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string multiple times

Posted on 2014-10-10
12
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Last Modified: 2014-10-12
Hi,

I am trying below challenge
http://codingbat.com/prob/p142270


I wrote code as below

public String stringTimes(String str, int n) {
  if(n>0){
 
  String str1=n*str;
  return str1;
  }
}


How to fix my code. Please advise
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Question by:gudii9
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12 Comments
 
LVL 86

Expert Comment

by:CEHJ
ID: 40374011
String str1=n*str;

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Makes no sense. What do you mean by it?
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LVL 86

Accepted Solution

by:
CEHJ earned 300 total points
ID: 40374013
Try

public String stringTimes(String str, int n) {
   StringBuilder sb = new StringBuilder(str.length()*n);
   for(int i = 0;i<n;i++) {
      sb.append(str);
   }
   return sb.toString();
}

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As i've mentioned to you many times before - avoid string concatenation
0
 
LVL 21

Expert Comment

by:Amitkumar Panchal
ID: 40374430
Agree with CEHJ. Provided one is the simplest way solve the challenge.
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LVL 84

Expert Comment

by:ozo
ID: 40374472
avoid string concatenation
Why?
What do you gain by doing StringBuilder concatenation instead?
0
 
LVL 27

Assisted Solution

by:dpearson
dpearson earned 200 total points
ID: 40374476
I think CEHJ is pointing out using StringBuilder would be better than concatenation like this:

public String stringTimes(String str, int n) {
   String result = "" ;
   for(int i = 0;i<n;i++) {
       result += str ;
   }
   return result ;
}

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which is inefficient because "result += str" requires allocating a a new sting and copying over the old string each time (unless the compiler is kind enough to rewrite this for you).

Doug
0
 
LVL 84

Expert Comment

by:ozo
ID: 40374503
Is efficiency the point of this exercise?  
If so, how much efficiency gain is there?
If the compiler reallocates in powers of 2, both methods would be linear in n.
What kinds of n values would we be using this on, and how much extra time are we talking about for the different methods?
How important is it to optimize that time in this application?
If you are that concerned about efficiency, should you even be writing this in Java?
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LVL 86

Expert Comment

by:CEHJ
ID: 40374537
Is efficiency the point of this exercise?  
The point of the exercise (one hopes) is to get better at programming. Getting better about programming is about doing things in the right, efficient way, not just about doing any old thing that works. If you don't understand why string concatenation can be inefficient, then inspect the bytecode
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LVL 86

Expert Comment

by:CEHJ
ID: 40374540
If you are that concerned about efficiency, should you even be writing this in Java?
It's exactly that kind of attitude that makes programmers who can't be bothered with efficiency think "I'm writing this in Java so i don't need to bother about memory or anything really, right?" Wrong.

Result? People saying "Java's just bloated and slow"
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LVL 84

Expert Comment

by:ozo
ID: 40374686
I would not want to suggest that efficiency concern are unimportant.
I could want to suggest that efficiency concerns are balanced among many other programming concerns
and that one should be cognizant of what concerns are driving particular decisions.
A blanket statement to avoid string concatenation, without without building an understanding
of what trade offs one is making, and why and when those should be of concern,
does not seem to be an effective way to get better at programming, especially when one is just staring to absorb the elements of programming.
Judging from the kinds of questions that have been asked so far, to expect the asker of this question to be able to understand why string concatenation can be inefficient by inspecting the bytecode, at this stage in their learning, seems unrealistic.
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Author Comment

by:gudii9
ID: 40375096
I would like to know how to see bytecode. I will start a new question on that as that is separate topic
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LVL 86

Expert Comment

by:CEHJ
ID: 40375536
Judging from the kinds of questions that have been asked so far, to expect the asker of this question to be able to understand why string concatenation can be inefficient by inspecting the bytecode, at this stage in their learning, seems unrealistic.
I know what you're getting at, but my rejoinder would be that if the theory and facts behind what happens in string concatenation are a little complex (and you could be right about that) then a policy of learning by rote and taking on trust (pro tem) the rule 'avoid string concatenation' is probably a good one.
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LVL 84

Expert Comment

by:ozo
ID: 40375652
I can see the usefulness of shortcuts like rote learning for getting quick solutions to problems that may be difficult to work out more completely.
We all use shortcuts all the time.  While they may not be the best in all situations, they do enable us to use our finite cognitive capacity to deal with so many circumstances we face every day.

But my general take is that I prefer trying to build more skills for evaluating situations, which may allow one to develop ones own set of shortcut rules, which could then be better applied in situations where a fixed set of rote rules may not have appropriately anticipated.
When there is potential for problems to be complex, as programming problems, and optimization problems can be,
then a policy of preferring understanding over rote trust seems to me to be a good one.
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