Please help me understand this piece of code in Java

Question

'm having issues understanding what exactly this piece of code does:

1.   HashMap<Integer, Integer> totals = new HashMap<>();
2.       
3.          for (int i = -NBR_STEPS; i <= NBR_STEPS; i++) {
4.              totals.put(i, 0);
5.          }
6.          System.out.println("");
7.          for (int i = 0; i < NBR_WALKS; i++) {
8.              int total_value = 0;
9.              for (int j = 0; j < NBR_STEPS; j++) {
10.                 int L = (int) (Math.random() * 2);
11.                 total_value += (L == 0) ? -1 : 1;
12.             }
13.             totals.put(total_value, totals.get(total_value) + 1);
14.         }
15. }

What I don't understand:

What does totals.put(i,0) do?
What does total_value += (L == 0) ? -1 : 1; do exactly?
What does totals.put(total_value, totals.get(total_value)+1); do?

I'm sorry that I'm asking this question, but I simply don't understand. Thank you:)

1. and 3. Did you read the doc for hashMap ? 2. Search for ternary operator. — Alexis C., Dec 11 '13 at 18:30
`HashMap#put`, ternary operator combined with the assignment operator and another `HashMap#put`. — Jeroen Vannevel, Dec 11 '13 at 18:31
I tried to read what hashmap does, but I'm not sure. It stores something that you can receive later on right? — Paludan, Dec 11 '13 at 18:31
Ok thank you @ZouZou and Jeroen Vannevel, I see the what to total_value does now. But I must admit the += operator confuses me^^ — Paludan, Dec 11 '13 at 18:33
@user2966573 `myVal += 1` is a shorthand to write `myVal = myVal + 1;` — Alexis C., Dec 11 '13 at 18:34
@ZouZou alright but how exactly does that work? What if myVal hasn't been defined ealier? It needs a value, like `int myVal = 0;` right? — Paludan, Dec 11 '13 at 18:37
the += operator adds -1 to itself if L == -1 else it adds 1. The expressione L==0 ? -1 : 1 is a special conditional operator in Java as as far as i know in c/c++ to — Diversity, Dec 11 '13 at 18:38
I'd really like to know where this piece of code is taken from — Kai Huppmann, Dec 11 '13 at 18:49

score 3 · Accepted Answer · edited May 23 '17 at 10:25

What does totals.put(i,0) do?

Stores the value 0 at key i in the hashmap totals.

What does total_value += (L == 0) ? -1 : 1; do exactly?

total_value += 1; means the same as total_value = total_value + 1

? is the ternary operator. The value of this statement (L == 0) ? -1 : 1; is -1 when (L == 0) is true and 1 when (L == 0) is false

total_value += (L == 0) ? -1 : 1; is the same as:

if (L == 0)
    total_value = total_value - 1; // subtract 1 from total_value 
else
    total_value = total_value + 1; // add 1 to total_value

What does totals.put(total_value, totals.get(total_value)+1); do?

Copies the value from hashmap location total_value+1 into hashmap location total_value. It essentially copies a value from the next location to the current location.

PKeidel · Answer 2 · 2013-12-11T19:04:47.450

1

EDIT: A Hashmap saves key-value pairs. So the first argument is the key, the second is the value.

a += 1; means the same as a = a + 1

a = b == 1 ? 1 : 2; means the same like:

if(b == 1)
  a = 1;
else
  a = 2;

edited Dec 11 '13 at 19:04

answered Dec 11 '13 at 18:34

PKeidel

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The first argument for put is the key, the second argument is the value. – dansalmo Dec 11 '13 at 18:37
Alright super thank you:) I think I understand my question now with the comments and this answer. Unfortunately someone thumbed me down:( – Paludan Dec 11 '13 at 18:39
1

"The second argument for put is the optional index where the element should be placed in the hashmap" ... couldn't be more wrong! There is no order of elements in a Hashmap, hence no index. As dansalmo said: put(key, value) – Kai Huppmann Dec 11 '13 at 18:42

score 1 · Answer 3 · answered Dec 11 '13 at 18:38

What does totals.put(i,0) do? It adds a key, value pair to the map. The variable 'i' is the key and '0' is the value associated with the key.

What does total_value += (L == 0) ? -1 : 1; do exactly? That is an If-else statement. The simplified version is as such.

if(L==0)
total_value += -1
else{
total_value += 1
}

What does totals.put(total_value, totals.get(total_value)+1); do?

It adds a key value pair to the hashmap. total_value is the key and totals.get(total_value)+1 is the value associated for the key.

Hope that helps.

score 1 · Answer 4 · answered Dec 11 '13 at 18:38

About the question n.2
A traditional if-else construct in C, Java and JavaScript is written:

if (a > b) {
    result = x;
} else {
    result = y;
}

This can be rewritten as the following ternary statement:

result = a > b ? x : y;

in your case the condition is (L==0) if this is true:

total_value=total_value+-1

and so...

total_value=total_value-1

if the condition is not true:

total_value=total_value+1

Please help me understand this piece of code in Java

4 Answers4