需要帮助来理解本课程的各个部分

Question

I need someone to elaborate on certain parts of this code.

class ContinueWithLabelDemo {
public static void main(String[] args) {

    String searchMe = "Look for a substring in me";
    String substring = "sub";
    boolean foundIt = false;

    int max = searchMe.length() - 
              substring.length();

test:
    for (int i = 0; i <= max; i++) {
        int n = substring.length();
        int j = i;
        int k = 0;
        while (n-- != 0) {
            if (searchMe.charAt(j++) != substring.charAt(k++)) {
                continue test;
            }
        }
        foundIt = true;
            break test;
    }
    System.out.println(foundIt ? "Found it" : "Didn't find it");
}
}

more specifically, I don't understand this part:

int n = substring.length();
        int j = i;
        int k = 0;
        while (n-- != 0) {
            if (searchMe.charAt(j++) != substring.charAt(k++)) {
                continue test;
            }
        }

Why is it necessary to declare j and k in this code at all? I know that there is a reason in it for the if statement

if (searchMe.charAt(j++) != substring.charAt(k++))

but I don't understand what the code is actually doing at this part.

Also, what does

while (n-- != 0)

mean?

Answer 1

while (n-- != 0)

This is just looping around, reducing n by 1 each time around the loop and ending when n (before) reducing it by 1 is not 0.

int n = substring.length();
int j = i;
int k = 0;
while (n-- != 0) {
    if (searchMe.charAt(j++) != substring.charAt(k++)) {
        continue test;
    }
}

This code is starting with j and k at different positions in the string, and then it is looping through comparing the character in the String at that position. j++ just says "use the current value of j, and then afterwards add 1 to it".

Answer 2

Starting with an n >= 0 (string length can not be negative):

while (n-- != 0) {
    ... 
}

is just a substitute for

for (int t = n ; t > 0; t--) {
    .... // use t instead of n, no change in your example
}

it iterates just n times (length of substring).

Answer 3

++ and -- are pre or post incrementor and decrementor in java.

Imagine the following code to understand the behaviour:

int a = 42
System.out.println(a++) // prints 42
System.out.println(a) // prints 43
System.out.println(++a) // prints 44
System.out.println(a) // prints 44

In fact, it adds or subtracts 1 before or after the statement is processed.

So in your situations, while (n-- != 0) means, that the condition is checked, wether n is not zero and after that decremented by 1.

To achieve the same, you could also write:

while (n != 0) {
    if (searchMe.charAt(j++) != substring.charAt(k++)) {
        continue test;
    }
    n = n - 1 // or n-- or n -= 1
}

Your second condition if (searchMe.charAt(j++) != substring.charAt(k++)) compares the character at the index j in searchMe against the character with the index k from substring and after that increments both indices to avoid two more lines, where these two are incremented.

Answer 4

This can be broken down in to the following:

outer: loop(/* for each character in searchMe to max */) {

    loop(/* for each character in substring
          * starting with current index in outer loop */) {

        if(/* this substring does not match */)
           continue /* with next outer loop iteration */;
    }

    /* if inner loop completed
     * the substring did match at this index so break */;
 }

Out of all the variables, n is actually the one that's not needed. I don't know why it's there except to try to confound. The while loop could just as easily read while(k < substring.length()) .

This kind of loop is necessary because to search for a substring you have to search starting at every character:

Loo
ook
ok 
k f
 fo
for
or 
r a
...
a s
 su
sub <- found it

Another way to express the same logic is the following but this creates a new object on each iteration:

int max = searchMe.length() - substring.length();

for(int i = 0; i <= max; i++) {
    String toTest = searchMe.substring(i, i + substring.length());

    if(toTest.equals(substring)) {
        foundIt = true;
        break;
    }
}

Answer 5

Well, this is some interesting code. To start off, the label on your break is unnecessary.

Next, to your main questions:

• n-- is a postfix decrement operator - the current value of n is evaluated, then it is decremented by 1. When n is evaluated next, it will have the value of n-1 .

  In the context of the code while(n-- != 0) implies that, when n == 1 , this loop will still execute, but the next time we see n , we will be viewing 0 instead.

• The statement:

   if (searchMe.charAt(j++) != substring.charAt(k++)) { continue test; } 

  ...indicates that, if the value in the position of the main search string doesn't match up with the value we're looking for, we need to immediately jump to the label test . This allows you to skip the execution of the following two statements after it, and continue looping and looking for positive matches.

  j is constantly set to what i is, but it is not always equal to i . If a partial positive match is found, then j will increment faster than i by virtue of the while loop around that statement.

Answer 6

 class ContinueWithLabelDemo { public static void main(String[] args) { String searchMe = "Look for a substring in me"; String substring = "sub"; boolean foundIt = false; int max = searchMe.length() - substring.length(); //the length of what we're going to search through, we //subtract the substring.Length because if the 3rd to //last character of searchMe is not equal to the first //character of substring then searchMe can not contain substring. test: for (int i = 0; i <= max; i++) { //repeat until we pass the 3rd to last character of searchMe int n = substring.length(); //resets n to the length of the substring int j = i; //resets j to equal i so we search for the next letter in searchMe int k = 0; //resets k to equal 0 so we search for the first letter in substring while (n-- != 0) { //repeat this loop until n = 0, each pass will subtract 1 from //n. this will loop a maximum of 3 times (the number of characters in substring) if (searchMe.charAt(j++) != substring.charAt(k++)) { //the if statement above will add 1 to j and add 1 to k after it checks to make sure the //characters are not equal to each other. if they are equal then it will take the new //j and k variables and repeat the while loop. continue test; //if statement true then loop through the 'test' again. } } foundIt = true; break test; } System.out.println(foundIt ? "Found it" : "Didn't find it"); } } 

Answer 7

Oracle doesn't know how to write simple things in a tutorial for beginners .

This part:

int n = substring.length();
int j = i;
int k = 0;
while (n-- != 0) {
    if (searchMe.charAt(j++) != substring.charAt(k++)) {
        continue test;
    }
}

can be changed to:

int n = substring.length();
for (int k = 0; k < n; k++) {
    if (searchMe.charAt(i + k) != substring.charAt(k)) {
        continue test;
    }
}

So, you don't need j and instead of that tricky while you can use a simple for which is more intuitive, because it iterates through the substring .