Print only changed values without the use of nested if statements - c++

Question

I have four different sensors and store the value they record in a struct. Now I want to print these values every minute. But only the values that changed compared to the last print!
struct SensorData{
uint8_t temp; //temperature
uint8_t pres; //presure
uint8_t hum; //humidity
uint8_t fuel; //fuel
};
I solved this with 15 ifs. But this is kinda hard to read and debug. So I wounder if there is a better and more elegant way to solve this problem.
A table of all posabilitys: 1 if the value is different to the value before and 0 if they are the same:
This is how I solved it with the ifs at the moment:
if(storeData.temp != sensorData.temp && storeData.pres != sensorData.pres && storeData.hum != sensorData.hum && storeData.fuel == sensorData.fuel){
//print case 1: temp,pres,hum
}
else{
if(storeData.temp != sensorData.temp && storeData.pres != sensorData.pres && storeData.hum == sensorData.hum && storeData.fuel != sensorData.fuel){
//print case 2: temp,pres,fuel
}
else{
if(storeData.temp != sensorData.temp && storeData.pres != sensorData.pres && storeData.hum == sensorData.hum && storeData.fuel == sensorData.fuel){
//print case 3: temp,pres,
}else{
if(storeData.temp != sensorData.temp && storeData.pres == sensorData.pres && storeData.hum != sensorData.hum && storeData.fuel != sensorData.fuel){
//print case 4: temp,hum,fuel
}else{
...
Like i mentioned I am looking for a way to solve this more elagant without the use of 15 ifs. Thank you for your help

Answer 1

Your table is just an enumeration of four bits representing the sensors. Hence you may have four comparisons and adjust the bits of a result accordingly:
#include <cstdint>
struct SensorData
{
enum Index
{
Temperature = 3,
Presure = 2,
Humidity = 1,
Fuel = 0
};
std::uint8_t temp; // temperature
std::uint8_t pres; // presure
std::uint8_t hum; // humidity
std::uint8_t fuel; // fuel
};
unsigned compare_sensors(const SensorData& a, const SensorData& b) {
unsigned result = 0;
result |= (a.temp != b.temp) << SensorData::Temperature;
result |= (a.pres != b.pres) << SensorData::Presure;
result |= (a.hum != b.hum) << SensorData::Humidity;
result |= (a.fuel != b.fuel) << SensorData::Fuel;
result = 15 - result; // adjust order
return result;
}
#include <iostream>
int main()
{
SensorData a = {};
SensorData b = {};
b.pres = 255;
b.fuel = 255;
std::cout << "Prssure/Fuel change is case " << compare_sensors(a, b) << ".\n";
}

Answer 2

I would use 4 ifs:
std::string output;
char delimiter = ',';
//Check if data changed
if (storeData.temp != sensorData.temp)
output += std::to_string(sensorData.temp) + delimiter;
if (storeData.pres != sensorData.pres)
output += std::to_string(sensorData.pres) + delimiter;
if (storeData.hum != sensorData.hum)
output += std::to_string(sensorData.hum) + delimiter;
if (storeData.fuel != sensorData.fuel)
output += std::to_string(sensorData.fuel) + delimiter;
//Check if nothing changed
if (output == "")
;//Skip using 'return'?
//Remove 'delimiter' from last case
output.erase(output.length() - 1, 1);
//Send 'output'
//....

Answer 3

You can just use 4 if statements.
Check change in each variable separately, use print statement inside each if block.
As per the constraints mentioned, I don't find any reason why you shouldn't do this.
If you want to print them in a single print statement:
Use 4 if statements as mentioned above, and use 4 variables to hold the values from each if statement. Finally, print it in a single line.
Another method will be to build 'printing string' or 'number' step by step(start with an empty string and build it inside each if statement).

Answer 4

You can use arrays for storing the values, and a loop to check which array elements have changed. Rough outline in C (may contain errors, did not run it through a compiler):
#include <stdbool.h>
char *sensorNames[] = {"temp", "pres", "hum", "fuel"};
uint8_t sensorData[4];
uint8_t storeData[4];
int i;
bool first
while (1) {
get_data(sensorData);
first = true;
for (i=0; i < sizeof(sensorData); i++) {
if (sensorData[i] != storeData[i]) {
// something changed
if (!first) puts(", "); // add a comma if not first item in line
printf("%s: %d -> %d", sensorNames[i], storeData[i], sensorData[i]);
storeData[i] = sensorData[i]; // remember value
first = false;
}
}
if (!first) puts("\n"); // add a line feed if there were any changes
}
Additional advantage: adding a 5th or 6th sensor is trivial :-)