Erratic sampleNumber variable

/**
 * The circuit:
 * Arduino UNO R3
 * SPI SD card shield attached (pins 10, 11, 12, 13)
 * - DS1307 RTC on SD card shield via I2C
 * Prototype shield attached
 * - LCD attached (pins 2, 3, 4, 5, 6, 7)
 * - BMP180 sensor via I2C (pins A4, A5)
 */

// include libraries
#include "Wire.h"
#include "RTClib.h"
#include "SparkFunBME280.h"
#include <SPI.h>
#include <SD.h>
#include <LiquidCrystal.h>

// DS1307 real time clock
RTC_DS1307 rtc;

// SD card
unsigned int cardPin = 10;
File dataFile;
char filename[16];
char timestamp[16];

// LCD display
LiquidCrystal lcd(7, 6, 5, 4, 3, 2);

// degree C symbol data
byte degreeCSymbol[8] = {
  0b01000,
  0b10100,
  0b01000,
  0b00011,
  0b00100,
  0b00100,
  0b00011,
  0b00000
};

// degree F symbol data
byte degreeFSymbol[8] = {
  0b01000,
  0b10100,
  0b01000,
  0b00011,
  0b00100,
  0b00111,
  0b00100,
  0b00000
};

// BMP280 sensor object
BME280 bmp280sensor;

// sensor data
float tempC;
float tempF;
float pressPa;
float presskPa;
float altM;
float altFt;

// CSV row counter
unsigned int sampleNumber = 0;

//  function to reset arduino at address 0
void (* resetFunc) (void) = 0;

// function to initialize serial connection
void initSerial()
{
  Serial.begin(115200);
  delay(2000);

  // program started
  Serial.println(F("Program Started\n"));
}

// function to initialize RTC on SD card shield
void initRTC()
{
  Serial.print(F("Initializing RTC module..."));
  if (!rtc.begin()) {
    // no RTC found
    Serial.println(F("RTC not found!"));
    // tell the user
    lcd.clear();
    lcd.print(F("   CHECK RTC!   "));
    delay(5000);
    // reset the arduino
    resetFunc();
  }
  // rtc was found but isn't running
  if (!rtc.isrunning()) {
    Serial.println(F("RTC found, but not running!"));
    // set the RTC date/time to when sketch complied
    rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
    Serial.println(F("RTC Date/Time adjusted to sketch compile time!"));
  }
  // uncomment to force RTC time update with each compile
  // rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
  Serial.println(F("RTC initalized! \n"));


}

// function to initalize SD card shield
void initSD()
{
  Serial.print(F("Initializing SD card..."));
  // see if the card is present and can be initialized:
  if (!SD.begin(cardPin)) {
    // card failed
    Serial.println(F("Card failed, or not present"));
    // tell the user
    lcd.clear();
    lcd.print(F(" CHECK SD CARD! "));
    delay(5000);
    // reset the arduino
    resetFunc();
  }
  Serial.println(F("card initialized! \n"));
}

// function to initalize LCD
void initLCD()
{
  Serial.print(F("Initializing LCD..."));
  lcd.begin(16, 2);
  Serial.println(F("16x2 display initialized!"));
  // initalize custom degree c symbol
  lcd.createChar(0, degreeCSymbol);
  Serial.println(F("- degree C character created."));
  // initalize custom degree f symbol
  lcd.createChar(1, degreeFSymbol);
  Serial.println(F("- degree F character created. \n"));
  // clear LCD to begin
  lcd.clear();
}

// function to initalize BMP280 sensor
void initBMP()
{
  // sensor settings
  bmp280sensor.settings.commInterface = I2C_MODE;
  bmp280sensor.settings.I2CAddress = 0x76; //0x77(default) or 0x76
  bmp280sensor.settings.runMode = 3; //Normal mode
  bmp280sensor.settings.tStandby = 0; // 0.5ms
  bmp280sensor.settings.filter = 0; //filter off
  bmp280sensor.settings.tempOverSample = 1; //oversampling 1
  bmp280sensor.settings.pressOverSample = 1; // oversampling 1
  bmp280sensor.settings.humidOverSample = 0; // skipped
  // init sensor
  Serial.print(F("Initalizing BME280... initialized at: 0x"));
  // calling .begin() causes the settings to be loaded
  delay(10); // make sure sensor had enough time to turn on. BME280 requires 2ms to start up.
  Serial.print(bmp280sensor.begin(), HEX);
  Serial.println(F("! \n"));
}

// setup the arduino
void setup()
{
  // initialize serial connection
  initSerial();

  // initalize on-board LED
  pinMode(13, OUTPUT);

  // initialize real time clock
  initRTC();

  // initialize SD card
  initSD();

  // initialize lcd
  initLCD();

  // initalize bmp280 sensor
  initBMP();

  // outputting CSV to seial connection and mirroring to SD card and LCD
  Serial.println(F("Generating CSV file on SD card and mirroring here: \n"));

