Question

I am trying to solve this String manipulation problem, where I need to find the smallest period of a given string.

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A string is said to have period k if it can be formed by concatenating one or more repetitions of another string of length k.

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For example, the string "abcabcabcabc" has period 3, since it is formed by 4 repetitions of the string "abc". It also has periods 6 (two repetitions of "abcabc") and 12 (one repetition of "abcabcabcabc"). Here’s my code :

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public static int getPeriod(String str){
    int len=str.length();   
    boolean flag=false;
    int i;
    
    for (i=0;i<len;i++){
        String s=str.substring(0,i);
        String tmp=str;

        while(tmp.length()>0){
            if(tmp.startsWith(s)){
                tmp=tmp.substring(0,i);
                flag=true;
            }
            
            else {
                flag=false;
                continue;
            }
        }
        
        if (flag==true)
            break;
    }
    
    return i;
 }

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I am forming a string s by looping through the original string, one character at a time. After, that I am checking if the original string can be completely exhausted by concatenating the string s any number of times, or not.

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ERROR:

The method always returns 0.

Why is that so ?

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EDIT : My algorithm

Lets consider the input string HoHoHo

First step: s=H
        tmp= HoHoHo
        tmp= oHoHo (after substringing tmp)
        'o' isn't the same as s, so we increase i



   Second step:s=Ho
            tmp= HoHoHo
            tmp= HoHo (after substringing tmp)
            tmp= Ho (after substringing tmp)
            tmp= "" (after substringing tmp)
       
Return the value of i, that is 2.

Answer 1

The code inside the while loop isn’t correct, it’s called during the first invocation of the for loop with i=0 and hence the first assignment to the tmp variable sets it to the empty string, the loop exits and you get 0. The flag assignments and the continue in the else are not correct too.
Try this:

public static int getPeriod(String str) {
    int len = str.length();
    int i;

    for (i = 1; i <= len/2; i++) {
        String period = str.substring(0, i);
        String tmp = str;
        boolean flag = true;

        while (flag && tmp.length() > 0) {
            if (tmp.startsWith(period)) {
                tmp = tmp.substring(i);
            } else {
                flag = false;
            }
        }

        if (flag == true) {
            return i;
        }
    }
    return 0;
}

Notice that the for loop starts from 1 and goes to len/2 because you don’t want to check for the zero length period and there can’t be periods longer than n/2.