Nested Statements

🎯 Syllabus & Goals 3 min

Cambridge 8.1.5 · Nested statements Paper 2 · Algorithms, Programming and Logic

By the end of this lesson you can:

1. Write an IF nested inside another IF, and the Python elif version.
2. Write a loop nested inside another loop, and state how many times the inner body runs.
3. Trace a nested algorithm that combines loops, selection, totalling and finding a maximum.

Textbook: Chapter 8, §8.1.5 (pp. 318–321)

Recap / Warm-Up 5 min

So far you have used each construct — sequence, selection, iteration — mostly on its own, or with an IF inside a loop for counting.

Quick starter

A clock shows hours and minutes. For each hour, the minutes go 0 to 59. How many times does the minute value change in 12 hours?

🧠 Key Concept 14 min

Selection and iteration statements can be placed inside each other: an IF inside a loop, a loop inside a loop, or an IF inside an IF. Nesting reduces the amount of code needed and makes a program simpler to test, because each level does one clear job.

1 · IF inside IF

Cambridge pseudocode has no "else if". To test a second condition only when the first is FALSE, put a new IF inside the ELSE. Python shortens this with elif.

IF Hours <= 1
  THEN
    Charge ← 0
  ELSE
    IF Hours <= 3
      THEN
        Charge ← 2
      ELSE
        Charge ← 5
    ENDIF
ENDIF

# Car park charge: an IF nested inside an ELSE
hours = int(input("Hours parked: "))
if hours <= 1:
    charge = 0
else:
    if hours <= 3:
        charge = 2
    else:
        charge = 5
print("Charge: $", charge)

Hours parked: 2
Charge: $ 2

2 · A loop inside a loop

The inner loop runs all its iterations for each iteration of the outer loop.

Worked Example 12 min

Example 1 · Nested FOR loops — a multiplication grid

DECLARE Row, Column : INTEGER
FOR Row ← 1 TO 2
  FOR Column ← 1 TO 3
    OUTPUT Row, " x ", Column, " = ", Row * Column
  NEXT Column
NEXT Row
OUTPUT "Finished"

# A loop inside a loop: every row is paired with every column
for row in range(1, 3):             # outer loop: 1, 2
    for column in range(1, 4):      # inner loop: 1, 2, 3
        print(row, "x", column, "=", row * column)
print("Finished")

1 x 1 = 1
1 x 2 = 2
1 x 3 = 3
2 x 1 = 2
2 x 2 = 4
2 x 3 = 6
Finished

Example 2 · Loops, selection and totalling together

A league has 2 teams, and each plays 3 matches. Input the points for every match. For each team output its total and its highest score; then output the team with the most points. Constants make it easy to test with small numbers and scale up later.

1. Start with the inner loop: one team's three matches, a total and a highest.build and test the innermost level first, as the textbook advises.
2. Wrap it in the outer loop over teams, resetting TeamTotal and TeamHigh at the start of each team.otherwise team 2 would carry on from team 1's figures.
3. After the inner loop, compare this team with the best so far.the comparison needs the finished team total.

CONSTANT NumberOfTeams ← 2
CONSTANT MatchesPerTeam ← 3
DECLARE Team, Match, Points : INTEGER
DECLARE TeamTotal, TeamHigh : INTEGER
DECLARE BestTeam, BestTotal : INTEGER
BestTeam ← 0
BestTotal ← -1
FOR Team ← 1 TO NumberOfTeams
  TeamTotal ← 0
  TeamHigh ← 0
  FOR Match ← 1 TO MatchesPerTeam
    OUTPUT "Team ", Team, ", match ", Match, " points: "
    INPUT Points
    TeamTotal ← TeamTotal + Points
    IF Points > TeamHigh
      THEN
        TeamHigh ← Points
    ENDIF
  NEXT Match
  OUTPUT "Team ", Team, " total: ", TeamTotal, " highest: ", TeamHigh
  IF TeamTotal > BestTotal
    THEN
      BestTotal ← TeamTotal
      BestTeam ← Team
  ENDIF
NEXT Team
OUTPUT "Best team is ", BestTeam, " with ", BestTotal, " points"

# Team points: nested loops with selection inside
NUMBER_OF_TEAMS = 2
MATCHES_PER_TEAM = 3
best_team = 0
best_total = -1
for team in range(1, NUMBER_OF_TEAMS + 1):
    team_total = 0
    team_high = 0
    for match in range(1, MATCHES_PER_TEAM + 1):
        points = int(input("Team " + str(team) + ", match " + str(match) + " points: "))
        team_total = team_total + points
        if points > team_high:
            team_high = points
    print("Team", team, "total:", team_total, "highest:", team_high)
    if team_total > best_total:
        best_total = team_total
        best_team = team
print("Best team is", best_team, "with", best_total, "points")

Team 1, match 1 points: 3
Team 1, match 2 points: 1
Team 1, match 3 points: 3
Team 1 total: 7 highest: 3
Team 2, match 1 points: 0
Team 2, match 2 points: 3
Team 2, match 3 points: 1
Team 2 total: 4 highest: 3
Best team is 1 with 7 points

Notice the third match of team 1: 3 > 3 is FALSE, so TeamHigh is not changed. For team 2, 4 > 7 is FALSE, so the best team stays as team 1.

Try It Yourself 12 min

📝 Exam Practice 10 min

Total ← 0
FOR X ← 1 TO 3
  FOR Y ← 1 TO X
    Total ← Total + Y
  NEXT Y
NEXT X
OUTPUT Total

🗝️ Recap & Key Terms 3 min

Nesting puts one construct inside another. The inner loop completes for every pass of the outer loop, so the inner body runs (outer × inner) times. Reset per-group values inside the outer loop, and close each block in the reverse order it was opened.

Nesting

The inclusion of one type of code construct inside another.

Nested loop

A loop inside another loop; the inner loop runs completely for each iteration of the outer loop.

Nested selection

An IF (or CASE) statement placed inside another IF, so a second condition is tested only on one path.

Initialise

To give a variable its starting value, e.g. Total ← 0, before it is used.

Homework 1 min

Task (≤ 15 min): Write pseudocode that outputs this triangle of stars using nested loops. The number of rows (here 4) is input by the user. [4]

*
**
***
****

DECLARE Rows, Row, Star : INTEGER
DECLARE Line : STRING
OUTPUT "How many rows? "
INPUT Rows
FOR Row ← 1 TO Rows
  Line ← ""
  FOR Star ← 1 TO Row
    Line ← Line & "*"
  NEXT Star
  OUTPUT Line
NEXT Row