3.4 Control Structures

• The three control structures: sequence, selection, iteration
• Boolean and compound expressions
• Selection: if, if-else, if-elif-else and nested if
• Iteration: while loops and for loops
• The break and continue statements
• Choosing the right control structure

• describe control structures in programming
• select the program control structure for a given situation
• use control structures in programming
• appreciate the application of control structures in programming

“How are Boolean expressions used in programming?”

“What is the importance of control structures in programming?”

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1. Selection: making decisions
   40 min · 1 practice tasks · quiz
   How a program chooses between paths using if, if-else and if-elif-else, tested by Boolean and compound expressions.
2. Iteration: loops, break and continue
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   Repeating steps with while and for loops, the danger of the infinite loop, and controlling loops with break and continue.
3. Sequence: steps in order
   40 min · 1 practice tasks · quiz
   Sequence is a control structure: do this, then that. A kiosk change program with no extra rules is pure sequence.
4. How Boolean expressions are used in control
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   Every if and while tests a Boolean expression from 3.3. True takes a path; False skips or takes the other.
5. Compound Boolean expressions in real rules
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   and, or and not inside if: teenager discounts, weekend fares, valid marks. Range uses and; alternatives use or.
6. The simple if: one optional path
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   if condition: indented block. True runs it; False skips it. Nothing else happens. Indentation is the syntax.
7. if-else: two paths, one of them always runs
   40 min · 1 practice tasks · quiz
   if-else chooses between two complete answers: pass or fail, enough tendered or ask for more, PIN right or wrong.
8. if-elif-else: several bands, first True wins
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   elif means else if. Tests run top to bottom. Grade bands and fare bands must be ordered from the most demanding down.
9. Nested if: a decision inside a decision
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   The inner if is reached only if the outer condition was True. Login then role; valid mark then pass. Indentation shows the nest.
10. Flowcharts for selection
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    Oval, parallelogram, rectangle, diamond. Every diamond has two labelled exits. Draw pass/fail and a grade chain before coding.
11. while loops: repeat until something happens
    40 min · 1 practice tasks · quiz
    while tests a Boolean before each round. Use it when the count is unknown: PIN, sum until 0. Update the variable or it never ends.
12. for loops and range: a known number of times
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    for walks through a collection. range(1, 6) is 1 to 5. Use for when you know the count or have a list of names.
13. Nested loops: a loop inside a loop
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    The inner loop finishes all its rounds for each outer round. Times tables, a seating grid, every learner on every day.
14. break: leave the loop immediately
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    break jumps out of the current loop, even if rounds remain. Use it to stop at the first match in a register.
15. continue: skip this round, not the loop
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    continue jumps to the next repetition. Use it to ignore absentees, hidden files, or a number you do not want to print.
16. Choosing the right control structure
    40 min · 1 practice tasks · quiz
    Match the situation: sequence if no choice and no repeat; if/elif for a choice; for a known count; while until something happens.
17. Combining sequence, selection and iteration
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    A real kiosk or class-fee program nests all three. That is why control structures are the grammar of programming.