HTTP requests from Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Arduino HTTP GET request code?

Consider this Arduino sketch snippet that makes an HTTP GET request to a server and prints the response code. What will be printed to the Serial Monitor?

Arduino

WiFiClient client;
HTTPClient http;

void setup() {
  Serial.begin(115200);
  WiFi.begin("SSID", "PASSWORD");
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
  }
  http.begin(client, "http://example.com");
  int httpCode = http.GET();
  Serial.println(httpCode);
  http.end();
}

void loop() {}

A 200

B 0

C -1

D 404

💡 Hint

HTTP status code 200 means success. The code variable holds the HTTP response code.

The http.GET() function returns the HTTP status code from the server. If the request is successful, it returns 200. Other values indicate errors or no connection.

🧠 Conceptual

intermediate

Which Arduino library is used for making HTTP requests?

You want to send HTTP GET and POST requests from your Arduino sketch. Which library provides the HTTPClient class to do this?

A Ethernet.h

B WiFi.h

C SPI.h

D HTTPClient.h

💡 Hint

Look for the library that has 'HTTP' in its name and provides HTTP methods.

The HTTPClient class is part of the HTTPClient.h library, which simplifies HTTP requests on Arduino.

🔧 Debug

advanced

Why does this Arduino HTTP POST code fail to send data?

Examine the code below. It tries to send JSON data via HTTP POST but the server never receives it. What is the main reason for failure?

Arduino

WiFiClient client;
HTTPClient http;

void setup() {
  Serial.begin(115200);
  WiFi.begin("SSID", "PASSWORD");
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
  }
  http.begin(client, "http://example.com/api");
  http.addHeader("Content-Type", "application/json");
  int httpCode = http.POST("{\"temp\": 25}");
  Serial.println(httpCode);
  http.end();
}

void loop() {}

A The HTTPClient.begin() is called with WiFiClient instead of WiFiClientSecure

B The WiFi connection is not established before the POST call

C The Content-Type header is missing

D The JSON string is not properly escaped

💡 Hint

Check the WiFi connection status before making HTTP requests.

If WiFi is not connected, HTTP requests will fail. The code waits for connection, so this is unlikely. But if the wait loop is skipped or WiFi fails, POST will fail.

📝 Syntax

advanced

Which option fixes the syntax error in this Arduino HTTP GET snippet?

Find the option that corrects the syntax error in this code snippet:

http.begin(client, "http://example.com")
int code = http.GET()
Serial.println(code)

A Change http.GET() to http.get()

B Remove parentheses from http.GET

C Add semicolons at the end of each line

D Replace double quotes with single quotes

💡 Hint

Arduino C++ requires statement terminators.

Each statement in Arduino C++ must end with a semicolon. Missing semicolons cause syntax errors.

🚀 Application

expert

How many bytes are sent in this Arduino HTTP POST request?

Given this Arduino code sending a JSON payload via HTTP POST, how many bytes are sent in the request body?

String payload = "{\"sensor\":\"temp\",\"value\":23}";
http.begin(client, "http://example.com/data");
http.addHeader("Content-Type", "application/json");
int code = http.POST(payload);
http.end();

A 28 bytes

B 26 bytes

C 30 bytes

D 24 bytes

💡 Hint

Count all characters including quotes and braces in the JSON string.

The payload string is exactly 28 characters long, so 28 bytes are sent in the POST body.

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