Why Do We Need Loops?¶
In programming, we often need to execute a block of code repeatedly. For example, to print numbers from 1 to 100 or calculate the sum of numbers from 1 to 100, manually writing the same code 100 times would be tedious and error-prone. This is where loop structures come in—they allow us to use concise code to “repeat an operation” instead of writing the same code repeatedly.
What is a for Loop?¶
The for loop is one of the most basic and commonly used loop structures in Java, especially suitable for scenarios with a known number of iterations (e.g., repeating 5 times, 10 times, etc.). Its core idea is to control the loop execution through three key components: initialization, condition judgment, and iteration update.
Syntax Structure of a for Loop¶
The basic syntax of a for loop is as follows:
for (initialization expression; condition judgment expression; iteration update expression) {
// Loop body: Code block to be executed repeatedly
}
Let’s break down these three components:
- Initialization Expression: Usually assigns an initial value to a loop variable (e.g.,
int i = 1), indicating where the loop starts. - Condition Judgment Expression: Determines whether the loop continues executing (e.g.,
i <= 100). If the condition is true, the loop body runs; if false, the loop exits. - Iteration Update Expression: Updates the loop variable after each iteration (e.g.,
i++, which is equivalent toi = i + 1), preparing for the next condition check.
First for Loop Example: Print Numbers from 1 to 5¶
Let’s use a for loop to print numbers from 1 to 5:
public class ForLoopDemo {
public static void main(String[] args) {
// Initialization: Start with i=1
// Condition: Continue looping while i <= 5
// Iteration: Increment i by 1 after each loop
for (int i = 1; i <= 5; i++) {
System.out.println("Current number: " + i);
}
}
}
Execution Process:
- First iteration: i=1, condition 1 <= 5 is true. Print Current number: 1, then i becomes 2.
- Second iteration: i=2, condition 2 <= 5 is true. Print Current number: 2, then i becomes 3.
- … until i=5: Print Current number: 5, then i becomes 6. Now, 6 <= 5 is false, so the loop ends.
Output:
Current number: 1
Current number: 2
Current number: 3
Current number: 4
Current number: 5
Classic Applications of for Loops¶
1. Calculate the Sum from 1 to 100¶
To compute the sum \(1 + 2 + 3 + \dots + 100\), we can use a for loop:
public class SumExample {
public static void main(String[] args) {
int sum = 0; // Stores the total sum
for (int i = 1; i <= 100; i++) {
sum = sum + i; // Add current i to sum in each iteration
}
System.out.println("The sum from 1 to 100 is: " + sum); // Result: 5050
}
}
2. Calculate 5 Factorial (5! = 5×4×3×2×1)¶
Factorial is another classic example, implemented with a for loop:
public class FactorialExample {
public static void main(String[] args) {
int factorial = 1; // Initialize factorial result to 1
int n = 5; // Calculate 5!
for (int i = 1; i <= n; i++) {
factorial = factorial * i; // Multiply current i to factorial in each iteration
}
System.out.println(n + "! = " + factorial); // Result: 120
}
}
Notes: Avoid “Infinite Loops”¶
The core of a loop lies in condition judgment and iteration update. Errors in these parts can cause an infinite loop (the program never terminates). Common mistakes include:
1. Incorrect Condition: For example, writing i < 5 instead of i <= 5 (or vice versa), or forgetting to update the loop variable (e.g., i = 1 without i++).
2. Missing Iteration Expression: If the iteration part is omitted (e.g., for (int i = 1; i <= 5; )), the loop variable i remains 1, so the condition i <= 5 is always true, leading to an infinite loop.
Summary¶
The for loop is a powerful tool in Java for handling repeated operations, controlled by three components: initialization, condition judgment, and iteration update. Key takeaways for beginners:
- Known Iterations: Ideal for scenarios with a fixed number of repetitions (e.g., printing 1 to 100).
- Avoid Infinite Loops: Ensure the condition judgment and iteration update correctly allow the loop to terminate.
- Focus on the Loop Body: The code inside the curly braces runs repeatedly; this is the core of the loop.
Mastering the for loop will help you easily handle “repetitive tasks” and lay the foundation for learning more advanced loops (e.g., nested loops, enhanced for loops).