WiFi with ESP8266/ESP32 in Arduino - Time & Space Complexity

Time Complexity: WiFi with ESP32

O(n)

Understanding Time Complexity

When using WiFi on ESP32, it's important to understand how the time to connect and send data changes as the program runs.

We want to know how the program's running time grows when it tries to connect or send data over WiFi.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

#include <WiFi.h>

const char* ssid = "MyNetwork";
const char* password = "MyPassword";

void setup() {
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
  }
}

void loop() {
  // main code
}
    

This code tries to connect the ESP board to a WiFi network by repeatedly checking the connection status until it succeeds.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The while loop that checks if WiFi is connected.
• How many times: It repeats until the connection is established, which depends on network conditions.

How Execution Grows With Input

Here, the "input" is the time or attempts needed to connect to WiFi, which can vary.

Input Size (n)	Approx. Operations
10 (attempts)	10 checks of WiFi status
100 (attempts)	100 checks of WiFi status
1000 (attempts)	1000 checks of WiFi status

Pattern observation: The number of operations grows directly with the number of connection attempts.

Final Time Complexity

Time Complexity: O(n)

This means the time to connect grows linearly with the number of attempts made to check the connection.

Common Mistake

[X] Wrong: "The connection will always happen instantly, so the loop runs only once."

[OK] Correct: WiFi connection depends on many factors like signal strength and network response, so the loop may run many times before connecting.

Interview Connect

Understanding how loops behave when waiting for hardware or network responses is a useful skill. It shows you can think about how long your code might take in real situations.

Self-Check

"What if we added a timeout to stop trying after a fixed number of attempts? How would the time complexity change?"

Practice

1. What is the main purpose of WiFi.begin(ssid, password) in ESP8266/ESP32 programming?

easy

A. To start connecting the device to a WiFi network

B. To disconnect from the current WiFi network

C. To check the current WiFi signal strength

D. To get the IP address of the device

Solution

1. Step 1: Understand the function purpose

   WiFi.begin(ssid, password) is used to initiate connection to a WiFi network using the given network name (SSID) and password.

2. Step 2: Compare with other options

   Disconnecting, checking signal strength, or getting IP address are done by other functions, not WiFi.begin.

3. Final Answer:

   To start connecting the device to a WiFi network -> Option A

4. Quick Check:

   WiFi.begin starts connection [OK]

Hint: Remember: begin means start connection [OK]

Common Mistakes:

• Confusing begin with disconnect
• Thinking it returns IP address
• Assuming it checks signal strength

2. Which of the following is the correct syntax to check if the ESP8266 is connected to WiFi?

easy

A. if (WiFi.connected())

B. if (WiFi.checkConnection())

C. if (WiFi.isConnected())

D. if (WiFi.status() == WL_CONNECTED)

Solution

1. Step 1: Recall the correct method to check WiFi connection

   The standard way is to use WiFi.status() and compare it to WL_CONNECTED to confirm connection.

2. Step 2: Verify other options

   Methods like WiFi.connected(), WiFi.isConnected(), or WiFi.checkConnection() do not exist in the ESP8266 WiFi library.

3. Final Answer:

   if (WiFi.status() == WL_CONNECTED) -> Option D

4. Quick Check:

   WiFi.status() == WL_CONNECTED checks connection [OK]

Hint: Use WiFi.status() with WL_CONNECTED constant [OK]

Common Mistakes:

• Using non-existent methods like WiFi.connected()
• Confusing status check syntax
• Forgetting to compare with WL_CONNECTED

3. What will the following code print if the ESP8266 successfully connects to WiFi?

WiFi.begin("MySSID", "MyPass");
while (WiFi.status() != WL_CONNECTED) {
  delay(500);
}
Serial.println(WiFi.localIP());

medium

A. The IP address assigned to the ESP8266

B. The SSID of the WiFi network

C. The WiFi password

D. Nothing, it will cause an error

Solution

1. Step 1: Analyze the connection loop

   The code waits until WiFi.status() returns WL_CONNECTED, meaning the device is connected to WiFi.

2. Step 2: Understand the output command

   WiFi.localIP() returns the IP address assigned to the device by the router, which is printed to Serial.

3. Final Answer:

   The IP address assigned to the ESP8266 -> Option A

4. Quick Check:

   WiFi.localIP() prints device IP [OK]

Hint: localIP() shows device IP after connection [OK]

Common Mistakes:

• Thinking it prints SSID or password
• Assuming it prints connection status
• Believing it causes an error

4. Identify the error in this ESP32 WiFi connection code:

WiFi.begin(ssid, password);
while (WiFi.status() = WL_CONNECTED) {
  delay(1000);
}
Serial.println("Connected!");

medium

A. Missing WiFi.begin() call

B. Incorrect delay value

C. Using single '=' instead of '==' in the while condition

D. No Serial.begin() before printing

Solution

1. Step 1: Check the while loop condition

   The condition uses a single '=' which is an assignment, not a comparison. It should be '==' to compare values.

2. Step 2: Confirm other parts

   WiFi.begin() is called, delay is valid, and Serial.begin() is not shown but not the main error here.

3. Final Answer:

   Using single '=' instead of '==' in the while condition -> Option C

4. Quick Check:

   Use '==' for comparison in conditions [OK]

Hint: Use '==' to compare, '=' assigns value [OK]

Common Mistakes:

• Confusing '=' with '==' in conditions
• Ignoring Serial.begin() setup
• Assuming delay value causes error

5. You want your ESP32 to connect to WiFi and print its IP address only after a successful connection, but also stop trying after 10 seconds if it fails. Which code snippet correctly implements this?

hard

A. WiFi.begin(ssid, password);
while (WiFi.status() != WL_CONNECTED) {
delay(500);
if (millis() > 10000) break;
}
Serial.println(WiFi.localIP());

B. WiFi.begin(ssid, password);
unsigned long start = millis();
while (WiFi.status() != WL_CONNECTED && millis() - start < 10000) {
delay(500);
}
if (WiFi.status() == WL_CONNECTED) {
Serial.println(WiFi.localIP());
} else {
Serial.println("Failed to connect");
}

C. WiFi.begin(ssid, password);
for (int i = 0; i < 20; i++) {
if (WiFi.status() == WL_CONNECTED) break;
delay(500);
}
Serial.println(WiFi.localIP());

D. WiFi.begin(ssid, password);
while (WiFi.status() == WL_CONNECTED) {
delay(10000);
}
Serial.println(WiFi.localIP());

Solution

1. Step 1: Understand the timeout logic

   WiFi.begin(ssid, password); unsigned long start = millis(); while (WiFi.status() != WL_CONNECTED && millis() - start < 10000) { delay(500); } if (WiFi.status() == WL_CONNECTED) { Serial.println(WiFi.localIP()); } else { Serial.println("Failed to connect"); } uses millis() to track elapsed time and stops trying after 10 seconds (10000 ms) if not connected.

2. Step 2: Check connection and print accordingly

   It prints the IP only if connected; otherwise, it prints a failure message. This matches the requirement exactly.

3. Final Answer:

   The code with 10-second timeout using millis() and conditional IP printing -> Option B

4. Quick Check:

   Use millis() for timeout and check status before printing [OK]

Hint: Use millis() to limit connection wait time [OK]

Common Mistakes:

• Not stopping connection attempts after timeout
• Printing IP without checking connection
• Using wrong while condition logic