Find the heights between the distances of a walking trail.

[Deepak Lal]

We have an array of heights, representing the altitude along a walking trail. Given start/end indexes into the array, return the sum of the changes for a walk beginning at the start index and ending at the end index. For example, with the heights {5, 3, 6, 7, 2} and start=2, end=4 yields a sum of 1 + 5 = 6. The start end end index will both be valid indexes into the array with start <= end.

sumHeights({5, 3, 6, 7, 2}, 2, 4) => 6
sumHeights({5, 3, 6, 7, 2}, 0, 1) => 2
sumHeights({5, 3, 6, 7, 2}, 0, 4) => 11

My code is as below:

public int sumHeights(int[] heights, int start, int end) {
  
  int total =0;
  int difference =0;
  for(int i=start;i<=end;i++){
    if(heights[i] > heights[i++]){
       difference =heights[i] - heights[i++];      
    }else if(heights[i++] > heights[i]){
       difference =heights[i++] - heights[i];    
    }
    total+=difference;
  }
  return total;
}

Could you let me know why im getting "Exception:java.lang.ArrayIndexOutOfBoundsException: 5 (line number:9)"
whats wrong with my code.Could you provide me with a fix for the same.

[Campbell Ritchie]

You want to go through that and see how many ++ operators you are using. Also write down the index you are accessing, starting with 0, and you will doubtless see you are going off the end of the array.

[Deepak Lal]

@Campbell,im unable to proceed further,Could you help me with the fix in the code that i have provided.i have checked the i++ operator with the previous index as well.As per the Question,The logic has been written,Could you help me fix this up for me.

[Campbell Ritchie]

Deepak Lal wrote: . . .

// omitted
  for(int i=start;i<=end;i++){
    if(heights[i] > heights[i++]){
       difference =heights[i] - heights[i++];
    }else if(heights[i++] > heights[i]){  
        difference =heights[i++] - heights[i];
 //omitted

. ..

You have the variable i nine times in that code snippet. Write it on a piece of paper and write down an array like this: {5, 3, 6, 7, 2}. That is one of your arrays from your earlier post. Now get a different coloured pencil and write the indices against the nine s numbers in that array.
Now get the same pencil and write the values of i as you go through that loop, starting at 3. Now get another colour of pencil and write against each occurrence of i which index in the array you are attempting to use. The value of i and the index are not necessarily the same. When you have done that, it will be obvious what has gone wrong.

[Campbell Ritchie]

Actually, I counted wrongly. You have eleven is.

[Deepak Lal]

Campbell,im still not getting it.could you help me to fix it up.Im a bit confused with this problem and struggling to get it right.
if you can provide me with code fix it would be appreciated.

[Deepak Lal]

If you can provide me with code fix it would be appreciated.
Im still confused....

[Rob Spoor]

Get rid of all those "i++" statements inside the loop, and replace them with "i + 1". The latter is only a value - the current value of "i" plus one. The former is a statement that modifies "i" and returns its old value. In your code "i" is increased 2 or 3 times within the loop body, then again in the for-loop increment.

So change it to this:

// omitted   
  for(int i=start;i<=end;i++){   
    if(heights[i] > heights[i+1]){ // no more i++
       difference =heights[i] - heights[i+1]; // no more i++
    }else if(heights[i+1] > heights[i]){ // no more i++
        difference =heights[i+1] - heights[i]; // no more i++
 //omitted

There is one easier way to do this though:

// omitted   
  for(int i=start;i<=end;i++){   
    difference = Math.abs(heights[i] - heights[i+1]);
 //omitted 

Math.abs turns a negative value into a positive value, so if height[i+1] > height[i] the subtraction returns a negative value which then gets turned positive again. For any two random values "x" and "y", Math.abs(x - y) == Math.abs(y - x).

[Campbell Ritchie]

Actually if you have a start and end, shouldn't the <= be changed to <