HTTP requests from Arduino - Time & Space Complexity

Time Complexity: HTTP requests from Arduino

O(n)

Understanding Time Complexity

When an Arduino sends HTTP requests, it performs steps repeatedly to communicate with a server.

We want to understand how the time it takes grows as the number of requests increases.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

#include <WiFi.h>
#include <HTTPClient.h>

void sendRequests(int n) {
  for (int i = 0; i < n; i++) {
    HTTPClient http;
    http.begin("http://example.com/data");
    int code = http.GET();
    http.end();
  }
}
    

This code sends n HTTP GET requests one after another to a server.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Sending an HTTP GET request inside the loop.
• How many times: Exactly n times, once per loop iteration.

How Execution Grows With Input

Each request takes roughly the same time, so total time grows as we send more requests.

Input Size (n)	Approx. Operations
10	10 HTTP requests
100	100 HTTP requests
1000	1000 HTTP requests

Pattern observation: The total time grows directly in proportion to the number of requests.

Final Time Complexity

Time Complexity: O(n)

This means if you double the number of requests, the total time roughly doubles too.

Common Mistake

[X] Wrong: "Sending multiple HTTP requests happens all at once, so time stays the same no matter how many requests."

[OK] Correct: Each request waits for a response before the next starts, so time adds up with each request.

Interview Connect

Understanding how repeated network calls affect time helps you design better programs and explain your choices clearly.

Self-Check

"What if we sent all HTTP requests at the same time using asynchronous calls? How would the time complexity change?"

Practice

1. What is the main purpose of using HTTPClient in Arduino sketches?

easy

A. To control digital pins

B. To connect to WiFi networks

C. To read analog sensor values

D. To send and receive HTTP requests easily

Solution

1. Step 1: Understand the role of HTTPClient

   The HTTPClient library is designed to simplify sending HTTP requests like GET and POST from Arduino.

2. Step 2: Differentiate from other libraries

   Connecting to WiFi is done by WiFiClient, not HTTPClient. Reading sensors or controlling pins is unrelated.

3. Final Answer:

   To send and receive HTTP requests easily -> Option D

4. Quick Check:

   HTTPClient = HTTP requests [OK]

Hint: HTTPClient handles web requests, not WiFi or sensors [OK]

Common Mistakes:

• Confusing HTTPClient with WiFi connection library
• Thinking HTTPClient reads sensors
• Assuming HTTPClient controls pins

2. Which of the following is the correct way to start a GET request using HTTPClient in Arduino?

easy

A. http.connect("http://example.com");

B. http.startGET("http://example.com");

C. http.begin("http://example.com");

D. http.open("http://example.com");

Solution

1. Step 1: Recall HTTPClient method for starting requests

   The correct method to initialize a request is begin() with the URL as argument.

2. Step 2: Check other options

   Methods like connect(), startGET(), or open() do not exist in HTTPClient library.

3. Final Answer:

   http.begin("http://example.com"); -> Option C

4. Quick Check:

   Use begin() to start HTTP request [OK]

Hint: Use begin() to start HTTP requests in Arduino [OK]

Common Mistakes:

• Using non-existent methods like connect() or open()
• Confusing begin() with WiFi connection functions
• Forgetting to pass the URL string

3. What will the following Arduino code print if the HTTP GET request to http://example.com/data returns status 200 and body "OK"?

HTTPClient http;
http.begin("http://example.com/data");
int httpCode = http.GET();
if (httpCode > 0) {
  String payload = http.getString();
  Serial.println(httpCode);
  Serial.println(payload);
} else {
  Serial.println("Error");
}
http.end();

medium

A. 200
OK

B. Error

C. 404
Not Found

D. 0

Solution

1. Step 1: Analyze the HTTP GET response code

   The code checks if httpCode > 0. Since the server returns 200, this condition is true.

2. Step 2: Print status code and response body

   The code prints the status code (200) and then the response body ("OK") on separate lines.

3. Final Answer:

   200 OK -> Option A

4. Quick Check:

   200 status means success, prints code and body [OK]

Hint: Status > 0 means success; print code and body [OK]

Common Mistakes:

• Assuming error when status is 200
• Confusing status code with response body
• Not calling http.end() to close connection

4. Identify the error in this Arduino code snippet for sending a POST request:

HTTPClient http;
http.begin("http://example.com/api");
http.addHeader("Content-Type", "application/json");
int httpCode = http.POST("{\"value\": 42}");
if (httpCode == 200) {
  Serial.println("Success");
}
http.end();

medium

A. Incorrect header name; should be "ContentType"

B. Missing WiFi connection before HTTPClient usage

C. POST method requires no argument

D. http.end() should be called before POST

Solution

1. Step 1: Check prerequisites for HTTP requests

   Before using HTTPClient, Arduino must be connected to WiFi. This code snippet does not show WiFi connection setup.

2. Step 2: Validate headers and method usage

   Header name "Content-Type" is correct, POST accepts a string argument, and http.end() is correctly called after POST.

3. Final Answer:

   Missing WiFi connection before HTTPClient usage -> Option B

4. Quick Check:

   WiFi must connect before HTTPClient [OK]

Hint: Always connect WiFi before HTTP requests [OK]

Common Mistakes:

• Skipping WiFi connection setup
• Using wrong header names
• Calling http.end() too early

5. You want to send sensor data as JSON to a server using Arduino's HTTPClient. Which sequence correctly sends a POST request with JSON data and handles the response?

hard

A. Connect WiFi -> http.begin(url) -> http.addHeader("Content-Type", "application/json") -> http.POST(jsonData) -> check httpCode -> http.end()

B. http.begin(url) -> http.POST(jsonData) -> http.addHeader("Content-Type", "application/json") -> check httpCode -> http.end()

C. Connect WiFi -> http.POST(jsonData) -> http.begin(url) -> http.addHeader("Content-Type", "application/json") -> check httpCode -> http.end()

D. Connect WiFi -> http.addHeader("Content-Type", "application/json") -> http.begin(url) -> http.POST(jsonData) -> check httpCode -> http.end()

Solution

1. Step 1: Establish WiFi connection first

   Arduino must connect to WiFi before making HTTP requests.

2. Step 2: Follow correct HTTPClient method order

   Start with http.begin(url), then set headers, then send POST with data, check response code, and finally call http.end().

3. Final Answer:

   Connect WiFi -> http.begin(url) -> http.addHeader("Content-Type", "application/json") -> http.POST(jsonData) -> check httpCode -> http.end() -> Option A

4. Quick Check:

   WiFi -> begin() -> headers -> POST -> end() [OK]

Hint: Connect WiFi first, then begin(), addHeader(), POST, end() [OK]

Common Mistakes:

• Calling POST before begin()
• Adding headers after POST
• Skipping WiFi connection