Stretch tasks Designing, Creating and Refining Algorithms

Round 1: Quiz Blitz

1. A diamond A diamond has exactly two ways out, which must be labelled yes and no.
2. A parallelogram Rectangles are processes, diamonds are decisions, rounded rectangles start and stop.
3. True That distinction is examined regularly.
4. 10 1 + 2 + 3 + 4 = 10. The loop includes 4.
5. 4 Five elements have indexes 0, 1, 2, 3 and 4. There is no index 5.
6. Comp substring(0,4) starts at index 0 and takes 4 characters: Comp.
7. 11 0 to 10 includes both ends: that is 11 values. Off by one errors start here.
8. Stopping a search as soon as the item is found; Moving an unchanging calculation out of a loop Efficiency means the same output with fewer steps or less memory.
9. total is used before it is given a value total must be initialised, for example total = 0, before the loop adds to it.
10. trace table A trace table shows what an algorithm actually does, not what you assume it does.
11. 24 x goes 3, 6, 12, 24. At 24 the condition x < 20 is false, so the loop stops.
12. True An unlabelled diamond is ambiguous, and in the exam it loses the mark.

Round 2: Quick Fire

1. • A function returns a value
   • A procedure carries out a task but does not return a value
   Model answer

   A function returns a value to the code that called it. A procedure carries out a task but does not return a value.

2. • Draw a column for every variable and for any output
   • Work through one line at a time, adding a row whenever a value changes
   Model answer

   Draw a column for every variable and one for any output. Then work through the algorithm one line at a time, writing a new row whenever a value changes, without skipping any iterations.

3. • The loop starts at 0, so it runs 11 times
   • It should be for i = 1 to 10
   Model answer

   The loop starts at 0, so it runs 11 times and prints 0 as well. It should be for i = 1 to 10.

4. • Stop searching as soon as the item is found
   • Move calculations that never change out of a loop
   • Replace repeated code with a loop or subroutine
   • Use a more efficient algorithm, such as binary search
   Model answer

   Stop searching as soon as the item is found instead of checking the rest of the list, and move any calculation that never changes outside the loop.

Round 3: Big Think

1. • i = 1 and 2: total becomes 1, then 3
   • i = 3: total becomes 6
   • i = 4: total becomes 10
   • It prints 6, then 10
   Model answer

   i = 1, total = 1. i = 2, total = 3. i = 3, total = 6, which is more than 5, so it prints 6. i = 4, total = 10, so it prints 10. The output is 6 and 10.

2. • Off by one errors in loops
   • Array indexes out of range
   • Assignment used instead of comparison
   • The wrong comparison operator at a boundary
   • A variable used before it is initialised
   • An infinite loop
   • The wrong data type
   • Steps in the wrong order
   Model answer

   Check for off by one errors, where a loop runs one too many or one too few times, and fix the start or end value. Check every array index is in range: an array of 5 items has indexes 0 to 4. Check that each variable is initialised before it is used, for example setting a total to 0 before adding to it. Check for infinite loops, where the condition can never become false, and make sure the loop variable changes inside the loop.

This or That

1. A rounded rectangle Terminals are rounded rectangles. Diamonds are decisions.
2. Hello Sam + joins two strings together, so the result is one string.
3. A 2D array Two sizes means two dimensions: 3 rows by 3 columns.
4. endwhile next ends a for loop. endwhile ends a while loop.
5. At least once The condition is checked at the end, so the body runs before it is ever tested.
6. Marks are given for the table A full table can earn marks even with one slip. A lone final answer cannot.
7. big 7 is greater than 5, so the first branch runs.
8. A for loop A for loop is count-controlled. A while loop repeats until a condition changes.

Memory Match

• A 4 for i = 1 to 5: A count-controlled loop
• B 6 while x < 10: A condition-controlled loop
• C 1 name.length: The number of characters in a string
• D 2 return: Sends a value back from a function
• E 3 Parallelogram: Flowchart symbol for input or output
• F 5 Diamond: Flowchart symbol for a decision

Sort It

1. Process total = total + price A calculation or assignment is a process: a rectangle.
2. Process count = 0 Setting a variable is a process.
3. Input/output Output a well done message Showing a message is output.
4. Input/output Display the total Output goes in a parallelogram too.
5. Input/output Ask the user for their name Reading input goes in a parallelogram.
6. Decision Is score > 50? A yes or no question goes in a diamond.
7. Process Calculate the average Calculations go in process boxes.
8. Terminal Stop Every flowchart ends with a terminal.
9. Terminal Start Start and stop go in rounded rectangles: terminals.
10. Decision Has the user typed quit? A question with two branches is a decision.
11. Decision Is the list empty? A decision diamond needs yes and no labels.