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29. Summary of for Loop Usage Techniques: Direct Control of Loop Count

1. Basic Structure of for Loop

The for loop is the most commonly used counting loop structure in C language, with the following syntax:

for (initialization; condition; increment) {
    // loop body code
}

Execution Flow

1. 1. Execute the initialization (only the first time)
2. 2. Check the condition; if false, exit the loop
3. 3. Execute the loop body code
4. 4. Execute the increment
5. 5. Return to step 2

2. Basic Usage Examples

1. Standard Counting Loop

for (int i = 0; i < 10; i++) {
    printf("%d ", i);
}
// Output: 0 1 2 3 4 5 6 7 8 9

2. Reverse Loop

for (int i = 9; i >= 0; i--) {
    printf("%d ", i);
}
// Output: 9 8 7 6 5 4 3 2 1 0

3. Common Techniques for for Loop

1. Multiple Variable Control

for (int i = 0, j = 10; i < j; i++, j--) {
    printf("%d %d\n", i, j);
}

2. Omission of Some Expressions

int i = 0;
for (; i < 10; ) {  // Equivalent to while loop
    printf("%d ", i++);
}

3. Infinite Loop

for (;;) {  // Classic infinite loop syntax
    // Must break internally
    if (condition) break;
}

4. Special Usage Techniques

1. Application of Comma Operator

for (int i = 1, sum = 0; i <= 100; sum += i, i++);

2. Complex Increment Expressions

for (int i = 2; i < 100; i *= 2) {
    printf("%d ", i);
}

5. Comparison with while Loop

Feature	for Loop	while Loop
Applicable Scenario	Known loop count	Condition-driven loop
Variable Scope	Can limit the scope of counting variable	Variable must be declared externally
Code Compactness	Loop control is centralized	Control statements are dispersed
Readability	Loop intent is clear at a glance	Condition changes are more flexible

6. Common Errors

1. Incorrect Semicolon Placement

for (int i = 0; i < 10; i++); {  // Error! Loop body is empty
    printf("%d", i);
}

Correct approach:

for (int i = 0; i < 10; i++) {
    printf("%d", i);
}

2. Array Out-of-Bounds Access

int arr[5];
for (int i = 0; i <= 5; i++) {  // arr[5] out of bounds
    arr[i] = i;
}

Correct approach:

for (int i = 0; i < 5; i++) {
    arr[i] = i;
}

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