Error With .toInt()

Projects Programming
1

With the code below, the .toInt() function is not working as expected. The variable moistureLevel is empty when printing to the Serial monitor:

String str = "POST /1000";
int moistureLevel;
float moisturePercentage;

void setup() {
  Serial.begin(9600);

  while (!Serial);
  
  moistureLevel = str.toInt();
  moisturePercentage = (moistureLevel / 3400) * 100;

  Serial.println("str: " + str);
  Serial.println("level: " + moistureLevel);
  Serial.println("percentage: " + String(moisturePercentage));
}

void loop() {
  // put your main code here, to run repeatedly:

}
2

From https://docs.arduino.cc/language-reference/en/variables/data-types/stringObject/Functions/toInt/

Description

Converts a valid String to an integer. The input String should start with an integer number. If the String contains non-integer numbers, the function will stop performing the conversion.

3

The string strdoes not start with a valid integer number.
(Posts crossed)

4

With this code I get the same issue:

String str = "POST /1000";
String strNumbers;
int moistureLevel;
float moisturePercentage;

void setup() {
  Serial.begin(9600);

  while (!Serial);
  
  strNumbers = str.substring(6);

  moistureLevel = strNumbers.toInt();

  Serial.println("str: " + str);
  Serial.println("strNumbers: " + strNumbers);
  Serial.println("level: " + moistureLevel);
  Serial.println("percentage: " + String(moisturePercentage));
}

void loop() {
  // put your main code here, to run repeatedly:

}
5 
Serial.println("level: " + moistureLevel);

You cannot concatenate a String and an integer like that. You must print them separately

6

I have a follow up question which is why does moisturePercentage print 0.00 after the line of code below?

moisturePercentage = (moistureLevel / 3400);

7

The calculation is being done using integers. Force it to use floats by using

moisturePercentage = (moistureLevel / 3400.00);
8

You can also print in one line by converting the integer element to string:

Serial.println("level: " + String(moistureLevel));

Not sure which is more efficient though.
Interesting question about the division. Casting one of the operands to a float - seems not to matter which - works as well. I guess that's what adding the .00 does automatically anyway.

9

Hi @mfusco ,

if you are interested in the different ways to format strings, integers and float data you may have a look at this comprehensive webpage

https://wolles-elektronikkiste.de/en/formatted-output

Enjoy!
ec2021

10

FYI, to see what happens when you try to concatenate a C-string and an integer

  for (int anInt = 0; anInt < 6; anInt++) {
    Serial.println("level: " + anInt);
  }

prints

level: 
evel: 
vel: 
el: 
l: 
:

The literal string resolves as a pointer to its contents, a const char *. If the integer is larger than than length of the string, you're asking to print whatever is out there in memory; could be nothing, or invisible, but more likely something.