find smallest element of an array and set its index to 0

Question

I am trying to create a C function that takes an array, finds the smallest element, and sets the value of that element to zero.

This is my function so far.

void find_minimum(double a[], int n) {

    int i, smallest = 0;

    smallest = a[0];

    for (i = 0; i < n; i++) {
        if (a[i] < smallest) {
            smallest = a[i];
        }
        a[i] = 0;
    }
}

When I run it, every index except the last one is zero, but I want only the smallest element to be zero.

move a[i] = 0 ; inside the if statement, and start comparison from the index 1 i.e. i=1 in for loop — Imran Ali
– Imran Ali, Commented Jun 14, 2016 at 21:28
@ImranAli no, then it will set the smallest-so-far to 0 each time it finds a new smallest — M.M
– M.M, Commented Jun 14, 2016 at 21:52
The title says "index", I guess from the description you actually meant "value" — M.M
– M.M, Commented Jun 14, 2016 at 21:53

Patrick Roberts · Accepted Answer · 2016-06-14 21:39:05Z

2

There's a few issues with your code, I'm surprised your compiler didn't explode on you.

void find_minimum(double a[], size_t n) {

  size_t i, index;
  double smallest;

  // sanity check to make sure we're not accessing outside allocated memory
  if (n > 0) {
    smallest = a[0];
    index = 0;
  } else {
    // nothing left to do here
    return;
  }

  // start at 1 because 0 is initial default
  for (i = 1; i < n; i++) {
    if (a[i] < smallest) {
      smallest = a[i];
      index = i;
    }
  }

  // assign only the smallest index
  a[index] = 0.0;
}

Feel free to post comments if you have any questions.

edited Jun 14, 2016 at 21:39

answered Jun 14, 2016 at 21:32

Patrick Roberts

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5 Comments

pm100 Over a year ago

you can remove second sanity check by doing an else return on the first one

Patrick Roberts Over a year ago

@pm100 good point, and a more DRY answer. I will update that, thank you. It's fixed now.

Jay Java Over a year ago

Hey Patrick. I implemented your code but it didn't change anything in the array. I appreciate your help tho. I know to start i at 1 instead of 0 now and also know about size_t

Patrick Roberts Over a year ago

@JayJava please provide code demonstrating how you call the function.

Jay Java Over a year ago

@PatrickRoberts follow up question, how could I alter this to set the 2, 3, or n lowest elements to zero? Im thinking either surrounding this code around a while loop or creating a new function and calling this one

Patrick Roberts · Accepted Answer · 2016-06-14 21:53:11Z

1

If we're assuming only one smallest like the OP and some answers, we can also keep track of just the index of the smallest, instead of the value of the smallest, to keep things simple:

void find_minimum(double a[], size_t n) {
    if (n <= 0) {
        return;
    }

    size_t smallest = 0;

    for (size_t i = 1; i < n; i++) {
        if (a[i] < a[smallest]) {
            smallest = i;
        }
    }

    a[smallest] = 0.0;
}

edited Jun 14, 2016 at 21:53

Patrick Roberts

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answered Jun 14, 2016 at 21:50

cdlane

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1 Comment

Jay Java Over a year ago

Thanks but this function didn't change any values of the array. I think I may have a complier issue because other solution didn't work

Bob__ · Accepted Answer · 2016-06-14 22:48:32Z

0

I'd like to address the case where there are more then one unique smallest value.

In such a case, two loops are needed. One to find out the smallest value over the entire array and the other to set to zero all the elements with that value.

I'll keep track of the minimum and maximum index where this value appears in the array to minimize the range of the second loop:

void set_smallest_to_zero(double a[], size_t n) {

    if ( !a || !n ) {
        return;
    }

    double smallest = a[0];
    size_t min_index = 0,               
           max_index = 0,
           i;

    // find the smallest values
    for ( i = 1; i < n; ++i ) {
        if ( a[i] > smallest ) {
            continue;
        } else if ( a[i] < smallest ) {
            smallest = a[i];
            min_index = i;
            max_index = i;
        } else {                       //  a[i] == smallest
            max_index = i;
        }
    }

    // set to zero all the the minimum values
    for ( i = min_index; i <= max_index; ++i ) {
        if ( a[i] == smallest ) {
            a[i] = 0.0;
        }
    }
}

HERE there is a live example.

edited Jun 14, 2016 at 22:48

answered Jun 14, 2016 at 22:34

Bob__

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1 Comment

Jay Java Over a year ago

Thanks. This works and appreciate you addresses other cases

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