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06-08-2016 , 10:33 PM

#1251

Loki

binary duo

Join Date: Nov 2015 Posts: 12,772

My first semester Java professor, halfway through the course, told us to stop returning False because if anyone ever sees you do that irl and they find out he was our professor, he would be extremely embarrassed.

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06-08-2016 , 10:54 PM

#1252

LBloom

journeyman

Join Date: Mar 2011 Posts: 239

One more question, and then I really am done tonight. One of my assignments for the week is to modify a program that shows the compound interest over 5 10 years at 5% interest to include 10 years at 6, 7, 8, 9 and 10% interest. I have done this and my code looks like this:

Code:

package compoundinterest;

public class CompoundInterest {

    public static void main(String[] args) 
    {
        double amount;
        double principal = 1000.0;
        
        System.out.printf("%s%20s%n", "Year", "Amount on deposit");
        
        for (double rate = 0.05; rate <= 0.10; rate += .01)
        {   
            System.out.println("Year by year growth for a " + rate * 100 
                               + "% interest rate.");
            System.out.printf("%s%20s%n", "Year", "Amount on deposit");
            
            for (int year = 1; year <= 10; ++year)
            {
                amount = principal * Math.pow(1.0 + rate, year);
                System.out.printf("%4d%,20.2f%n", year, amount);
                
                if (year == 10)
                    System.out.println();
            }
        }
    }
}

My question is about the line I included to print "Year by year growth for an x% interest rate." When I run this program, it outputs 5.0%, 8.0%, 9.0%, and 10.0%, but 6 and 7 both get output as something like "6.000000001%". For the purpose of my assignment it doesn't even matter because I could just delete that line, but why is this happening? And why for only 6 and 7? WTF?

I guess I could also just output the rate as "0.06" instead of 6%, but I still just want to understand this.

Last edited by _dave_; 06-09-2016 at 02:12 AM. Reason: code tags

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06-08-2016 , 11:07 PM

#1253

RustyBrooks

Carpal \'Tunnel

Join Date: Feb 2006 Posts: 24,646

Not every number can be well represented as a floating point number. The result is you can input something like "6" and it'll get stored as the nearest thing that CAN be represnted, i.e. 6.0000000001%

This is about python but it's the same idea in most languages:
http://effbot.org/pyfaq/why-are-floa...inaccurate.htm

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06-09-2016 , 02:13 AM

#1254

_dave_

_Pooh_Bah_

Join Date: Feb 2005 Posts: 13,154

LBloom, put your code in [code][/code] tags

Quote

06-09-2016 , 06:44 AM

#1255

adios

Carpal \'Tunnel

Join Date: Sep 2002 Posts: 22,558

Quote:

Originally Posted by LBloom

Code:

package compoundinterest;

public class CompoundInterest {

    public static void main(String[] args) 
    {
        double amount;
        double principal = 1000.0;
        
        System.out.printf("%s%20s%n", "Year", "Amount on deposit");
        
        for (double rate = 0.05; rate <= 0.10; rate += .01)
        {   
            System.out.println("Year by year growth for a " + rate * 100 
                               + "% interest rate.");
            System.out.printf("%s%20s%n", "Year", "Amount on deposit");
            
            for (int year = 1; year <= 10; ++year)
            {
                amount = principal * Math.pow(1.0 + rate, year);
                System.out.printf("%4d%,20.2f%n", year, amount);
                
                if (year == 10)
                    System.out.println();
            }
        }
    }
}

Generally speaking, you should generate values that match the precision of your calculations. In this case you are working with a precision of 10**-2. You should format your output accordingly. You should also round to the nearest 10**-2. For example, 6.0051 should rounded to 6.01 and 6.00001 should be rounded to 6.00. Working with fractional values in programming is a little messy but can be very interesting, at least to me.

Last edited by adios; 06-09-2016 at 06:51 AM.

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06-09-2016 , 07:27 AM

#1256

Wolfram

Carpal \'Tunnel

Join Date: Jan 2006 Posts: 15,066

I would avoid floating point numbers in control structures as much as possible.

