Iteration: repeating instructions with loops

边玩边学

回答这些题目来赚取能量，然后钓鱼探索。无需账号。

给教育者：面向 Iteration: repeating instructions with loops（KS3 Computing，Computer Science）的即用型课程幻灯片, 复习笔记——用在你的课堂上，或将该知识点作为学习者可实时游玩的互动课堂活动来运行。

课程笔记

What is Iteration?

Iteration means repeating a set of instructions (a block of code) more than once.
It is one of the three basic building blocks of programming, alongside sequence (doing things in order) and selection (making choices).
Loops let you write the same code once and run it many times, saving time and reducing errors.
Each repetition of the loop is called an iteration.

A loop repeats while a condition holds, or for a fixed number of repetitions.

Count-Controlled Loops (for loops)

A count-controlled loop (often a for loop) repeats a fixed number of times.
It uses a loop variable (also called a counter) that changes each time the loop runs.
The loop variable often starts at a value, continues while a condition is true, and increases or decreases by a step.
Example: `for i in range(5):` runs the indented code 5 times, with `i` taking values 0, 1, 2, 3, 4.
Use a for loop when you know exactly how many times you want to repeat.

A count-controlled loop runs a fixed number of times, here outputting the numbers 1 to 5.

Condition-Controlled Loops (while loops)

A condition-controlled loop (often a while loop) repeats as long as a condition is true.
The condition is checked before each repetition; if it is false at the start, the loop body never runs.
Example: `while x < 10:` repeats until `x` becomes 10 or more.
Use a while loop when you do not know in advance how many times the loop will run.
The loop must eventually make the condition false, or it will run forever.

A flag controls the loop: it keeps asking until the correct password is entered.

Loop Variables and Updating

The loop variable is a variable that changes each iteration, often used to control the loop or to track progress.
In a for loop, the loop variable is automatically updated by the loop.
In a while loop, you must update the variable yourself inside the loop body.
Common update: `counter = counter + 1` (or `counter += 1`) to increase the counter by 1.
The loop variable can be used inside the loop, for example to calculate sums or to index into a list.

Infinite Loops and How to Avoid Them

An infinite loop is a loop that never ends because the condition never becomes false.
Infinite loops can freeze a program or cause it to run forever.
To avoid them, make sure the condition can become false (e.g., update the loop variable).
Check that the loop variable moves towards the stopping condition (e.g., increases if the condition is `<`).
Test your loop with small values to see if it behaves as expected.

A loop boundary that is too small misses the last item; one that is too large processes extra items.

Choosing the Right Loop

Use a for loop when the number of repetitions is known in advance (e.g., repeat 10 times, or iterate over a list).
Use a while loop when the number of repetitions depends on a condition that may change (e.g., keep asking until the user enters a valid answer).
For loops are often clearer and less error-prone for fixed counts.
While loops are more flexible for unknown numbers of repetitions.
Sometimes a for loop can be written as a while loop, and vice versa, but one is usually more natural.

Combining Loops with Selection

You can put an if statement inside a loop to make decisions during each iteration.
This lets you process each item differently based on its value.
Example: `for num in numbers: if num % 2 == 0: print(num)` prints only even numbers.
You can also put a loop inside an if statement, or nest loops inside loops (nested loops).
Combining loops and selection allows you to solve complex problems like searching, counting, and filtering.

Common Loop Patterns

Counting: use a loop to count how many times something happens.
Summing: accumulate a total by adding each value in a loop.
Searching: loop through items to find a specific one.
Input validation: use a while loop to keep asking for input until it is valid.
Nested loops: a loop inside another loop, often used for grids or tables.

Key Terminology

Iteration: one repetition of a loop.
Loop body: the block of code that is repeated.
Loop condition: the boolean expression that controls a while loop.
Loop variable: the variable that changes each iteration.
Infinite loop: a loop that never terminates.

幻灯片

Step through every slide for this topic — plus flashcards and revision notes — with a free account.

练习题

免费预览——61 题中的 8 题。注册即可查看全部。

1.What is the term for the control flow that repeats a block of instructions a set number of times?
Easy
- Acount-controlled loop
- Bcondition-controlled loop
- Cinfinite loop
- Dselection
2.A 'while' loop is an example of a count-controlled loop.
Easy
True or false?
3.Which of the following is a typical use of a count-controlled (for) loop?
Easy
- ARepeating until the user enters 'quit'
- BPrinting numbers 1 to 10
- CChecking a password until correct
- DWaiting for a button press
4.What is the main danger of a condition-controlled loop if the condition never becomes false?
Easy
- AIt runs too slowly
- BIt becomes an infinite loop
- CIt skips the block
- DIt uses too much memory
5.Which of the following are examples of iteration? (Select all that apply)
Easy
- Afor loop
- Bwhile loop
- Cif statement
- Drepeat-until loop
6.In a 'for' loop, the loop variable automatically increases (or decreases) each time the loop runs.
Easy
True or false?
7.Which loop would be most appropriate to keep asking for a password until the correct one is entered?
Medium
- Afor loop
- Bwhile loop
- Crepeat-until loop
- Dboth while and repeat-until
8.What is the value of 'total' after this code runs? total = 0; for i in range(1, 4): total = total + i
Medium
- A3
- B6
- C10
- D4

