C Code returning random characters at end.

Question

After I print out the final output it returns a random string of characters.
The code takes in an input that is replaced with a flipped alphabet version of the text.
https://code.sololearn.com/cXgYh7bpYRN6/#c

Answer 1

You can add
output[i] = '\0';
After your loop to correct the issue.

It's printing out the garbage values left behind in those memory locations, since it doesn't see the termination of the C string.

Answer 2

You forgot to terminate the string, i.e., forgot to add '\0' at the end of the string. So printf() has no idea until when to stop printing the string.