boolean useSerial = true; 

void sendSerial() { 
    if(useSerial) { 

        Serial.print(" "); 
        Serial.println(fft_log_out[j]); 
    } 
} 

void loop() { 
    ...... 
    if(Serial.available()) { 
        char a = Serial.read(); 
        if(a == 'n') 
            useSerial = true; 
        else 
            useSerial = false; 
    } 
    ...... 
} 

이렇게 답변주셨는데요

/*
fft_adc.pde
guest openmusiclabs.com 8.18.12
example sketch for testing the fft library.
it takes in data on ADC0 (Analog0) and processes them
with the fft. the data is sent out over the serial
port at 115.2kb.  there is a pure data patch for
visualizing the data.
*/

// do #defines BEFORE #includes
#define LOG_OUT 1 // use the log output function
#define FFT_N 256 // set to 256 point fft
#include <FFT2.h>

boolean useSerial = true; 

void sendSerial() { 
    if(useSerial) { 

        Serial.print(" "); 
        Serial.println(fft_log_out[j]); 
    } 
} 

void setup() {
  Serial.begin(1200); // use the serial port
  TIMSK0 = 0; // turn off timer0 for lower jitter - delay() and millis() killed
  ADCSRA = 0xe5; // set the adc to free running mode
  ADMUX = 0x40; // use adc0
  DIDR0 = 0x01; // turn off the digital input for adc0
  pinMode(A0, OUTPUT);
  pinMode(12, INPUT);
  Serial.println("reset!");
}

void loop() {

 if(Serial.available()) { 
        char a = Serial.read(); 
        if(a == 'n') 
            useSerial = true; 
        else 
            useSerial = false; 
    } 
  while(1) { // reduces jitter
    cli();  // UDRE interrupt slows this way down on arduino1.0
    for (int i = 0 ; i < 512 ; i += 2) { // save 256 samples
      while(!(ADCSRA & 0x10)); // wait for adc to be ready
      ADCSRA = 0xf5; // restart adc
      byte m = ADCL; // fetch adc data
      byte j = ADCH;
      int k = (j << 8) | m; // form into an int
      k -= 0x0200; // form into a signed int
      k <<= 6; // form into a 16b signed int
      fft_input[i] = k; // put real data into even bins
      fft_input[i+1] = 0; // set odd bins to 0
    }
    // window data, then reorder, then run, then take output
    fft_window(); // window the data for better frequency response
    fft_reorder(); // reorder the data before doing the fft
    fft_run(); // process the data in the fft
    fft_mag_log(); // take the output of the fft
    sei(); // turn interrupts back on
     for (uint8_t j = 0; j < FFT_N/2; j++) {
      Serial.print(" ");
      Serial.println(fft_log_out[j]);
      delay(1000);
    }
  }
}

제가 합쳐본 건 인데요 

이렇게 합치는 게 맞나요?ㅜㅜ 자꾸 빨간 부분에서

 exit status 1
'j' was not declared in this scope 

라고 오류뜨네요 ㅜㅜ

찾아보니까  선언부는 선언부끼리 void setup() 끼리 void loop()끼리 해라고 되어있는데 boolean useSerial = true; 랑 void sendSerial() 은 처음 보네요ㅜㅜ 이거는 어디다 넣어야 되는 거죠?