How to SD card data store in an array.

I pretty new on arduino . So please helpe out. I am using arduino mega2560 using library SD.h read data data from the sd card but still I am store in an array . If I am trying to store data, received only data in ascii format.

thanks

#include <SD.h>
#include <SPI.h>
File sdcard_file;
int CS_pin = 53; 
String data = "";
uint32_t voltage[10] = {220, 220, 222, 223, 224, 225, 226, 227, 228, 229};
uint32_t current[10] = {1,2,2, 3,35, 53, 2, 4, 2, 2};
struct filename
{
  String file1; String file2; String file3, file4;
};
String buff;
char arr[200];
char arr2[200];
char ar1[10];
char ar2[10];
void setup() {
 Serial.begin(9600); //Setting baudrate at 9600
 pinMode(CS_pin, OUTPUT);//declaring CS pin as output pin
 if (SD.begin())
 {
 Serial.println("SD card is initialized and it is ready to use");
 } else
 {
 Serial.println("SD card is not initialized");
 return;
 }
 }
void loop() 
{
  while(Serial.available()>0)
  {
    char receiveData = Serial.read();
    Serial.println(receiveData);
    if(receiveData == '1')
    {
      SD.remove("log.csv");
      sdcard_file = SD.open("log.csv", FILE_WRITE); //Looking for the data.txt in SD card
 
      if (sdcard_file) { //If the file is found
      Serial.println("Writing to file is under process");
      for(int i =0; i<10; i++)
      {
        data += String(voltage[i]);
        data += ",";
        data += String(current[i]);
        sdcard_file.println(data); //Writing to file
        data = "";
      }
      sdcard_file.close(); //Closing the file

    }
    }
    else if(receiveData == '2')
    {
      sdcard_file = SD.open("log.csv");
      Serial.println("Reading from the file");
      while (sdcard_file.available()) {
        buff = sdcard_file.read();
        strcpy(arr, buff.c_str());
      //Serial.write(arr);
      Serial.print(arr);
       }
     sdcard_file.close();
    
     int j=0,k=0;
     for(int i=0; i<sizeof(arr);i++)
     {
      arr2[i] = arr[i]-0;
     }
     Serial.println(arr2);
    }
    receiveData =0;
  }
}

In your code when receivedata is 2, replace buff = sdcard_file.read(); with buff = sdcard_file.readString();.
It will work flawlessly.
Thanks.