}

// loop logic
void loop()
{

  // generate the filename
  generateFilename();

  // open the file
  dataFile = SD.open(filename, FILE_WRITE);

  // if the file is available, write to it:
  if (dataFile)
  {
    // build CSV data
    if (sampleNumber == 0)
    {
      // build a first-row of column headers
      dataFile.print("Sample,");
      dataFile.print("Timestamp,");
      dataFile.print("T(deg C),");
      dataFile.print("T(deg F),");
      dataFile.print("P(Pa),");
      dataFile.print("P(kPa),");
      dataFile.print("Alt(m),");
      dataFile.print("Alt(ft),");
      dataFile.println("");

      Serial.print(F("Sample,"));
      Serial.print(F("Timestamp,"));
      Serial.print(F("T(deg C),"));
      Serial.print(F("T(deg F),"));
      Serial.print(F("P(Pa),"));
      Serial.print(F("P(kPa),"));
      Serial.print(F("Alt(m),"));
      Serial.print(F("Alt(ft),"));
      Serial.println(F(""));
    }
    else
    {
      // sample number
      dataFile.print(sampleNumber);
      dataFile.print(F(","));

      Serial.print(sampleNumber);
      Serial.print(F(","));

      // timestamp
      generateTimestamp();

      dataFile.print(timestamp);
      dataFile.print(F(","));

      Serial.print(timestamp);
      Serial.print(F(","));

      // read and display data
      displayData(0, 0, 1, 6); // temp,   C,    1 of 6
      displayData(0, 1, 2, 6); // temp,   F,    2 of 6
      displayData(1, 0, 3, 6); // press,  Pa,   3 of 6
      displayData(1, 1, 4, 6); // press,  kPa,  4 of 6
      displayData(2, 0, 5, 6); // alt,    m,    5 of 6
      displayData(2, 1, 6, 6); // alt,    ft,   6 of 6

      // print new line
      dataFile.println("");

      Serial.println(F(""));
    }

    // increase CSV counter
    sampleNumber++;

    // close the file
    dataFile.close();

  }
  // if the file isn't open, pop up an error:
  else
  {
    // couldn't open file!
    Serial.println(F("ERROR: file can't be saved!"));
    // tell the user
    lcd.clear();
    lcd.print(F(" CAN'T SAVE SD! "));
    delay(5000);
    // reset the arduino
    resetFunc();
  }

}

// generate a non-duplicate filename
void generateFilename()
{
  // get the time
  DateTime now = rtc.now();

  // format the filename with MMDDYYYY.CSV format
  sprintf(filename,"%02u%02u%04u.CSV",now.month(),now.day(),now.year());

  // if this is a new loop and file already exists
  if (sampleNumber == 0 && SD.exists(filename))
  {
    // counter
    int n = 0;

    // check for next file on SD card
    while(SD.exists(filename))
    {
      // increase counter
      n++;

      // format filename with trailing number
      sprintf(filename,"%02u%02u-%02u.CSV",now.month(),now.day(), n);
    }
    // display filename
    Serial.print(F("Filename is: "));
    Serial.println(filename);
  }
  // this is a new file and new loop, output filename
  else if (sampleNumber == 0)
  {
    // display filename
    Serial.print(F("Filename is: "));
    Serial.println(filename);
  }
}

// generate timestamp for CSV
void generateTimestamp()
{
  // create DateTime object from RTC.now()
  DateTime now = rtc.now();

  // format the date MM-DD-YYYY HH:MM:SS
  sprintf(timestamp,"%02u-%02u-%04u %02u:%02u:%02u",now.month(),now.day(),now.year(),now.hour(),now.minute(),now.second());

}

// read, format, and display requested measurments
void displayData(unsigned int measurment, unsigned int unit, unsigned int posX, unsigned int posY)
{
  // measurment begins
  digitalWrite(13, HIGH );

  // what data are we displaying?
  switch (measurment)
  {
    // requesting Temperature
    case 0:
    if (unit == 0)
    {
      // requesting celsius
      lcd.print(F("Temperature: "));
      // update progress on LCD
      lcd.setCursor(13, 0);
      lcd.print(posX);
      lcd.print(F("/"));
      lcd.print(posY);
      // read sensor
      tempC = bmp280sensor.readTempC();
      // move cursor to new line
      lcd.setCursor(0, 1);
      // print measurement to LCD
      lcd.print(tempC,2);
      // print measurement to SD
      dataFile.print(tempC);
      dataFile.print(",");
      // print measurement to Serial
      Serial.print(tempC);
      Serial.print(F(","));
      // print degree character
      lcd.write((uint8_t)0);

