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Dry Run

Dry Run. You can test your program without using a computer by dry running it on paper You act as the computer – following the instructions of the program, recording the valves of the variables at each stage You can do this with a table . Dry Run.

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Dry Run

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  1. Dry Run • You can test your program without using a computer by dry running it on paper • You act as the computer – following the instructions of the program, recording the valves of the variables at each stage • You can do this with a table

  2. Dry Run • The table with have column headed with the names of the variables in the program • Each row in the table will be labelled with a line number form the program. • The entries of each row in th e table will be values of the variables after the execution of the statement on that line • You don’t need a row for every line in the program, just those lines where some significant change to the state of the program occurs e.g a variable gets a new value

  3. Dry Run • In this table you can record all relevant changes to the variables as the program progresses, thereby test the logic of the program / algorithm • Do a dry run before you code your program on computer this way any logic errors will come to light during the dry run

  4. Example of a Dry run • L1 Declare two variables , first num second num • L2 Initialise both variables to 0 • L3 first num = 0 second num = 0 • L4 Ask user to enter first number • L5 Assign user input to first num variable • L6 Ask user to enter second number • L7 Assign user input to second num variable • L8 Add first num to second num • L9 Print result • Do a Dry run for this program assuming the user enters 17 for first number and 24 for the second

  5. A sample Dry Run table

  6. Relational Operators • Give examples • < less than • > greater than • <= Less than or equal to a<=b • >= greater than or equal c>=d • == equals • != not equals 5!=6 • Boolean expression – TRUE or FALSE

  7. If then Else statements • If <TEST> • <STATEMENT 1 > • Else • <STATEMENT 2 > • <TEST> is a boolean expression • Is <TEST? Has a false valve then <STATEMENT 2 > is executed • Is <TEST? Has a false valve then <STATEMENT 1 > is executed • Note the indentation – this makes it easier to read

  8. Nesting Layout If (mark > 69) grade = “A” Else If (mark > 59) grade = “B” Else If (mark > 49) grade = “C” Else If (mark > 39) grade = “D” Else grade = ‘F’ In this alternative the psuedo code crawls across • To make your algorithm easier to read nest the statements If (mark > 69) grade = “A” Else If (mark > 59) grade = “B” Else If (mark > 49) grade = “C” Else If (mark > 39) grade = “D” Else grade = ‘F’

  9. Nesting of if-else and while loops Int number Number =3 For loop : 1 to 3 if number = 1 print ‘red’ else if number = 2 print ‘green’ else if number = 3 print ‘yellow’ endif Loop end

  10. More Complex Loop Tests • Sometimes we may want to allow the user to terminate the program/loop • We can do this by prompting the user with a question e.g imagine a user is entering a series of numbers which will be added to together, like student marks. We can ask the user the question (Do you want to add another student mark? ) • We assign the user response to a variable named e.g. continue Declare variable continue Initialise the variable Continue = ‘X’ Ask the user (Do you want to add another student mark? ) Assign/store the user response to variable continue Continue := user response While (continue = ‘Y’) Loop Body…

  11. More Complex Loop Tests • Sometimes you may want to include more than one condition in your loop tests • In such cases you can use boolean connectives (&& and ||) • E.g. • while (continue = ‘y’) && (item number < 10) • Where item number = number of items to be added

  12. Problem solving • Understanding the problem • The plan – decide how to tackle problem - algorithms • Test and Evaluate – dry run and test plans

  13. Test Plans • Do before you code! • Do a test plan for every branch test e.g. if the else and loop statements • Extreme values such as 0 or negative numbers are tested in addition to ‘normal’ values. Testing the bounds is most efficient way for testing for run time errors

  14. The following psuedo code / algorithm prints the larger of two numbers • Do a test plan to find it – a series of valves to be input in variables first and second .Run the program several times Int first Int second Int max Sring str Prompt user to enter an integer (please enter a number) Store the number into variable first Prompt user to enter a second integer (please enter a second number) Store the second number into variable second Max = first if (second > max) max = second Print message to screen saying (larger of the two numbers is max)

  15. Test plan example

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