Arduino IDE: size(array) ends in error

Using Arduino Programming Questions
1

Hi folks,

I have a simple question. How can I read out the length of an array for a "for loop"?
I would be very happyto get insight to it.

This works with C++ on VS2022 (console application)

#include <iostream>
std::string topics[5] = { "hellopl", "Testpl" };
int main()
{
    for (int i = 0; i < size(topics); i++) {
        std::cout << topics[i];
    }
}

While this on Arduino IDE throws an error:

#include <iostream>
String topics[2]           = {"Hi", "test"};

void loop() {
for (int i = 0; i < size(topics); i++) {
    Serial.println(topics[i]);
  }
}

error: 'size' was not declared in this scope
  109 |   for (int i = 0; i < size(topics); i++) {
      |                       ^~~~
2

Welcome to the forum

Do you want to know the size of the array in bytes or the number of elements in the array ?

String topics[2] = { "Hi", "test" };

void setup()
{
    Serial.begin(115200);
    Serial.print("bytes used by the array = ");
    Serial.println(sizeof(topics));
    Serial.print("number of elements in the array = ");
    Serial.println(sizeof(topics) / sizeof(topics[0]));
}

void loop()
{
}
1 Like
3

try

for (int i = 0; i < sizeof(topics)/sizeof(String); i++)
4

Hi,

Thanks for the quick answers!

The "possible" elements of an array. In my sample its "2" -> just for the loop of priting all defined topics in the array :slight_smile:

5

I assume that you mean the actual number of elements in the array as it will not change

6

you need the array identifier (a pointer to the start of the array in menory) and the type of elements, e.g.

  float data[] = { 1.5, 3.142, 9.6, 2.7, 1034.0 };
  for(int i = 0; i < sizeof(data) / sizeof(float); i++)
    Serial.println(data[i]);

prints

1.50
3.14
9.60
2.70
1034.00
2 Likes
7

Hmm. When I think about it, it might be the best to return the actual entries of the array instead of the possible, yes thats correct and agree with you

8

Not sure how that's possible since the function size() is indeed undeclared in your code.

Did you mean std::size()?
Or, you could use std:array which has a size() member function.

Both of these work on an ESP32 with the Arduino core:

#include <string>

std::string topics[5] = { "hellopl", "Testpl" };

void setup() {
  Serial.begin(115200);
  delay(1000);
  
  for (int i = 0; i < std::size(topics); i++) {
    Serial.println(topics[i].c_str());
  }
}

void loop() {}
#include <string>
#include <array>

std::array<std::string, 5> topics = { "hellopl", "Testpl" };
void setup() {
  Serial.begin(115200);
  delay(1000);
  
  for (int i = 0; i < topics.size(); i++) {
    Serial.println(topics[i].c_str());
  }
}

void loop() {}
9

You can also use a for-each loop, which iterates through each element of an array.

String topics[2] = {"Hi", "test"};

void setup() {
  Serial.begin(115200);
  for (auto &i : topics) {
    Serial.println(i);
  }
}

void loop() {
}
1 Like
10

Note that both "size" functions above return the container's size (5) not the number of assigned elements (2). Will your container have a variable number of elements? If so, std::vector might be a better choice.

11

I used @horace solution, because this was indeed the solution I've looking for.
empty array's printing is not that usefull after all ^^

12

I would favour using

sizeof(topics) / sizeof(topics[0]

It does the same thing but will work with data of any type