerLimit OR userInput >
upperLimit DO SEND "Input must be between "& lowerLimit & " and
" & upperLimit TO DISPLAY RECEIVE userInput FROM (INTEGER) KEYBOARDEND WHILE
END PROCEDURE
Input ValidationPROCEDURE GetValidInput()
REPEAT
RECEIVE userInput FROM (INTEGER) KEYBOARD
IF userInput < lowerLimit OR userInput > upperLimit THEN SEND "Input must be between " & (STRING) lowerLimit & " and " & (STRING)
upperLimit TO DISPLAY END IF
UNTIL userInput >= lowerLimit AND userInput <= upperLimit
END PROCEDURE
This algorithm is less efficient than the previous one because the input is being checked twice.
Input validation (String)
PROCEDURE GetValidInput()
SEND ["Please enter Y or N"] TO DISPLAYRECEIVE userInput FROM (STRING) KEYBOARDWHILE userInput ≠ ["Y"] AND userInput ≠ ["N"] DO SEND "Input must be Y or N " TO DISPLAY RECEIVE userInput FROM (STRING) KEYBOARDEND WHILE
END PROCEDURE
Input Validation with boolean flag
PROCEDURE GetValidInput()validInput = falseREPEAT RECEIVE userInput FROM (STRING) KEYBOARD IF length(userInput) < lengthLimit THEN validInput = true ELSE SEND "Input must be less than " &
(STRING) lengthLimit & " characters" TO DISPLAY END IFUNTIL validInput = trueEND PROCEDURE
Input Validation with boolean flag
In this version of the algorithm the boolean variable validInput is set to false at the beginning, and the conditional loop only terminates once it has been set to true.
This version of the algorithm is useful if you want to check a number of different conditions in the input string
Input Validation (Function)
FUNCTION ValidItem(lowerLimit, upperLimit)RETURNS INTEGER
RECEIVE userInput FROM (INTEGER) KEYBOARD WHILE userInput < lowerLimit OR userInput > upperLimit
DO SEND "Input must be between "& lowerLimit " and "
&upperLimit" TO DISPLAY RECEIVE userInput FROM (INTEGER) KEYBOARD END WHILE RETURN userInput
END FUNCTION
Input Validation (Function)
We could call this function with actual parameters, 1 and 50 to return a number between 1 and 50:
numberToUse = ValidItem(1,50)
or we could call it with the actual parameters 1 and inputRange which is a variable which has a value assigned elsewhere in the program:
RECEIVE inputRange FROM (INTEGER) KEYBOARDnumberToUse = ValidItem(1,inputRange)
This call would return a value between 1 and inputRange.
Iterating through arrays
• Finding the maximum / minimum value
• Counting occurrences
• Linear search
Finding the Maximum
SET largest TO the value of the first item in the array
FOR each of the remaining items in the array DO
IF item_value > largest THEN
SET largest TO item_value
END IF
END FOR
print largest
Finding the MaximumPROCEDURE FindMax()
SET maximumValue TO numbers[0]FOR counter FROM 1 TO 9 DO IF maximumValue < number THEN SET maximumValue TO number END IFEND FORSEND "The largest value was "& (STRING) maximumValue TO DISPLAY
END PROCEDURE
Finding the Maximum
PROCEDURE FindMax()
SET maximumValue TO numbers[0]FOR counter FROM 1 TO 9 DO IF maximumValue < number THEN SET maximumValue TO number END IFEND FORSEND "The largest value was "& (STRING) maximumValue TO DISPLAY
END PROCEDURE
Finding the Minimum
SET smallest TO the value of the first item in the array
FOR each of the remaining items in the array
IF item_value < smallest THEN
SET smallest TO item_value
END IF
END FOR
print smallest
Finding the MinimumPROCEDURE FindMin()
SET minimumValue TO numbers[0]FOR counter FROM 1 TO 9 DO IF minimumValue > numbers[counter] THEN SET minimumValue TO numbers[counter] END IFEND FORSEND "The smallest value was " & (STRING) minimumValue TO DISPLAY
END PROCEDURE
Finding the MinimumPROCEDURE FindMin()
