Temp Sensors Max and Min - c++

Question

(Referring to the last if statement)I have some code here that keeps giving me the T_High always the same as T. If I am looking for the highest temperature that the code has seen, I am not sure if I need to add another variable and adjust my code or what my current problem is. I have tried looking this up online about max and mins, but even with multiple different sources I was not able to get it to work. I understand that after my if statement I have them being equaled to each other, but I thought that the if statement would take care of this. This is in particle IDE (very similar to Arduino).`
#include <math.h>
const int thermistor_output = A1;
void setup() {
Serial.begin(9600);
}
void loop() {
float T_Low = 999;
float T_High;
int x=0;
int thermistor_adc_val;
double a,b,c,d,e,f,g,h,i,j,T;
double output_voltage, thermistor_resistance, therm_res_ln, temperature_celsius;
while (x < 10 )
{
thermistor_adc_val = analogRead(thermistor_output);
output_voltage = ( (thermistor_adc_val * 3.3) / 4095.0 );
thermistor_resistance = ( ( 3.3 * ( 10.0 / output_voltage ) ) - 10 ); /* Resistance in kilo ohms */
thermistor_resistance = thermistor_resistance * 1000 ; /* Resistance in ohms */
therm_res_ln = log(thermistor_resistance);
/* Steinhart-Hart Thermistor Equation: */
/* Temperature in Kelvin = 1 / (A + B[ln(R)] + C[ln(R)]^3) */
/* where A = 0.001129148, B = 0.000234125 and C = 8.76741*10^-8 */
temperature_celsius = ( 1 / ( 0.001129148 + ( 0.000234125 * therm_res_ln ) + ( 0.0000000876741 * therm_res_ln * therm_res_ln * therm_res_ln ) ) ); /* Temperature in Kelvin */
temperature_celsius = temperature_celsius - 273.15; /* Temperature in degree Celsius */
T = temperature_celsius * 1.8 + 29; /* Temperature in degree Fahrenheit */
delay(100);
x ++ ;
if (x==1){a=T;}
if (x==2){b=T;}
if (x==3){c=T;}
if (x==4){d=T;}
if (x==5){e=T;}
if (x==6){f=T;}
if (x==7){g=T;}
if (x==8){h=T;}
if (x==9){i=T;}
}
j=a+b+c+d+e+f+g+h+i;
T=j/9;
x=0;
delay(2000);
if (T > T_High) {
T_High = T;
}
Particle.publish ("Temp", String(T));
Particle.publish ("High", String(T_High));
}
`

Answer 1

T_High is a local variable in loop(). It exists only until the loop() function exits. In the next iteration there will be a new instance of the T_High variable that has no relation to the previous one. Therefore you are not saving T_High's value throughout calls to loop().
Move the variable into file scope (i.e. outside the function definitions), so that it has static storage duration and survives throughout the whole program execution.
Also, you never initialize T_High to a value or assign to it before you do the test T > T_High. At this point T_High has indeterminate value and using an indeterminate value in almost any way causes the program to have undefined behavior, as is the case here.
If you move the variable to file scope, this particular problem will be remedied automatically, because static storage duration objects are always zero-initialized, but still it is advisable to initialize T_High to a sensible value, e.g.:
float T_High = 0;
or whatever works for your use case.