Moving Negative Numbers to the Beginning of Array

Given an array of positive and negative numbers, write a program to move the numbers such that all negative numbers appear before positive numbers

Complexity $\Theta(n)$

No. of swaps in the worst case = <math> \lfloor n/2 \rfloor</math>

<syntaxhighlight lang="c" name="negative_positive">

1. include<stdio.h>

int main() {

       int i, n, pi, ni, count = 0;
       int a[1000];
       printf("Enter size of array: ");
       scanf("%d",&n);
       printf("Enter numbers of array\n");
       for(i=0; i<n; i++)
       {
               scanf("%d",&a[i]);
       }
       ni = n-1;
       /*Making ni point to the rightmost negative number*/
       while(a[ni] >= 0)
               ni--;
       pi = 0;
       /*Making pi point to the leftmost positive number*/
       while(a[pi] < 0)
               pi++;
       /*Looping till either negative or positive numbers exhaust*/
       while(ni > pi)
       {
               /*Swapping a[ni] and a[pi]*/
               int temp = a[pi];
               a[pi] = a[ni];
               a[ni] = temp;
               /*Moving ni leftwards to the next negative number*/
               while(a[ni] >= 0)
                       ni--;
               /*Moving pi rightwards to the next positive number*/
               while(a[pi] < 0)
                       pi++;

       }

       for(i=0; i<n; i++)
       {
               printf("%d ", a[i]);
       }

} </syntaxhighlight>

Explanation

We are scanning the array from two ends. We scan for positive numbers from left and negative numbers from right and swaps them. As soon as one of them finishes, the algorithm stops. There are <math>n</math> elements, so there canʼt be more than <math>n/2</math> positive numbers and more than <math>n/2</math> negative numbers.

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