Return value of 0 if value is less than zero

Question

Background: I am using an analog pressure sensor with a range from 0-100PSI returning voltage values .44v to 4.44v. It's open air (or 0 PSI) voltage on the signal pin is .47 volts. I have a function that subtracts that .47v (to make zero) and then multiplies it by 25 (25 PSI per volt). Due to signal noise or other factors, sometimes my function returns a negative number, which wreaks havoc on my math. I would like to return a value of 0 when my offset math returns a negative number, but I'm having a problem with it.

The code that I wrote that doesn't work:

double EngineOilPressure() {
    //insert oil pressure code here
    int reading = analogRead(sensorPin3);
    float voltage = reading * 5.0;
    voltage /=1024.0;

    Serial.print(voltage); Serial.println(" volts");

    if (voltage-.47 < 0)
    {
        float voltadj = 0;
    }
    else
    {
        float voltadj = (voltage-.47);
    }

    float pressureP = voltadj * 25;
    return PSIToPascal(pressureP);
}

The error that I get is

dynamic_engine.pde: In function 'double EngineOilPressure()':
dynamic_engine.pde:41:23: error: 'voltadj' was not declared in this scope
Error compiling.

The code that works but sometimes returns a very wrong value:

double EngineOilPressure() {
    int reading = analogRead(sensorPin3);
    float voltage = reading * 5.0;
    voltage /=1024.0;

    Serial.print(voltage); Serial.println(" volts");

    float pressureP = (voltage - .47) * 25;
    return PSIToPascal(pressureP);
}

Answer 1

The scope of each of the two different and unrelated voltadj variables that your code (as below) declares is limited to within its enclosing set of braces.

if (voltage-.47 < 0)
{
    float voltadj = 0;
}
else
{
    float voltadj = (voltage-.47);
}

Instead perhaps say

float voltadj = voltage-.47;
if (voltadj < 0) voltadj = 0;

or perhaps

float voltadj = voltage < .47? 0 : voltage-.47;

or

float voltadj = constrain(voltage-0.47, 0.0, 5.0);

or

float voltadj = max(0.0, voltage-0.47);

Also see max() and constrain() reference pages at arduino.cc.

Answer 2

Another method, though one that is quite cryptic to understand, would be:

float pressureP = voltage > 0.47 ? (voltage - 0.47) * 25 : 0;

That is known (by me at least) as an inline conditional. The ? acts as an if operator, and the : as an else. Basically it's saying:

Voltage is greater than 0.47
Is that true?
    then subtract 0.47 and multiply by 25 and assign it
otherwise:
    assign 0

The format is simply:

value = {condition} ? {true value or expression} : {false value or expression};

If can be quite useful when you want to conditionally calculate or assign something. @jwpat7 makes mention of a max() function above. That max() function can itself be written with this inline conditional format:

maxval = a > b ? a : b;

Or: If a is greater than b then assign a otherwise assign b.

Answer 3

The Arduino way is to use constrain

float pressureP = constrain ((voltage - 0.47) * 25 , 0 [The Maximum your code can handle]);

This will return the value ((voltage - 0.47) * 25) but never less than 0 and never more than The maximum you provide.