Question

can someone explain to me what this code does:
public class D {
static double dp[][]=new double[51][51];static {
dp[1][1]=1;
for (int i=2;i<=50;i++)
for(int j=1;j<=50;j++)
dp[i][j]=dp[i-1][j-1]+dp[i-1][j]*j;
}
public static void main(String args[]){
System.out.println("enterhow many lines:s");
Scanner sc=
new Scanner(System.in);
while (sc.hasNext()) {
int n=sc.nextInt();
if(n==0) break;
double ans=0;
for(int i=1;i<=n;i++)
ans+=dp[n][i];
System.out.printf("%d %.0f\n",n,ans);
}
}}

Answer 1

alrighty.... Where did you see this? Crazy haha, lets begin.

Answer 2

static double dp[][]=new double[51][51];static {
dp[1][1]=1;
for (int i=2;i<=50;i++)
for(int j=1;j<=50;j++)
dp[i][j]=dp[i-1][j-1]+dp[i-1][j]*j;
}

not too sure why static, but it's basically creating 2 dimensional array starting at 1 not 0 for some reason. Then it's basically storing dp[i][j]=dp[i-1][j-1]+dp[i-1][j]*j; so for the first one it goes dp[2][1] = dp[1][0] + dp[0][1] * 1;

Answer 3

now the main is asking for you to enter a number n.

if(n==0) break;
double ans=0;
for(int i=1;i<=n;i++)
ans+=dp[n][i];
System.out.printf("%d %.0f\n",n,ans);
}

n cannot be 0, it starts at 1, then it runs through and saying ans (which is 0 at first) is = to ans + dp[n][i] which at first would be lets say n = 5, then ans = ans + dp[5][1] + dp[5][2] etc. Then prints it out.

Answer 4

so if it starts at dp [1][1]. what is stored in the zero? (or does that not matter since we dont actually use it.
in addition: if i put in 5. then it does dp[5[ 2] but what is [5][2]/ where did we initialze any values?

Answer 5

you can start anywhere, which is why n cannot = 0.

dp[1][1]=1; is the initialization.

Answer 6

however it shows a 0 atsome point in that loop, which it's most likely going to be 0, or null.

Answer 7

so when you put in a number, what does it do? what does the dp[][]actually do?

Answer 8

dp[i][j]=dp[i-1][j-1]+dp[i-1][j]*j;

Answer 9

dp[2][1] = dp[1][0] + dp[0][1] * 1;

Answer 10

dp[2][1] = 0 dp[1][1] = 1 * 1 = 1.

then dp[2][2] = = dp[2][1] + dp[1][2] * 2 =.....

Answer 11

it seems like it's printing out the line numbers.