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C Exercises: Find the first missing positive integer from a given unsorted integer array

C Programming Practice: Exercise-28 with Solution

Write a C programming to find the first missing positive integer from a given unsorted integer array.

C Code:

//https://github.com/begeekmyfriend/leetcode/blob/master/041_first_missing_positive/missing_positive.c
#include <stdio.h>
#include <stdlib.h>
static inline void swap_val(int *x, int *y)
{
    int tmp = *x;
    *x = *y;
    *y = tmp;
}

static int first_Missing_Positive_num(int* nums, int nums_size)
{
    if (nums_size < 1) {
        return 1;
    }

    int i = 0;
    while (i < nums_size) {
        if (nums[i] != i + 1 && nums[i] > 0 && nums[i] <= nums_size && nums[i] != nums[nums[i] - 1]) {
            swap_val(nums + i, nums + nums[i] - 1);
        } else {
            i++;
        }
    }

    for (i = 0; i < nums_size; i++) {
        if (nums[i] != i + 1) break;
    }
    return i + 1;
}

int main()
{
    int i, count = 4;
    int nums[4] = {3,4,-1,1};
      
    printf("%d\n", first_Missing_Positive_num(nums, count));
    return 0;
}

Sample Output:

2

Flowchart:

C Programming Flowchart: Find the first missing positive integer from a given unsorted integer array.

C Programming Code Editor:

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C Programming: Tips of the Day

C Programming - Why do all the C files written by my lecturer start with a single # on the first line?

In the very early days of pre-standardised C, if you wanted to invoke the preprocessor, then you had to write a # as the first thing in the first line of a source file. Writing only a # at the top of the file affords flexibility in the placement of the other preprocessor directives.

From an original C draft by the great Dennis Ritchie himself:

12. Compiler control lines

[...] In order to cause [the] preprocessor to be invoked, it is necessary that the very first line of the program begin with #. Since null lines are ignored by the preprocessor, this line need contain no other information.

That document makes for great reading (and allowed me to jump on this question like a mad cat).

I suspect it's the lecturer simply being sentimental - it hasn't been required certainly since ANSI C.

Ref : https://bit.ly/2Mb8OVZ