Java for loop only executing once

Question

Sorry to ask such a basic question but other questions on here don't seem to fix the problem and I've been staring at it for quite a while now. I'm writing some code to find the smallest common multiple for the numbers from 1 to 20. From debugging, the outer for loop only runs once and I can't figure out why. Can someone please point out where I've gone code blind.

public class ED5 {
public static void main(String[] args){
    int smallestCommonMultiple = 1;
    //excluding 1 because it has no effect on the result and would just mean extra work
    int[] numbers = {2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20};
    //initially set to true for the first loop
    boolean iIsFactor = true;

    //for each potential divisor from 2-20 (note only the prime divisors will ever cause a division)
    for(int i = 2; i < 21; i++){
        while(iIsFactor){
            //reset flag for the start of each new run through the array
            iIsFactor = false;

            //for each element of the array
            for(int j=0; j<19; j++){
                //if the current divisor is a factor of that array entry
                if(numbers[j]% i == 0){
                    //divide the current entry by the current divisor
                    numbers[j] = numbers[j]/i;
                    //indicate that at least one entry had a factor of i
                    iIsFactor= true;
                }
            }//end for loop for each array pass

            if(iIsFactor){
                smallestCommonMultiple *= i;
            }
        }//end while loop for the divisor
    }//end for loop for all the divisors

    //output result
    System.out.println("The smallest common multiple of the numbers from 1 to 20 is:");
    System.out.println(smallestCommonMultiple);
}

}

I've tried it multiple times and stepped through with the debugger and the outer for loop only runs once. The print is only supposed to occur once. — Victoria Clutton
– Victoria Clutton, Commented Jun 16, 2015 at 15:31
You mean for(int i = 2; i < 21; i++)? That definitely loops 19 times. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Jun 16, 2015 at 15:32
I put a System.out.println call immediately after the for, outside the while loop, and it prints for every value of i from 2 through 20. The while loops does no iterations for values of i other than 2. That could be what you are seeing. — Patricia Shanahan
– Patricia Shanahan, Commented Jun 16, 2015 at 15:34

Patricia Shanahan · Accepted Answer · 2015-06-16 15:47:22Z

4

The underlying problem has been identified in another answer. This answer is about avoiding the confusion that prevented the OP from seeing which loop was not executing, and therefore failing to find the bug.

When an inner loop is the entire body of an outer loop, it can be unclear which loop is not executing. Printouts and breakpoints that are inside the inner loop are useless for this purpose. The simplest solution is to add a statement at the start of the outer loop. If that statement executes multiple times, it is the inner loop that is not executing. The added statement could be almost anything, but a printout of a key variable for the loop iteration is particularly useful:

for (int i = 2; i < 21; i++) {
  System.out.println("for-loop, i=" + i);
  while (iIsFactor) {

Running the program with that added statement made it obvious that the outer loop was doing the full set of iterations, and the problem had to be with the inner loop.

answered Jun 16, 2015 at 15:47

Patricia Shanahan

26.2k4 gold badges42 silver badges75 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Victoria Clutton Over a year ago

That's an incredibly helpful comment, thank you so much.

Jordi Castilla · Accepted Answer · 2015-06-16 15:26:57Z

2

Your while and boolean declaration is not correct,

for(int i = 2; i < 21; i++){
    //reset flag for the start of each new run through the array
    iIsFactor = false;
    while(!iIsFactor){

answered Jun 16, 2015 at 15:26

Jordi Castilla

27.1k8 gold badges73 silver badges113 bronze badges

2 Comments

Jorge Casariego Over a year ago

That's right! Result after that change is "The smallest common multiple of the numbers from 1 to 20 is: 1" ideone.com/Fe1Ioz

Tom Over a year ago

@JorgeCasariego You wrote while(iIsFactor) instead of while(!iIsFactor).

Mike · Accepted Answer · 2015-06-16 15:27:43Z

2

while(iIsFactor){
        //reset flag for the start of each new run through the array
        iIsFactor = false;

After this, the next itteration of the loop, that while statement becomes false.

answered Jun 16, 2015 at 15:27

Mike

211 bronze badge

Collectives™ on Stack Overflow

Java for loop only executing once

3 Answers 3

1 Comment

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related