🔁 C Language Loop Structures (Part 2): Detailed Explanation of while and do-while Loops

Author: IoT Smart Academy

💡 1. Why do we need while?

Sometimes we do not know how many times a loop should execute, for example:

• The user keeps entering data until they input 0 to end
• Repeatedly entering a password until it is correct
• The sensor continuously samples until a threshold is triggered

In this case, <span>for</span> loops are inconvenient because the number of iterations is unknown. ✅ while loops are more flexible!

🧱 2. Basic Syntax of while

while (condition) {
    loop statements;
}

📌 Execution Logic:

Check → True → Execute → Check again → Execute again → … → False → End

✅ Example 1: Output numbers from 1 to 5

#include <stdio.h>
int main() {
    int i = 1;
    while (i <= 5) {
        printf("%d ", i);
        i++;
    }
    return 0;
}

Output:

1 2 3 4 5

📍 Note:

• Remember to update the variable <span>i++</span>
• Otherwise, the condition will always be true → infinite loop!

✅ Example 2: Calculate the sum from 1 to 100

#include <stdio.h>
int main() {
    int sum = 0, i = 1;
    while (i <= 100) {
        sum += i;
        i++;
    }
    printf("Total sum: %d\n", sum);
    return 0;
}

Output:

Total sum: 5050

✅ Example 3: Password verification program (Typical application)

#include <stdio.h>
#include <string.h>

int main() {
    char password[20];
    while (1) {
        printf("Please enter the password:");
        scanf("%s", password);
        if (strcmp(password, "iot2025") == 0) {
            printf("Login successful!\n");
            break;
        } else {
            printf("Incorrect password, please try again.\n");
        }
    }
    return 0;
}

📍 Features:

• Infinite loop <span>while(1)</span>
• Combined with <span>break</span> to achieve “until correct”

🧩 3. do-while Loop (Executes at least once)

do {
    loop statements;
} while (condition);

📌 Execution Order:

Execute first → Check → If satisfied, continue → Otherwise, exit

✅ Example 1: Basic structure

#include <stdio.h>
int main() {
    int i = 1;
    do {
        printf("%d ", i);
        i++;
    } while (i <= 5);
    return 0;
}

Output:

1 2 3 4 5

Similar to while, but executes at least once ✅

✅ Example 2: Input numbers until 0 is entered

#include <stdio.h>
int main() {
    int n;
    do {
        printf("Please enter a number (input 0 to end):");
        scanf("%d", &n);
        if (n != 0)
            printf("You entered: %d\n", n);
    } while (n != 0);
    printf("Program ended.\n");
    return 0;
}

📌 Suitable for “do first, then check” situations.

✅ Example 3: Average score input (Dynamic loop)

#include <stdio.h>
int main() {
    int count = 0;
    float score, sum = 0;
    do {
        printf("Input score (negative number to end):");
        scanf("%f", &score);
        if (score >= 0) {
            sum += score;
            count++;
        }
    } while (score >= 0);

    if (count > 0)
        printf("Average score: %.2f\n", sum / count);
    else
        printf("No valid scores entered.\n");

    return 0;
}

📍 while is a typical application for unknown iterations.

⚙️ 4. Summary of Loop Control

Control Statement	Function	Example
<span>break</span>	Immediately exit the entire loop	Exit after correct password
<span>continue</span>	Skip the current loop, proceed to the next	Skip negative inputs
<span>while(1)</span>	Infinite loop	Commonly used in main menus, control programs

🚫 5. Common Errors Summary

Error Type	Example	Description
Infinite Loop	<span>while(1){}</span>	Condition always true with no break
Condition Not Updated	<span>i</span> not incremented	Condition always true
Logical Error	<span>do…while(i>5)</span>	Ends immediately after one execution

🧠 6. Classroom Exercises (Analysis part to be published tomorrow)

📝 Exercise 1

Use a while loop to calculate the factorial of 1~n (n!)

Input: 5 Output: 120

📝 Exercise 2

Use do-while to input several positive numbers and output the maximum value. Input a negative number to end.

📝 Exercise 3 (Challenge)

Write a number guessing game:

• The system sets a random number between 1 and 100
• The user keeps inputting until they guess correctly
• Each time, prompt “too high” or “too low”

Hint: <span>#include <stdlib.h></span>, <span>rand()</span>, <span>srand(time(0))</span>

✅ 7. Summary

Loop Type	Characteristics	Scenario
<span>for</span>	Known number of iterations	Accumulation, traversal
<span>while</span>	Unknown number of iterations, check first	Dynamic loops, real-time input
<span>do-while</span>	Executes at least once	Menu loops, input validation

🔜 Next Article Preview

💥 Comprehensive Applications of C Language Loops

• Upper right triangle of multiplication table
• Fibonacci sequence
• Comparison of accumulation and multiplication
• Random number exercises
• Comprehensive case of break + continue

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