How do I manipulate strings using a for loop?

While attempting the problem 1768. Merge Strings Alternately on leetcode, I ran in to an issue with my for loop.

The goal is to merge two strings (word1 and word2) by alternating the letters of both strings (example: word1 = "abc", word2 = "def", the solution should then be "adbecf"). If the strings are of different lengths the end of the longer string should be appended to the merged string (example: word1 = "abc", word2 = "defgh", the solution to this should be "adbecfgh").

My code ran into no issues when it came to appending the end of longer strings. What it won’t do is merge the beginning of the two strings alternately.

I tried using a for loop to append the letters one by one:

/*declare string*/
string merged = "";

/*add letters to string in alternating order*/
for (int i = 0; i <= word1.Length ^ i <= word2.Length; i ++)
{
    merged.Append(word1[i]);
    merged.Append(word2[i]);
}

I also tried adding the letters "arithmetically":

/*declare string*/
string merged = "";

/*add letters to string in alternating order*/
for (int i = 0; i <= word1.Length ^ i <= word2.Length; i ++)
{
    merged += word1[i];
    merged += word2[i];
}

Whenever I tried any of these methods merged returned as an empty string (merged = ""). Can anyone explain to me what I have done wrong?

>Solution :

Your actual problem has nothing to do with string manipulation and is caused by your for condition: ^ is the exclusive or operator, so the condition is initially false, because both <= checks will always evaluate true at first (since 0 is less than or equal the length of any string), and true ^ true is false. The result is execution never enters the loop, hence why merged is still an empty string at the end. As a tip, you would have realised this very quickly if you had stepped in your code line by line with a debugger.

You should use && instead. Also, you want to compare with <, not <=, because strings in C# are just like arrays, they end one index before their length:

for (int i = 0; i < word1.Length && i < word2.Length; i++)

Now that this is fixed, let’s talk string manipulation.

In your first case, you’re actually using the LINQ method Append, which is available because a string is a sequence of chars. This doesn’t actually append anything, but rather returns an enumerable sequence that, when enumerated, will give the items of both one after the other.

In your second case, you’re using string‘s operator +=, which, since C# strings are immutable, creates a new copy every time. It works, but it’s inefficient. So, instead, you should use StringBuilder, which is a mutable string. You can do various operations on it, and when you’re done editing your string, use ToString() to get a final immutable copy as a string:

var builder = new StringBuilder();

for (int i = 0; i < word1.Length && i < word2.Length; i++)
{
    builder.Append(word1[i]);
    builder.Append(word2[i]);
}

var merged = builder.ToString();

It’s much faster.