返回字符串“hi”出现在给定字符串中的任何位置的次数

Question

I wrote the following Java code, and it returned a time out error. I'm not exactly sure what that means nor why the code doesn't run

public int countHi(String str) {
  int pos = str.indexOf("hi"); 
  int count = 0;
  while(pos!=-1)
  {
    count++;
    pos = str.substring(pos).indexOf("hi");
  }
  return count;
}

I know an alternative solution, using a for loop but I really thought this would work too.

Answer 1

You're getting into an infinite loop because pos never advances past the first match as the first match will be included in the substring.

You can fix by using this overridden version of indexOf() inside your while loop:

pos = str.indexOf("hi", pos + 1);

Or use a do... while loop to avoid having to repeat the call to indexOf() :

public static int countHi(String str) {
    int pos = -1, count = -1;

    do {
        count++;
        pos = str.indexOf("hi", pos + 1);
    } while (pos != -1);

    return count;
}

Answer 2

str.substring(pos) Output the substring from the given index. therefore in your code while loop never travel through your whole string and its stop at the first "hi".Use this.

while(pos!=-1){
   count++;
   str = str.substring(pos+2);
   pos = str.indexOf("hi");
}

str variable store 2nd half of the string (use +2 for travel two more indexes for end of the hi) then check pos variable store that index of "hi" appear in the new string.

Answer 3

Just for added fun......

If the supplied substring (ie: "hi") is to be counted and it doesn't matter where it is located within the input string (single word or part of a word), you can use a one liner and let the String.replace() method do the job for you by actually removing the desired substring you want to count from the initial input string and calculating what remains of that input string (this does not modify the initial input string):

String inputString = "Hi there. This is a hit in his pocket";
String subString = "hi";
int count = (inputString.length() - inputString.replace(subString, "").
                length()) / subString.length())

//Display the result...
System.out.println(count);

Console will display: 3

You will note that the above code is letter case sensitive and therefore in the example above the substring "hi" differs from the word "Hi" because of the uppercase "H" so "Hi" is ignored. If you want to ignore letter case when counting for the supplied substrings then you can use the same code but utilize the String.toLowerCase() method within it:

String inputString = "Hi there. This is a hit in his pocket";
String subString = "hi";
int count = (inputString.length() - inputString.toLowerCase().
             replace(substring.toLowerCase(), "").
             length()) / substring.length())

//Display the result...
System.out.println(count);

Console will display: 4

If however the supplied substring you want to count is a specific word (not part of another word) then it gets a little more complicated. One way you can do this is by utilizing the Pattern and Matcher Classes along with a small Regular Expression . It could look something like this:

String inputString = "Hi there. This is a hit in his pocket";
String subString =   "Hi";
String regEx = "\\b" + subString + "\\b";

int count = 0;  // To hold the word count
// Compile the regular expression
Pattern p = Pattern.compile(regEx);
// See if there are matches of subString within the 
// input string utilizing the compiled pattern
Matcher m = p.matcher(inputString);
// Count the matches found
while (m.find()) {
    count++;
}

//Display the count result...
System.out.println(count);

Console will display: 1

Again, the above code is letter case sensitive. In other words if the supplied substring was "hi" then the display in console would of been 0 since "hi" is different from "Hi" which is in fact contained within the input string as the first word. If you want to ignore letter case then it would just be a matter of converting both the input string and the supplied substring to either all upper case or all lowercase, for example:

String inputString = "Hi there. This is a hit in his pocket";
String subString =   "this is";
String regEx = "\\b" + subString.toLowerCase() + "\\b";

int count = 0;  // To hold the word count
// Compile the regular expression
Pattern p = Pattern.compile(regEx);
// See if there are matches of subString within the 
// input string utilizing the compiled pattern
Matcher m = p.matcher(inputString.toLowerCase());
// Count the matches found
while (m.find()) {
    count++;
}

//Display the count result...
System.out.println(count);

Console will display: 1

As you can see in the two most recent code examples above the Regular Expression (RegEx) of "\\\\bHi\\\\b" was used (in code, a variable was used in the place of Hi ) and here is what it means: