base64 encoding .txt file

Projects Programming
12

So, i need to modifie code, that there are no posibility that two files are open at a time or just find a library that supports access to more than one file at a time.

Somehow i almost found solution to my second problem , but still it is not perfect, with some issues.
here is code:

#include <SD.h>
#include <SPI.h>

float u = 0;
File base;
int i = 30.0;
int sk = 0;
String inData;

void setup() {
   Serial.begin(115200);
   Serial.print("Initializing SD card...");
   pinMode(10, OUTPUT);
  digitalWrite(10, HIGH);   
  if (!SD.begin(4)) {
    Serial.println("initialization failed!");
    return;
  }
  Serial.println("initialization done.");
}
void loop()
{
  char var = Serial.read();
switch (var) {

case 'u':
   {
     Serial.println("lasa");
             base = SD.open("base.txt", FILE_READ);
            while(base.available())    {
              char a = base.read();
           inData += a;
           sk++;
           u = (float)sk/i;
             Serial.println(sk);
             Serial.println(u);        
           if (u == 1 || u == 2 || u == 3 || u == 4 || u == 5 || u == 6 || u == 7 || u == 8 || u == 9){
             Serial.print(inData);
             Serial.println();
             inData = 0;
           } 
           }
           base.close();
   }
    break; 
}
}