How would I subtract from a for loop in java without it ending up with negative one?

Question

So I'm trying to print a string backward, but it doesn't start where I want it to. Whenever I run the code it says it starts at -1. Here's my code:

public static String reverse(String str)
    {
        //This method will return a string that reverses the letters in 'str'
        //Example:
        //   reverse("abcdef") returns "fedcba"
        String result="";

        for(int i = str.length()-1; i >= 0; i--){
           result = result + str.substring(i-1, i);
        }

        return result;
    }

How would I get it to subtract 1 instead of it thinking it's a negative one?

Try changing str.substring(i-1, i) to str.substring(i, i+1) — Abra
– Abra, Commented Nov 11, 2020 at 4:59

Parth Manaktala · Accepted Answer · 2020-11-11 06:57:56Z

2

Well, there is a logic issue in the code.

Correct implementation should be

//using subString the way you are trying
public static String reverse(String str)
    {
       String result="";

        for(int i = str.length()-1; i >= 0; i--){
           result = result + str.substring(i, i+1);
        }

        return result;
    }

Alternative Approaches using charAt

public static String reverse(String str)
    {
        String result="";

        for(int i = str.length()-1; i >= 0; i--){
           result +=str.charAt(i);
        }

        return result;
    }

Using String Builder(Probably the best method if you are not writing he code for learning purpose)

String reversed = new StringBuilder(str).reverse().toString();

answered Nov 11, 2020 at 6:57

Parth Manaktala

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Comments

Jeyming Wu · Accepted Answer · 2020-11-11 05:25:20Z

0

Why not splice characters one by one

for(int i = str.length() - 1; i >= 0; i--){
    result += str.charAt(i);
}

answered Nov 11, 2020 at 5:25

Jeyming Wu

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