C: Count positive/negative numbers and 0's in array

Question

I'm trying to write a program in C to count the number of positive and negative numbers in an array as well as the number of zeroes.

I have written the following code:

int A[15], pos, neg, nul, i;

[...]

pos = 0;
neg = 0;
nul = 0;

for (i = 0; i <= 15; i++) {
    if (A[i] > 0) {
        pos++;
    }
    if (A[i] < 0) {
        neg++;
    }
    if (A[i] == 0) {
        nul++;
    }
}

However, the counts are always wrong. Specifically, the count for pos and nul is always off by 1. An array of 15 positive numbers and 1 zero gives pos = 16 and neg = 0, while an array of 15 zeroes and 1 positive number gives pos = 0 and nul = 16.

What's going on here and what to do about it?

How did you put 16 items into an array of 15? ("An array of 15 positive numbers and 1") — Vlad Feinstein, Mar 24 '20 at 20:57
You are reading past your buffer. You only have space for 15 elements, but you are continuing to the 16th element — Christian Gibbons, Mar 24 '20 at 20:57
The standard `for` loop used with an array of size `n` is `for (i=0; i — user3386109, Mar 24 '20 at 21:02

Ardent Coder · Answer 1 · 2020-03-24T21:10:05.517

Problem:

Your array size is 15 but you traverse it 16 times due to:

for (i=0; i<=15; i++)

Values of i: {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}:

A total of 16 indices, where A[15] is undefined behaviour.

Solution:

Change that loop header to:

for (i = 0; i < 15; i++)

This will ensure that the loop runs 15 times with inbound array indices.

Bonus:

An array of n elements is generally traversed as:

for (i = 0; i < n; ++i)
{
    // process a[i] here
}

score 3 · Answer 2 · answered Mar 24 '20 at 21:03

3

the problem is here for (i=0; i<=15; i++) .You are passing boundaries of your array.

it should be for (i=0; i<15; i++)

answered Mar 24 '20 at 21:03

hanie

1,863
3
9
19

score 1 · Answer 3 · edited Apr 05 '20 at 19:29

1

(i=0; i<=15; i++) will pass your arrays boundaries.

Your array size is 15 but since you are starting i from 0 your loop will run 16 times.

The solution is to write it as:

(i=0; i<15; i++)

edited Apr 05 '20 at 19:29

Martijn Pieters

1,048,767
296
4,058
3,343

answered Mar 25 '20 at 09:19

qqqqqkks

62
16

C: Count positive/negative numbers and 0's in array

3 Answers3