Bouncing-ball simulation

Question

I posted a couple questions about a week ago, but have almost gotten it working. This program is supposed to simulate the following:

1. Ask the user how many balls to drop
2. Drop one ball at a time
3. Allow it to bounce ten times moving either once to the right or once to the left each bounce
4. Then print out the even numbers (since a ball could never land on an odd) -10 - 10 with a 'o' to represent each ball.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
/* Prototype for drop_Balls, int parameter is number of balls being dropped */
void drop_balls(int); 
/* Gets the number of balls that need to be dropped by the user */
int get_num_balls(); 
int main()
{
 drop_balls(get_num_balls());
}
int get_num_balls() 
{
 int num_balls;
 printf("How many balls should be dropped? ");
 scanf("%d", &num_balls);
 /* Ensure that it is atleast one ball */
 while(num_balls <= 0) 
 {
 printf("You have to drop at least one ball! \n ");
 printf("How many balls should be dropped? ");
 scanf("%d", &num_balls);
 }
 /* Return the number of balls that will be dropped */
 return num_balls; 
}
void drop_balls(int num_balls)
{
 /* Keeps track of how many balls landed where */
 int ball_count[21]; 
 /* What number ball are we on */
 int ball_num = 0; 
 /* Seed the generator */
 srand(time(NULL)); 
 /* Do the correct # of balls */
 for(ball_num = 0; ball_num < num_balls; ball_num++ ) 
 {
 /* Start at 10 since its the middle of 21 */
 int starting_point = 10; 
 /* Bounce each ball 10 times */
 for(starting_point = 10; starting_point > 0; starting_point--) 
 {
 int number;
 /* Create a random integer between 1-100 */
 number = rand() % 100; 
 /* If its less than 50 bounce to the right once */
 if(number >= 50) 
 {
 starting_point++;
 }
 /* If its greater than 50, bounce to the left once */
 else 
 {
 starting_point--;
 }
 }
 /* Add one to simulate one ball landing there */
 ball_count[starting_point]++; 
 }
 /* Start at the -10 spot */
 int ball_place = -10; 
 int x;
 /* Go through the 21 spots, but skip the odd ones, since they never get balls */
 for(x = 0; x < 20; x+2)
 {
 printf("\n %d: ", ball_place);
 int l = 0;
 /* Print out an 'o' for each ball */
 for(l = 0; l < ball_count[x]; l++)
 {
 printf("o");
 }
 /* Increase the spot, taking it from -10 - 10 */
 ball_place = ball_place + 2;
 }
}

• svick: So, what's the question? Is your code working the way you want it to? If not, then your question is off topic here.

Answer 1

Comments

You should use // comments for single-line comments.

I am not sure you are using comments properly. Comments shouldn't explain how, it should explain why.

/* Prototype for drop_Balls, int parameter is number of balls being dropped */
void drop_balls(int); 

One can see that this is a prototype. void drop_balls(int number_of_balls); would probably provide as much information in a more compact way.

/* Return the number of balls that will be dropped */
return num_balls; 

This does not look like a useful comment to me.

/* Keeps track of how many balls landed where */
int ball_count[21]; 

A bit of explanation on the 21 here would be useful... but it is not provided.

This leads me to the next section:

Magic numbers

You probably don't want to have hard-coded numbers all over the place. You could create a constant with value 21 and then:

• starting_point = 10 would become starting_point = N /2.

• for(x = 0; x < 20; x+2) would become for(x = 0; x < N; x+2).

• int ball_place = -10; would become int ball_place = - N/2;

Code organisation

It would probably be better now to call srand in rand_balls as it shouldn't be its responsibility. You can do it from main() if you want.

Define variable in the smallest possible scope. For instance:

for(int ball_num = 0; ball_num < num_balls; ball_num++ )

Also, how is this loop:

for(starting_point = 10; starting_point > 0; starting_point--)

supposed to behave? Don't we always have the variable set to 0 or -1 at the end?

• Steve: I agree with what you said about the commenting, I will also go through and change them to make more sense once I get it working. The constants I know make it look sloppy, I was going to go through and change that once I got everything working. That loop moves it once to the left or right every pass through, so I think its doing what its supposed to.
• Gareth Rees: Can you explain your advice, "You should use // comments for single-line comments."? What if the OP needs the code to compile with the -ansi flag?
• Jamal: Shouldn't srand() always be called in main()? It doesn't look like it should be an option.

Answer 2

In addition to @Josay's answer:

This code contains some duplication which you can avoid:

printf("How many balls should be dropped? ");
scanf("%d", &num_balls);
/* Ensure that it is atleast one ball */
while(num_balls <= 0) 
{
 printf("You have to drop at least one ball! \n ");
 printf("How many balls should be dropped? ");
 scanf("%d", &num_balls);
}

Either refactor for the number of balls into it's own method:

int ask_for_number_of_balls()
{
 printf("How many balls should be dropped? ");
 scanf("%d", &num_balls);
}

And then your loop becomes:

int num_balls = ask_for_number_of_balls();
while (num_balls <= 0)
{
 printf("You have to drop at least one ball! \n ");
 num_balls = ask_for_number_of_balls();
}

Or you get a bit clever:

int num_balls = -1;
while (1)
{
 printf("How many balls should be dropped? ");
 scanf("%d", &num_balls);
 if (num_balls > 0) { break; }
 printf("You have to drop at least one ball! \n ");
}

I would probably prefer the refactored method over clever (clever is harder to read and maintain).

I'm not sure about this loop:

for(starting_point = 10; starting_point > 0; starting_point--)

You randomly add one or subtract one from starting_point, but the running condition for that loop is starting_point > 0. So, as long as the starting point is greater than 0, this loop will run. This means that at the end, starting_point will always be 0 afterwards (not to mention that your loop could run for a long time if your random numbers happen to be greater than 50 most of the time).