复习笔记

## Iteration: Repeating Instructions with Loops ### What is Iteration? Iteration is a way to repeat a block of code multiple times. It is one of the three basic building blocks of programming (sequence, selection, iteration). Instead of writing the same instructions over and over, you use a loop. ### Types of Loops #### Count-Controlled Loops (`for`) - Used when you know **how many times** you want to repeat. - A **loop variable** counts the repetitions. - Example (Python): ```python for i in range(5): print("Hello") ``` This prints "Hello" 5 times. #### Condition-Controlled Loops (`while`) - Used when you repeat **while a condition is true**. - The loop continues until the condition becomes false. - Example (Python): ```python count = 0 while count < 5: print("Hello") count = count + 1 ``` This also prints "Hello" 5 times. ### Loop Variables - A loop variable is a variable that changes each time the loop runs. - It is often used to control how many times the loop repeats or to track progress. - In a `for` loop, the loop variable is updated automatically. - In a `while` loop, you must update it yourself (or the loop may never end). ### Infinite Loops and How to Avoid Them - An **infinite loop** is a loop that never ends because the condition is always true. - This can happen if you forget to update the loop variable in a `while` loop. - To avoid infinite loops: - Make sure the condition eventually becomes false. - Update the loop variable inside the loop. - Use a `for` loop when you know the number of repetitions. ### Choosing the Right Loop - Use a `for` loop when you know exactly how many times to repeat. - Use a `while` loop when you don't know the number of repetitions, but you have a condition to check. ### Combining Loops with Selection - You can put `if` statements inside loops to make decisions for each repetition. - Example: Print only even numbers from 1 to 10. ```python for i in range(1, 11): if i % 2 == 0: print(i) ``` ### Key Points to Remember - Loops save time and make code shorter. - Always ensure loops terminate (end) unless you intentionally want an infinite loop (rare). - Practice tracing loops to understand how they work.

Iteration: Repeating a block of code multiple times, often using loops.
Count-controlled loop: A loop that repeats a fixed number of times, typically using a `for` loop.
Condition-controlled loop: A loop that repeats as long as a condition is true, typically using a `while` loop.
Loop variable: A variable that changes each time the loop runs, often used to control the number of repetitions.
Infinite loop: A loop that never ends because its condition is always true.
Selection: A programming construct that makes a decision based on a condition, such as an `if` statement.

闪卡

What is iteration in programming?

Iteration is the repetition of a block of instructions, often using loops, to execute the same code multiple times.

Name the two main types of loops in KS3 Computing.

Count-controlled loops (for loops) and condition-controlled loops (while loops).

What is a count-controlled loop?

A loop that repeats a fixed number of times, typically using a counter variable. In Python, a for loop is often used.

What is a condition-controlled loop?

A loop that repeats as long as a condition is true. In Python, a while loop is used.

What is a loop variable?

A variable used to control the number of iterations in a loop, often incremented or decremented each time the loop runs.

Give an example of a for loop in Python that prints numbers 1 to 5.

for i in range(1, 6): print(i)

Give an example of a while loop in Python that prints numbers 1 to 5.

i = 1 while i ≤ 5: print(i) i = i + 1

What is an infinite loop?

A loop that never ends because its condition is always true or it has no exit condition. It can cause a program to hang.

How can you avoid an infinite loop?

Ensure the loop's condition eventually becomes false, for example by updating the loop variable inside the loop.

When should you use a for loop instead of a while loop?

Use a for loop when you know the exact number of iterations in advance. Use a while loop when the number of iterations depends on a condition.

What is the purpose of the range() function in Python?

It generates a sequence of numbers, often used in for loops to specify the number of iterations.

What does range(5) produce?

It produces the numbers 0, 1, 2, 3, 4 (five numbers starting from 0).

What does range(1, 5) produce?

It produces the numbers 1, 2, 3, 4 (stops before 5).

What does range(1, 10, 2) produce?

It produces 1, 3, 5, 7, 9 (starts at 1, ends before 10, step of 2).

What is the difference between a for loop and a while loop?

A for loop iterates over a sequence or a fixed number of times; a while loop repeats as long as a condition is true.

How can you combine loops with selection (if statements)?

You can place an if statement inside a loop to make decisions for each iteration, e.g., printing only even numbers.

Write a Python loop that prints only even numbers from 1 to 10.

for i in range(1, 11): if i % 2 == 0: print(i)

What is a nested loop?

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

Give an example of a nested loop that prints a 3x3 grid of asterisks.

for i in range(3): for j in range(3): print('×', end=' ') print()

What is the role of the loop variable in a for loop?

It takes each value from the sequence (e.g., range) in turn, allowing you to use that value in the loop body.

What happens if you forget to update the loop variable in a while loop?

The loop may become infinite because the condition never becomes false.

What is the output of this code? for i in range(3): print(i)

0 1 2

What is the output of this code? i = 0 while i < 3: print(i) i += 1

0 1 2

How do you break out of a loop early in Python?

Use the break statement. It immediately exits the loop and continues with the next statement after the loop.

What does the continue statement do in a loop?

It skips the rest of the current iteration and jumps to the next iteration of the loop.

Unlock all 61 questions, flashcards & more

创建免费账号，即可查看该知识点的每一道题、幻灯片、闪卡和复习笔记。

历年真题

该知识点的历年真题练习即将推出。

即将推出