SET minimumValue TO numbers[0]FOR counter FROM 1 TO 9 DO IF minimumValue > numbers[counter] THEN SET minimumValue TO numbers[counter] END IFEND FORSEND "The smallest value was " & (STRING) minimumValue TO DISPLAY
END PROCEDURE
Counting Occurrences
Ask for target_value to count
SET number_found TO zero
FOR every item in the array
IF equal to target_value THEN add 1 to number_found
END FOR
Print number_found
Counting OccurrencesPROCEDURE CountOccurrences()
RECEIVE itemToFind FROM (INTEGER) KEYBOARD
SET numberFound TO 0
FOR EACH number FROM numbers DO
IF number = itemToFind THEN
SET numberFound TO numberFound + 1
END IF
END FOREACH
SEND "There were " & (STRING) numberFound) & "occurrences of " & (STRING) itemToFind & " in the list" TO DISPLAY
END PROCEDURE
Counting OccurrencesPROCEDURE CountOccurrences()
RECEIVE itemToFind FROM (INTEGER) KEYBOARD
SET numberFound TO 0
FOR EACH number FROM numbers DO
IF number = itemToFind THEN
SET numberFound TO numberFound + 1
END IF
END FOREACH
SEND "There were " & (STRING) numberFound) & "occurrences of " & (STRING) itemToFind & " in the list" TO DISPLAY
END PROCEDURE
Counting Occurrences
PROCEDURE CountOccurrences()
RECEIVE itemToFind FROM (INTEGER) KEYBOARD
SET numberFound TO 0
FOR counter FROM 0 TO 9 DO
IF number[counter] = itemToFind THEN
SET numberFound TO numberFound + 1
END IF
END FOR
SEND "There were " & (STRING) numberFound) & "occurrences of " & (STRING) itemToFind & " in the list" TO DISPLAY
END PROCEDURE
Linear Search
Input target_value from userSET found to falseSET Counter TO 0REPEAT IF array[counter] equals target_value THEN SET found to
true END IFadd 1 to counterUNTIL foundIF Counter > list_length THEN print "Not found"Print "Found at position" counter
Linear SearchPROCEDURE linearSearch()RECEIVE itemToFind FROM (INTEGER) KEYBOARD
SET found TO falseSET arraySize TO highestIndexSET counter TO 0
REPEAT IF numbers[counter] = itemToFind THEN SET found to true END IF SET counter TO counter + 1UNTIL found OR counter > arraySizeIF found THEN SEND (STRING) itemToFind &" found at position" & (STRING) counter - 1) TO DISPLAYELSE SEND "Item not found" TO DISPLAYEND IFEND PROCEDURE
Linear SearchPROCEDURE linearSearch()RECEIVE itemToFind FROM (INTEGER) KEYBOARD
SET found TO falseSET arraySize TO highestIndexSET counter TO 0
REPEAT IF numbers[counter] = itemToFind THEN SET found to true END IF SET counter TO counter + 1UNTIL found OR counter > arraySizeIF found THEN SEND (STRING) itemToFind &" found at position" & (STRING) counter - 1) TO DISPLAYELSE SEND "Item not found" TO DISPLAYEND IFEND PROCEDURE
Linear SearchPROCEDURE linearSearch()RECEIVE itemToFind FROM (INTEGER) KEYBOARD
SET found TO falseSET arraySize TO highestIndexSET counter TO 0
REPEAT IF numbers[counter] = itemToFind THEN SET found to true END IF SET counter TO counter + 1UNTIL found OR counter > arraySizeIF found THEN SEND (STRING) itemToFind &" found at position" & (STRING) counter - 1) TO DISPLAYELSE SEND "Item not found" TO DISPLAYEND IFEND PROCEDURE
Linear Search
PROCEDURE linearSearch()RECEIVE itemToFind FROM (INTEGER) KEYBOARD
SET found TO falseSET arraySize TO highestIndexSET counter TO 0
REPEAT SET found TO numbers[counter] = itemToFind SET counter TO counter + 1UNTIL found OR counter > arraySize
IF found THEN SEND (STRING) itemToFind &" found at position" & (STRING) counter - 1) TO DISPLAYELSE SEND "Item not found" TO DISPLAYEND IFEND PROCEDURE
Example from SQA paper 2010
A subprogram in building management software is used to find the range of temperatures in a building in one day. The temperature is recorded every 15 minutes within a 24 hour period and stored in a list.
