Please tell me why its wrong (LCM & GCD )

Question

Question : Given an array of n numbers, find LCM of it.

Since LCM(a,b) = a*b / GCD(a,b), here is my original code:

class GFG {

      // the simple 2 integers recursive to find their GCD.

    static int getGcd(int a, int b){
        while(a != b){
            if(a >b){
                a = a-b;
            }else{
                b= b-a;
            }
        }
        return a;
    }

    // find GCD in an array that has more than two integers.

    static int Gcd(int[] newArray){

     int gcd = newArray[0];
       for(int i=1; i<newArray.length; i++){
        gcd = getGcd(gcd,newArray[i]);
     }

        return gcd;
    }

    static int Lcm(int[] newArray){
        int multi =1;
        for(int i=0; i< newArray.length; i++){
            multi = multi* newArray[i];
        }
        return (multi/gcd);
    }

    public static void main (String[] args) {
        int[] newArray = { 2, 7, 3, 9, 4 };
        System.out.println(Gcd(newArray));
        System.out.println(Lcm(newArray));
    }
}

But when I run this code, it shows some error:

prog.java:33: error: cannot find symbol
        return (multi/gcd);
                      ^
  symbol:   variable gcd

I don't know how to fix it. Please help me to correct my code... Thanks!!!

I expect gcd to be the greatest common divisor in this array — 吳芯緰, Jul 11 '18 at 03:37

Yihang Wang · Accepted Answer · 2018-07-11T04:27:00.550

There is no gcd defined in the Lcm() function. The gcd last defined was in Gcd() function, while this is not a global variable Lcm() cannot use it.

Maybe you mean this?

static int Lcm(int[] newArray){
    int multi =newArray[0];
    int gcd = 0;
    for(int i=1; i< newArray.length; i++){
        gcd = getGcd(multi, newArray[i]);
        if(gcd == 1) {
            multi = multi * newArray[i];
        }else {
            multi = multi * newArray[i] / gcd;
        }
    }
    return multi;
}

Use this could generate the result. You could just have a think about this. We start from 2, multi is 2, and the next value is 7. The gcd for 2 and 7 is 1, so the lcd will be 14. Then we use 14 to compare 3, gcd is also 1, so just multi them is 42. Then use 42 compare 9, we found that 9 have the gcd with 42 is 3, so we use 42/3 is 14, then use 14 and 9, gcd is 1, give multi 14x9 is 126. Then use 126 and 4, the gcd is 2, so multi is 126/2 = 63, and 63 and 4 gcd is 1, so finally multi is 63x4 is 252.

If you have a look of any two numbers, no matter is [3,7],[50,5],[20,10],[100,10000],[52,57],[3,81], the lcm is always a*b/gcd. In this case we could get the answer.

Oh! it works! Thank you! But when I compile the code, the LCM of array { 2, 7, 3, 9, 4 } should be 252, and my answer is wrong :( Is there any wrong on my thinking logic ? — 吳芯緰, Jul 11 '18 at 03:34
Sorry launching just now. Give me seconds, I will work it out for you. — Yihang Wang, Jul 11 '18 at 03:58

score 0 · Answer 2 · answered Jul 11 '18 at 03:23

I hope This Logic May helpful to u .

// Java Program to find LCM of n elements
public class GFG {

public static long lcm_of_array_elements(int[] element_array)
{
    long lcm_of_array_elements = 1;
    int divisor = 2;

    while (true) {
        int counter = 0;
        boolean divisible = false;

        for (int i = 0; i < element_array.length; i++) {

            // lcm_of_array_elements (n1, n2, ... 0) = 0.
            // For negative number we convert into
            // positive and calculate lcm_of_array_elements.

            if (element_array[i] == 0) {
                return 0;
            }
            else if (element_array[i] < 0) {
                element_array[i] = element_array[i] * (-1);
            }
            if (element_array[i] == 1) {
                counter++;
            }

            // Divide element_array by devisor if complete
            // division i.e. without remainder then replace
            // number with quotient; used for find next factor
            if (element_array[i] % divisor == 0) {
                divisible = true;
                element_array[i] = element_array[i] / divisor;
            }
        }

        // If divisor able to completely divide any number
        // from array multiply with lcm_of_array_elements
        // and store into lcm_of_array_elements and continue
        // to same divisor for next factor finding.
        // else increment divisor
        if (divisible) {
            lcm_of_array_elements = lcm_of_array_elements * divisor;
        }
        else {
            divisor++;
        }

        // Check if all element_array is 1 indicate 
        // we found all factors and terminate while loop.
        if (counter == element_array.length) {
            return lcm_of_array_elements;
        }
    }
}

// Driver Code
public static void main(String[] args)
{
    int[] element_array = { 2, 7, 3, 9, 4 };
    System.out.println(lcm_of_array_elements(element_array));
}
}

I've already seen this solution, and I think its a little bit hard to understand, so I try to write the code through my logic. But when I compile the code, the LCM of array { 2, 7, 3, 9, 4 } should be 252, and my answer is wrong :( Is there any wrong on my thinking logic ? Thank you — 吳芯緰, Jul 11 '18 at 03:39

Sivabalakrishnan · Answer 3 · 2018-07-11T03:54:37.810

Logic For GCD of two numbers is ,

// Java program to find GCD of two numbers
class Test
{
// Recursive function to return gcd of a and b
static int gcd(int a, int b)
{
    // Everything divides 0 
    if (a == 0 || b == 0)
       return 0;

    // base case
    if (a == b)
        return a;

    // a is greater 
    //u r missing the logic here.....................
    if (a > b)
        return gcd(a-b, b);
    else
        return gcd(a, b-a);
}

// Driver method
public static void main(String[] args) 
{
    int a = 98, b = 56;
    System.out.println("GCD of " + a +" and " + b + " is " + gcd(a, b));
}
}

Please see the logic, you should iterate till a=b , u r iterating only once.

Please tell me why its wrong (LCM & GCD )

3 Answers3