Tracing selection with a trace table

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Quick Questions

Shade in the A or B circle for each question.

1. A trace table has x = 7. The algorithm says: if x > 10 then print "big". What do you record for the condition?

A TRUE, so print "big"

B FALSE, so skip the print line

2. Which step comes FIRST when you trace an if statement?

A Check the condition using the current values

B Run the lines inside the if block

3. score = 50. The code says: if score >= 50 then print "Pass". What goes in the condition column?

A FALSE

B TRUE

4. In an if-then-else, the condition is TRUE. What happens to the else branch?

A It is skipped

B It also runs after the then branch

5. age = 12. The code says: if age > 12 then print "Teen" else print "Kid". What is printed?

A Kid

B Teen

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1if score >= 50 then		AThe condition was worked out wrongly. 3 > 5 is FALSE, so the then branch should be skipped.
2else		B\| age \| age >= 13 \| message \|
3The trace table has a column to record the condition result for each check		CBoth branches were followed. Only the branch that matches the condition result is followed.
4If the condition is FALSE, skip the then branch and do not change its variables in the table		DThe branch that is skipped when the condition was TRUE, and followed only when it was FALSE
5x = 3, x > 5 is TRUE, so follow the then branch		EWork out score >= 50 with the current value of score, then write TRUE or FALSE in the trace table
6temp = 30, temp > 25 is TRUE: do then AND else		Fx <- 4 if x > 10 then x <- 0

Definitions

trace an if

Fill in the Blanks

Complete each line of code by filling in the missing parts.

Trace row: score=45, score >= 50 is _____

If the condition is _____ , skip the then branch

x=4 | x > 10 | FALSE | then _______

age=15 | age >= 18 | FALSE | ____ runs

n <- 7 __ n > 5 then print "Big"

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A score ← 45

B if score ← 50 then

C print "Pass"

D else

E print "Try again"

F end if

Question 2

A 31 ← temp

B if temp > 30 then

C print "Hot"

D end if

Question 3

A age ← 12

B if age < 13 then

C print Junior

D else

E print "Senior"

F end if

Question 4

A mark ← 72

B if mark ≥ 50 then

C print "Pass"

D end

E print "Done"

Question 5

A money ← 20

B if money ≥ 15 then

C money ← money - 15

D print "Bought

E end if

Answer Key

Tracing selection with a trace table

Section A: Quick Questions

B — FALSE, so skip the print line

A — Check the condition using the current values

B — TRUE

A — It is skipped

A — Kid

Section B: Match the Pairs

if score >= 50 then → Work out score >= 50 with the current value of score, then write TRUE or FALSE in the trace table

else → The branch that is skipped when the condition was TRUE, and followed only when it was FALSE

The trace table has a column to record the condition result for each check → | age | age >= 13 | message |

If the condition is FALSE, skip the then branch and do not change its variables in the table → x <- 4 if x > 10 then x <- 0

x = 3, x > 5 is TRUE, so follow the then branch → The condition was worked out wrongly. 3 > 5 is FALSE, so the then branch should be skipped.

temp = 30, temp > 25 is TRUE: do then AND else → Both branches were followed. Only the branch that matches the condition result is followed.

Section C: Definitions

trace an if — Check the condition using the current values, record TRUE or FALSE in the trace table, then follow only the branch that is true and skip the other branch.

Section D: Fill in the Blanks

Trace row: score=45, score >= 50 is FALSE

If the condition is FALSE
, skip the then branch

x=4 | x > 10 | FALSE | then skipped

age=15 | age >= 18 | FALSE | else runs

n <- 7
if n > 5 then
  print "Big"

Section E: Spot the Error

B — The condition uses the assignment arrow ←, which stores a value. A condition has to compare, so it should be: if score ≥ 50 then. Without a comparison there is no TRUE or FALSE to record in the trace table.

A — The assignment is backwards. The variable name goes on the left of the arrow and the value on the right: temp ← 31. Without this, the trace table has no starting value for temp.

C — Text to be printed must be inside quotation marks. Without them, Junior looks like a variable name. It should be: print "Junior".

D — The selection block must be closed with end if. Without it, we can't tell where the true branch stops and where the code that always runs (print "Done") begins.

D — The text is missing its closing quotation mark. It should be: print "Bought".