C Exercises: Check two lines are parallel or not

C Basic Declarations and Expressions: Exercise-138 with Solution

Write a C program to test whether two lines are parallel or not. The four points are P(x1, y1), Q(x2, y2), R(x3, y3) and S(x4, y4), check PQ and RS are parallel are not.

Input:
−100 <= x1, y1, x2, y2, x3, y3, x4, y4 <= 100
Each value is a real number with at most 5 digits after the decimal point.

Sample Solution:

C Code:

#include <stdio.h>
main() {
  double x1, x2, y1, y2, x3, y3, x4, y4;
  int i, n;

  printf("Input P(x1,y1):\n");
  scanf("%lf %lf", &x1, &y1);
  printf("\nInput P(x2,y2):\n");
  scanf("%lf %lf", &x2, &y2);
  printf("\nInput P(x3,y3):\n");
  scanf("%lf %lf", &x3, &y3);
  printf("\nInput P(x4,y4):\n");
  scanf("%lf %lf", &x4, &y4);

  if ((x1 == x2) && (x3 == x4))
    printf("\nPQ and RS are parallel!\n");
  else if ((x1 == x2) || (x3 == x4))
    printf("\nPQ and RS are not parallel!\n");
  else if (((y1 - y2) / (x1 - x2) - (y3 - y4) / (x3 - x4)) == 0.0)
    printf("\nPQ and RS are parallel!\n");
  else
    printf("\nPQ and RS are not parallel!\n");
  return (0);
}

Sample Output:

Input P(x1,y1):
5
7

Input P(x2,y2):
3
6

Input P(x3,y3):
8
9

Input P(x4,y4):
5
6

PQ and RS are not parallel!

Flowchart:

C programming Code Editor:

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