Change/declare a constant in setup and use it in loop

Question

I want to change or declare a constant in the setup() and then, I want to access it in the loop().

I have searched a lot, but the only questions I found could be solved by declaring the constant in the beginning of the program. That doesn't work for me because I am using the Adafruit BMP388 and I can't use the pressure before setup().

I also tried declaring the variable before setup() and changing it inside setup(), but it didn't change when I used it in loop().

How can I solve this?

Answer 1

You are not going to get good answers without showing your code, but maybe this is a good guess of what you want:

why not try something roughly like this (untested and incomplete)?

Adafruit_BMP3XX bmp;
float startPressure;
float difference;

void setup() {
  bmp.begin_I2C();
  startPressure = bmp.readPressure();
} 

void loop() {
  difference = startPressure - bmp.readPressure();
}

Or, better:

Adafruit_BMP3XX bmp;

void setup() {
  bmp.begin_I2C();
} 

void loop() {
  static const float startPressure = bmp.readPressure();
  static float difference;
  difference = startPressure - bmp.readPressure();
}