C++ programing question

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Hi. I wrote a program to find the sum of the of the gaps between adjacent entries of an array. The gap has to be absolute value between two numbers. Here is my program:

  int sumGaps(int a[], int cap) { 
      int gap, sum; 
      for (int i = 1; i < cap; i++) { 
          int gap = a[i] - a[i - 1]; 
          if (gap < 0) gap = -gap; sum = sum + gap; 
      } 
      return sum; 
  }

  int main() { 
      int x[5] = {3, 1, 4, 1, 5}; 
      cout << sumGaps(x, 5) << endl; // prints 12 corresponding to the sum of
                                        gaps 2 + 3 + 3 + 4.
      return 0; 
  }

As you can see in the main function, the correct answer is 12. However, when I ran the program, it gave me the answer of 4196783. I figured out that if I changed to declare the variable "gap" inside the for loop, it would give the correct answer. Why is this? Can anybody explain for me why declaring a variable inside vs declaring it outside the for loop make the answer change?
Thank you.

Last edited on

helios (17575)

What does the code that doesn't work look like?

Also, this is unrelated to your question, but avoid doing things like line 5.

if (gap < 0) gap = -gap; sum = sum + gap;

Use proper formatting in your code.

1
2
3

if (gap < 0)
    gap = -gap;
sum = sum + gap;

lastchance (6980)

sum isn't initialised.

There is also an abs() function to simplify your code.
sum += abs( gap );

It doesn't matter whether you declare gap outside the loop or not: you must be doing something else wrong.

Last edited on

keskiverto (10402)

This is what you do:

int sum; // uninitialized
// value of sum is undefined

sum = sum + 2; // undefined + 2
// value of sum is still undefined

Whether you read value 4196771 or 0 from undefined variable makes no difference.

In fact, consider yourself lucky that you got the 4196783 as hint of a problem and extremely unlucky when unrelated code change happened to produce the expected 12 by accident.

Javi87 (2)

Very nice response! Thx

icy1 (596)

Since you're starting at index 1 and have no need to store temporary variables, can refactor to not have "gap":

// Expects array to have at least 2 elements
int SumGaps(int a[], int a_size)
{ 
    int sum = 0;
    for (int i=1; i<a_size; i++) 
        sum += abs(a[i] - a[i-1]);

    return sum;
}

keskiverto (10402)

One can naturally make it look more complicated:

// adjacent_difference example
#include <iostream>     // std::cout
#include <numeric>      // std::adjacent_difference, accumulate

int main () {
  constexpr size_t FV {5};
  int val[FV] {3, 1, 4, 1, 5};
  int result[FV];

  std::adjacent_difference (val, val+FV, result, [](int x, int y){return abs(x-y);} );
  std::cout << "adjacent_difference: ";
  for (auto x : result) std::cout << x << ' ';
  std::cout << '\n';

  std::cout << "accumulate: ";
  std::cout << std::accumulate( result+1, result+FV, 0 );
  std::cout << '\n';

  return 0;
}

lalaland5 (2)

Thank you all for your responses.
*keskiverto* you are right. The program printed 4196771 as the result and I couldn't explain why. Now I understand. Thank again

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