    }
    else if (unit == 1)
    {
      // requesting ferinheight
      lcd.print(F("Temperature: "));
      // update progress on LCD
      lcd.setCursor(13, 0);
      lcd.print(posX);
      lcd.print(F("/"));
      lcd.print(posY);
      // read sensor
      tempF = bmp280sensor.readTempF();
      // move cursor to new line
      lcd.setCursor(0, 1);
      // print measurement to LCD
      lcd.print(tempF,2);
      // print measurement to SD
      dataFile.print(tempF);
      dataFile.print(",");
      // print measurement to Serial
      Serial.print(tempF);
      Serial.print(F(","));
      // print degree character
      lcd.write((uint8_t)1);
    }
    break;

    // requesting Pressure
    case 1:
    if (unit == 0)
    {
      // requesting Pascals
      lcd.print(F("Pressure: "));
      // update progress on LCD
      lcd.setCursor(13, 0);
      lcd.print(posX);
      lcd.print(F("/"));
      lcd.print(posY);
      // read sensor
      pressPa = bmp280sensor.readFloatPressure();
      // move cursor to new line
      lcd.setCursor(0, 1);
      // print measurement to LCD
      lcd.print(pressPa,2);
      // print measurement to SD
      dataFile.print(pressPa);
      dataFile.print(",");
      // print measurement to Serial
      Serial.print(pressPa);
      Serial.print(F(","));
      // print unit of measure
      lcd.print(F(" Pa"));
    }
    else if (unit == 1)
    {
      // requesting Kilo-Pascals
      lcd.print(F("Pressure: "));
      // update progress on LCD
      lcd.setCursor(13, 0);
      lcd.print(posX);
      lcd.print(F("/"));
      lcd.print(posY);
      // read sensor
      presskPa = (bmp280sensor.readFloatPressure() / 1000);
      // move cursor to new line
      lcd.setCursor(0, 1);
      // print measurement to LCD
      lcd.print(presskPa,2);
      // print measurement to SD
      dataFile.print(presskPa);
      dataFile.print(",");
      // print measurement to Serial
      Serial.print(presskPa);
      Serial.print(F(","));
      // print unit of measure
      lcd.print(F(" kPa"));
    }
    break;

    // requesting Altitude
    case 2:
    if (unit == 0)
    {
      // requesting Meters
      lcd.print(F("Altitude: "));
      // update progress on LCD
      lcd.setCursor(13, 0);
      lcd.print(posX);
      lcd.print(F("/"));
      lcd.print(posY);
      // read sensor
      altM = bmp280sensor.readFloatAltitudeMeters();
      // move cursor to new line
      lcd.setCursor(0, 1);
      // print measurement to LCD
      lcd.print(altM,2);
      // print measurement to SD
      dataFile.print(altM);
      dataFile.print(",");
      // print measurement to Serial
      Serial.print(altM);
      Serial.print(F(","));
      // print unit of measure
      lcd.print(F(" m"));

    }
    else if (unit == 1)
    {
      // requesting Feet
      lcd.print(F("Altitude: "));
      // update progress on LCD
      lcd.setCursor(13, 0);
      lcd.print(posX);
      lcd.print(F("/"));
      lcd.print(posY);
      // read sensor
      altFt = bmp280sensor.readFloatAltitudeFeet();
      // move cursor to new line
      lcd.setCursor(0, 1);
      // print measurement to LCD
      lcd.print(altFt,2);
      // print measurement to SD
      dataFile.print(altFt);
      dataFile.print(",");
      // print measurement to Serial
      Serial.print(altFt);
      Serial.print(F(","));
      // print unit of measure
      lcd.print(F(" ft"));
    }
    break;
  }

  // measurment ends
  digitalWrite(13, LOW );

  // delay for displaying on LCD
  delay(5000);

  // clear LCD for next measurment
  lcd.clear();
}

/*
SERIAL OUTPUT:

Program Started

Initializing RTC module...RTC initalized!

Initializing SD card...card initialized!

Initializing LCD...16x2 display initialized!
- degree C character created.
- degree F character created.

Initalizing BME280... initialized at: 0x58!

Generating CSV file on SD card and mirroring here:

Filename is: 1120-13.CSV
Sample,Timestamp,T(deg C),T(deg F),P(Pa),P(kPa),Alt(m),Alt(ft),
1,11-20-2017 16:36:14,24.43,75.97,94446.63,94.45,607.64,1993.56,
12603,11-20-2017 16:36:44,24.43,75.97,94446.63,94.45,608.13,1998.42,
13371,11-20-2017 16:37:14,24.43,75.96,94453.24,94.45,608.27,1995.63,
12603,11-20-2017 16:37:45,24.43,75.97,94450.84,94.45,609.23,1996.36,
13371,11-20-2017 16:38:15,24.44,75.97,94445.41,94.45,608.49,1997.16,
12603,11-20-2017 16:38:45,24.42,75.99,94459.75,94.45,608.42,1996.94,
13371,11-20-2017 16:39:15,24.42,75.97,94450.84,94.46,608.49,1997.98,
12603,11-20-2017 16:39:45,24.42,75.99,94446.17,94.46,608.42,1996.94,
13371,11-20-2017 16:40:15,24.43,75.99,94451.60,94.45,608.42,1996.12,

*/