C# Different probabilities in if statements

Question

I am writing a program where I put different students in different classrooms. I'm having some trouble figuring out how to use probabilities in C#, and programming in general. I want it to be a 5% probability to become a student in Alpha, 10& for Omega & 60% for Nightwalkers. I don't understand what numbers to put in.My method right now:

 public string AssignClassroom() { int rand = random.Next(0, 100); { if (rand < 5) // or >95%? { student.Classroom = "Alpha"; } if (rand < 10) // or >90? { student.Classroom = "Omega"; } if (rand < 60) // or >40? { student.Classroom = "Nightwalkers"; } } }

@RoyiNamir Fixed it. So the rest looks right? I can do it this way? What I'm thinking is, say rand = 96. Then the student won't be assigned to any classroom.. — evesare
– evesare, Commented Dec 1, 2021 at 15:02
So, your desired probabilities add up to 75%. What's meant to happen to the other 25% of students? That's a maths problem, not a programming one. — Damien_The_Unbeliever
– Damien_The_Unbeliever, Commented Dec 1, 2021 at 15:08
In a real world, you should probably split on the count of students. Your algorithm (even if it's fixed) will put students with a certain probability into certain rooms. Since there's no guarantee that the random numbers will be evently distributed, you could still end up squeezing all into room Alpha. — PMF
– PMF, Commented Dec 1, 2021 at 15:12

Dmitrii Bychenko · Accepted Answer · 2021-12-01 15:09:20Z

You should add up figures in ifs:

 if (rand < 5) { student.Classroom = "Alpha"; } else if (rand < 10 + 5) { student.Classroom = "Omega"; } else if (rand < 60 + 10 + 5) { student.Classroom = "Nightwalkers"; }

Note, that 5, 10, 60 are differencies:

 0 .. 5 .. 15 .. 75 | 5 | | | -> 5% top students go to the 1st class | 10 | | -> 10% next go to the 2nd | 60 | -> 60% goes to the 3d

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