Use pseudocode to design one algorithm to find both the highest and lowest temperatures in this list.
Example from SQA paper 2010
PROCEDURE findMaxMin()SET min TO temp[0]SET max TO temp[0]FOR counter FROM 0 TO 95 If temp[counter] > max THEN SET max TO temp[counter] If temp[counter] < min THEN SET min TO temp[counter] END FOR SEND "Maximum = " & max TO DISPLAYSEND "Minimum = " & min TO DISPLAYEND PROCEDURE
Example from SQA paper 2011
RightIT, a software company, is currently developing a cash machine program for a bank. The cash machine will offer five options to customers.
The options selected during a day are stored as a list. The bank would like the software to calculate the number of times the mobile top-up option appears on this list. Use pseudocode to design an algorithm to carry out this calculation.
Example from SQA paper 2011
PROCEDURE countTopups()
SET total TO 0
FOR EACH item FROM options
IF item = mobile_top_up THEN
SET total TO total + 1
END IF
END FOR
SEND "There were "& total&" mobile topups" to DISPLAY
END PROCEDURE
Another Counting Occurrences example
• At the end of each round of an archery competition, those players who have a score which is the same or greater than the qualifying score are selected for the next round. The software is required to count the number of players who have qualified. Use pseudocode to design an algorithm to carry out this calculation.
• State the data structure which will be used to store the players’ scores.
Another Counting Occurrences example
Data structure is an integer array
PROCEDURE countQualifiers()
SET Number_qualified TO 0
FOR each archer_score FROM scores
IF archer_score >= qualification_score THEN
SET Number_qualified TO Number_qualified +1
ENDIF
END FOR
SEND "Number of qualifiers = "& Number_qualified TO DISPLAY
END PROCEDURE
Linear search example
Customers are given a unique ID number when they register with their employer. Customer ID’s are created using the first 3 letters of the customers surname and the last 3 letters of their postcode. This ID is stored in a 1-D array. For example: Mr J Brown with a postcode of EH15 6NW has a customer ID of BRO6NW
Using a design notation with which you are familiar, write an algorithm that would find a particular customer ID from the array.
Linear search example
PROCEDURE findEmployee()SET found TO falseSET counter TO 0RECEIVE target_ID FROM (STRING) KEYBOARDREPEAT SET found TO employee[counter] = target_ID SET counter TO counter + 1UNTIL found OR counter > total_employeesIF found THEN SEND ["Found at position " & counter] TO DISPLAYELSE SEND ["Not found"] TO DISPLAYEND IFEND PROCEDURE
Linear search example
PROCEDURE findEmployee()SET found TO falseSET counter TO 0RECEIVE target_ID FROM KEYBOARDREPEAT SET found TO employee[counter] = target_ID SET counter TO counter + 1UNTIL found OR counter > total_employeesIF found THEN SEND ["Found at position " & counter] TO DISPLAYELSE SEND ["Not found"] TO DISPLAYEND IFEND PROCEDURE
Cycle Race: What Data Structure?
• Race times: real array
• Names: string array
• Nationalities: string array
• Qualifiers: boolean array
Cycle Race: What standard algorithm?
• Find the best race time
• Find how many cyclists were from Scotland
• Find the name of the winner
• Find Bradley’s time
• Find how many qualified
Best race time
• Best race time – Find the minimum from race_times array
• Name of winner– match to names array
• Cyclists from Scotland– Count occurrences from nationalities array
• Bradley’s time– Linear search names array then match to race_times array
• Qualifiers– Count occurrences from qualifiers array