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Repetition is the best method for memory; spend one minute each day to remember the basics of C language.

“Series of 100 Essential Knowledge Points for C Language Beginners“

27. Summary of while Loop Usage Techniques: Repeat Execution When Conditions Are Met

I. Basic Structure of while Loop

The while loop is the most basic conditional loop structure in C language, with the following syntax:

while (condition) {
    // loop body code
}

Execution Flow

1. 1. Calculate the value of the condition expression
2. 2. If the result is true (non-zero), execute the loop body code
3. 3. Repeat steps 1-2 until the condition is false (zero)

II. Simple Usage Examples

1. Basic Counting Loop

int i = 0;
while (i < 5) {
    printf("%d ", i);
    i++;
}
// Output: 0 1 2 3 4

2. User Input Validation

int input;
printf("Please enter a number between 1 and 100:");
scanf("%d", &input);

while (input < 1 || input > 100) {
    printf("Invalid input, please re-enter:");
    scanf("%d", &input);
}

III. Syntax Explanation

1. Loop Condition

• • Can be any scalar type expression (integer, float, pointer, etc.)
• • Non-zero values are true, zero values are false

2. Loop Control

• • Ensure that the loop condition can become false after a finite number of iterations
• • Otherwise, it will lead to an infinite loop

3. Loop Body

• • Curly braces can be omitted for a single statement (but not recommended)
• • Must include statements that change the loop condition

IV. Special Usage Techniques

1. Empty Loop Body

// Wait for the condition to be met
while (!isReady());  // Note the semicolon

2. Complex Conditions

char ch;
while ((ch = getchar()) != EOF && ch != '\n') {
    // Process each character until the end of the line
}

3. Multiple Variable Control

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

V. Common Errors

1. Infinite Loop

int x = 1;
while (x > 0) {  // Condition is always true
    printf("%d", x);
    x++;         // Will eventually overflow
}

Correct approach:

int x = 1;
while (x > 0 && x < 100) {
    printf("%d", x);
    x++;
}

2. Missing Iteration Operation

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

Correct approach:

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

VI. Tips

1. 1. Clarify Loop Conditions:

   // Not recommended
   while (x) {...}
   
   // Recommended
   while (x != 0) {...}

2. 2. Limit the Scope of Loop Variables:

   { // Use a block to limit the scope of i
       int i = 0;
       while (i < 10) {...}
   }

3. 3. Add Comments for Complex Loops:

   // Process input until a specific marker is encountered
   while ((ch = getchar()) != '#') {
       // ...
   }

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