Re SD card datafile open question

This code is from the example folder for the SD lib.
Here the file is opened, writen to and closed in the loop section.

Q… is it possible to ex open the file just one time in ex setup section
do the write stuff in loop, and close it lats say after 1000 writes ??
Or is the datafile open procedure so fast that it dont matter??
I nead this as fast as possibel…
Have tried this but just getting … not declared in this scope… problem


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

Serial.print(“Initializing SD card…”);

// 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:
for (int analogPin = 0; analogPin < 3; analogPin++) {
int sensor = analogRead(analogPin);
dataString += String(sensor);
if (analogPin < 2) {
dataString += “,”;

// 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 =“datalog.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”);


Newer mind,problem sorted..