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“Essential Knowledge Points for C Language Beginners: 100 Articles Series“

28. Summary of do-while Loop Usage Techniques: A Loop that Executes at Least Once

1. Basic Structure of do-while Loop

The do-while loop is the only loop structure in C language that executes first and checks the condition afterward. The syntax is as follows:

do {
    // Loop body code
} while (condition);

Execution Flow

1. Execute the loop body code first
2. Calculate the value of the condition expression
3. If the result is true (non-zero), return to step 1
4. If the result is false (zero), exit the loop

2. Key Differences from while Loop

Feature	do-while	while
Number of Executions	At least once	May not execute at all
Condition Check	After the loop body	Before the loop body
Applicable Scenarios	Situations that must execute	Situations that are conditionally prior

3. Usage Examples

1. Menu Selection

char choice;
do {
    printf("1. Start Game\n");
    printf("2. Load Save\n");
    printf("3. Exit\n");
    printf("Please choose: ");
    scanf(" %c", &choice);
} while (choice < '1' || choice > '3');

2. Password Verification

int password;
do {
    printf("Please enter password (4 digits): ");
    scanf("%d", &password);
} while (password != 1234);

4. Usage Tips

1. Simplifying Input Validation

int value;
do {
    printf("Please enter a positive number: ");
    scanf("%d", &value);
} while (value <= 0);

2. Multi-condition Control

int x = 0, y = 10;
do {
    printf("%d %d\n", x++, y--);
} while (x < 5 && y > 0);

3. Simulating Other Loops

// Simulating for loop
int i = 0;
do {
    printf("%d ", i++);
} while (i < 5);

5. Common Mistakes

1. Forgetting the Semicolon

do {
    // ...
} while (condition)  // Error! Missing semicolon

Correct syntax:

do {
    // ...
} while (condition);

2. Improper Loop Condition

int x = 5;
do {
    printf("%d", x++);
} while (x < 5);  // Condition not satisfied initially, but still executes once

3. Infinite Loop

int x = 0;
do {
    printf("%d", x);
} while (x == 0);  // Condition is always true

Correct syntax:

int x = 0;
do {
    printf("%d", x++);
} while (x < 5);

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Essential Knowledge Points for C Language Beginners: Summary of do-while Loop Usage Techniques

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