AnalogVoltage and write to SD card

So I have Arduino uno with an sd card shield, and want to measure an analog voltage on pin A0 and write it to the sd card.

I looked the examples on the arduino program and tried to make a sketch for that. The sketch I ended up with is not giving any problem while compiling but it’s not working when i run it.

Needless to say my programming skills are very low…

This is the sketch I used:

  SD card datalogger
 This example shows how to log data from three analog sensors 
 to an SD card using the SD library.
 The circuit:
 * analog sensors on analog ins 0, 1, and 2
 * SD card attached to SPI bus as follows:
 ** MOSI - pin 11
 ** MISO - pin 12
 ** CLK - pin 13
 ** CS - pin 4
 created  24 Nov 2010
 modified 9 Apr 2012
 by Tom Igoe
 This example code is in the public domain.

#include <SD.h>

// On the Ethernet Shield, CS is pin 4. Note that even if it's not
// used as the CS pin, the hardware CS pin (10 on most Arduino boards,
// 53 on the Mega) must be left as an output or the SD library
// functions will not work.
const int chipSelect = 4;

void setup()
 // Open serial communications and wait for port to open:
   while (!Serial) {
    ; // wait for serial port to connect. Needed for Leonardo only

  Serial.print("Initializing SD card...");
  // make sure that the default chip select pin is set to
  // output, even if you don't use it:
  pinMode(10, OUTPUT);
  // see if the card is present and can be initialized:
  if (!SD.begin(chipSelect)) {
    Serial.println("Card failed, or not present");
    // don't do anything more:
  Serial.println("card initialized.");

void loop()
  // make a string for assembling the data to log:
  String dataString = "";

  // read three sensors and append to the string:
    int sensor = analogRead(A0);
    float voltage = sensor * (5.0 / 1023.0);
    dataString += String(voltage);

  // open the file. note that only one file can be open at a time,
  // so you have to close this one before opening another.
  File dataFile ="datalog10.txt", FILE_WRITE);

  // if the file is available, write to it:
  if (dataFile) {
    // print to the serial port too:
  // if the file isn't open, pop up an error:
  else {
    Serial.println("error opening datalog.txt");

changed the filename to datalog9.txt and now works.

8.3 filenames....