FOR loop with STEP (7c)

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Quick Questions

Shade in the A or B circle for each question.

1. Which loop counts down from 10 to 1?

A FOR Count ← 10 TO 1

B FOR Count ← 10 TO 1 STEP -1

2. Which loop outputs only the even numbers 2, 4, 6, 8, 10?

A FOR Num ← 2 TO 10 STEP 2

B FOR Num ← 2 TO 10 STEP 1

3. How many times does FOR N ← 1 TO 10 STEP 3 run?

A 4 times (N is 1, 4, 7, 10)

B 3 times (N is 3, 6, 9)

4. Which statement about STEP is correct?

A STEP can be negative to count down

B STEP must always be a positive number

5. FOR X ← 1 TO 10 STEP 2: what is the last value of X used inside the loop?

A 10

B 9

Match the Pairs

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

Left column		Right column
1FOR Count ← 2 TO 10 STEP 2		AFOR Number ← 3 TO 1 STEP -1 OUTPUT Number NEXT Number
2FOR Count ← 5 TO 1 STEP -1		BThis outputs 1, 3, 5, 7, 9 (odd numbers) not the even numbers. To output even numbers it should start at 2: FOR Count ← 2 TO 10 STEP 2.
3STEP sets how much the loop variable changes each time round, so STEP 2 gives the even numbers 2, 4, 6, 8, 10		CThere is no STEP -1, so the loop counts up by 1 by default. Because 10 is already past 1, the loop body never runs. It should be FOR Count ← 10 TO 1 STEP -1.
4A negative STEP counts down, and the loop ends when the value goes past value2 (below 1 here)		DFOR Number ← 2 TO 10 STEP 2 OUTPUT Number NEXT Number
5FOR Count ← 10 TO 1 OUTPUT Count NEXT Count		ECounts in twos: Count takes the values 2, 4, 6, 8, 10, so the loop runs 5 times
6FOR Count ← 1 TO 10 STEP 2 OUTPUT Count NEXT Count		FCounts down: Count takes the values 5, 4, 3, 2, 1, so the loop runs 5 times

Definitions

STEP

Fill in the Blanks

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

FOR Num ← 2 TO 10 ____ 2 OUTPUT Num NEXT Num

FOR Count ← 10 TO 1 STEP __ OUTPUT Count NEXT Count

FOR Num ← 0 TO 10 STEP _ OUTPUT Num NEXT Num

FOR Num ← 10 TO 0 STEP -2 OUTPUT Num NEXT ___

FOR Count ← 20 __ 0 STEP -5 OUTPUT Count NEXT Count

Spot the Error

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

Question 1

A OUTPUT "Even numbers"

B FOR Number ← 2 TO 10 STEP 2

C OUTPUT Number

D NEXT Count

Question 2

A OUTPUT "Countdown"

B FOR Count ← 5 TO 1 STEP 1

C OUTPUT Count

D NEXT Count

Question 3

A DECLARE Num : INTEGER

B FOR Num ← 10 TO 0 STEP -2

C Num ← Num - 2

D NEXT Num

Question 4

A Total ← 0

B FOR Num ← 2 TO 10 STEP 2

C Total ← Num

D NEXT Num

E OUTPUT Total

Question 5

A DECLARE Odd : INTEGER

B FOR Odd ← 1 TO 9 BY 2

C OUTPUT Odd

D NEXT Odd

Answer Key

FOR loop with STEP (7c)

Section A: Quick Questions

B — FOR Count ← 10 TO 1 STEP -1

A — FOR Num ← 2 TO 10 STEP 2

A — 4 times (N is 1, 4, 7, 10)

A — STEP can be negative to count down

B — 9

Section B: Match the Pairs

FOR Count ← 2 TO 10 STEP 2 → Counts in twos: Count takes the values 2, 4, 6, 8, 10, so the loop runs 5 times

FOR Count ← 5 TO 1 STEP -1 → Counts down: Count takes the values 5, 4, 3, 2, 1, so the loop runs 5 times

STEP sets how much the loop variable changes each time round, so STEP 2 gives the even numbers 2, 4, 6, 8, 10 → FOR Number ← 2 TO 10 STEP 2 OUTPUT Number NEXT Number

A negative STEP counts down, and the loop ends when the value goes past value2 (below 1 here) → FOR Number ← 3 TO 1 STEP -1 OUTPUT Number NEXT Number

FOR Count ← 10 TO 1 OUTPUT Count NEXT Count → There is no STEP -1, so the loop counts up by 1 by default. Because 10 is already past 1, the loop body never runs. It should be FOR Count ← 10 TO 1 STEP -1.

FOR Count ← 1 TO 10 STEP 2 OUTPUT Count NEXT Count → This outputs 1, 3, 5, 7, 9 (odd numbers) not the even numbers. To output even numbers it should start at 2: FOR Count ← 2 TO 10 STEP 2.

Section C: Definitions

STEP — Sets how much the FOR loop variable changes each time round. STEP 2 counts in twos and STEP -1 counts down. The loop stops when the value goes past the end value.

Section D: Fill in the Blanks

FOR Num ← 2 TO 10 STEP 2
    OUTPUT Num
NEXT Num

FOR Count ← 10 TO 1 STEP -1
    OUTPUT Count
NEXT Count

FOR Num ← 0 TO 10 STEP 2
    OUTPUT Num
NEXT Num

FOR Num ← 10 TO 0 STEP -2
    OUTPUT Num
NEXT Num

FOR Count ← 20 TO 0 STEP -5
    OUTPUT Count
NEXT Count

Section E: Spot the Error

D — NEXT must name the same loop variable that the FOR line uses. The loop variable is Number, so the line should be NEXT Number.

B — Counting down from 5 to 1 needs a negative step. STEP 1 counts up, so the value is already past the end value and the loop never runs. It should be STEP -1.

C — STEP -2 already lowers Num by 2 each time round, so the loop body must not change it by hand. The body should output the value with OUTPUT Num.

C — This replaces Total with the current value each time round instead of adding to it. To total the even numbers it should be Total ← Total + Num.

B — The keyword that sets the change in the loop variable is STEP, not BY. The line should be FOR Odd ← 1 TO 9 STEP 2.