Code:

for (int percentage = 5; percentage <= 10; percentage += 1)
{
  double rate = percentage / 100.0;
  // rest of code
}

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06-09-2016 , 08:41 AM

#1257

Loki

binary duo

Join Date: Nov 2015 Posts: 12,772

Yeah, doubles/floats and comparisons can be tricky. Heaven forfend you ever try to check for equality instead of inequality.

Quote

06-09-2016 , 04:22 PM

#1258

LBloom

journeyman

Join Date: Mar 2011 Posts: 239

A C++ question:

Speaking of doubles, floats, integers...

In a program I'm making for a homework assignment, there is a point where I have to add the value of two doubles say x and y (both of which have a fractional part of exactly .5) and store the value in a new variable, z. During future calculations in this program, I'm going to want to only get integer values. If I got a float value, I would have to eliminate the decimal part anyway. So my question is, is it appropriate to just save myself some trouble and just declare z as an int? Since the numbers are x.5 and y.5, they will always equal a whole number, and tests appear to show no problems, except a warning message I get from visual studio that some data may be lost.

Alternatively, I could wait until I produce the final double value and just cast it into an integer, which seems like maybe it is better for (in this case extremely hypothetical) reuse.

If it helps the program is to determine how many "moshers" of a given size can fit in a mosh pit of a given size with given sized barriers, so end result must be a whole number obviously.

Quote

06-09-2016 , 05:41 PM

#1259

kazana

Pooh-Bah

Join Date: May 2006 Posts: 5,658

The warning is only shown if you do not explicitly tell it you want a float to be an integer (and accept potential loss of precision).

So instead of just assigning the value of two floats added together, you can cast the result to integer. That makes it obviously clear what you are doing. Both for the compiler and others looking at the code.

E.g.

Code:

double x = 1.5;
double y = 3.5;
int foo = (int)(x+y); // no warning, you seem to be aware of what's happening
int bar = x+y; // warning, precision lost

Quote

06-09-2016 , 08:03 PM

#1260

adios

Carpal \'Tunnel

Join Date: Sep 2002 Posts: 22,558

Quote:

Originally Posted by LBloom

Quote:

Originally Posted by kazana

Code:

double x = 1.5;
double y = 3.5;
//int foo = (int)(x+y);
int foo = static_cast<int>(x+y); // no warning, you seem to be aware of what's happening
int bar = x+y; // warning, precision lost

A little more descriptive, results the same

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06-10-2016 , 03:17 AM

#1261

ChrisV

Carpal \'Tunnel

Join Date: Jul 2004 Posts: 40,336

In other C-style languages, for instance C#, not explicitly casting to int would be a compile error. Generally better to be explicit about what you're doing.

Quote

06-10-2016 , 07:00 AM

#1262

Loki

binary duo

Join Date: Nov 2015 Posts: 12,772

java/android question: doing some basic intro app progamming wile insomnic. The code as follows:

Code:

mPrevButton = (Button) findViewById(R.id.prev_button);
        mPrevButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mCurrentIndex = (mCurrentIndex - 1) % mQuestionBank.length;
                updateQuestion();
            }
        });
    }

    private void updateQuestion() {
        int question = mQuestionBank[mCurrentIndex].getTextResId();
        mQuestionTextView.setText(question);
    }

gives me:

Quote:

java.lang.ArrayIndexOutOfBoundsException: length=5; index=-1

when I try to wrap around from the first question to the last one, backwards. (i.e. go from array index 0 to length - 1)

now, google assures me that (0-1) % 5 == 4, so a 5 length array should not be out of bounds if it's trying to access index 4. But it seems to be trying to access index -1 if I'm reading that error right.

What's going on here? I would try to debug but i've no clue how to do that in androind studio/intelliJ or whoever makes it, and also i'm very sleepy/foggy headed

edit:

ugh

found a similar issue in javascript, apparently the solution is:

Code:

mCurrentIndex = (((mCurrentIndex - 1) % mQuestionBank.length) + mQuestionBank.length) % mQuestionBank.length;

which is really beautiful and elegant

Last edited by Loki; 06-10-2016 at 07:16 AM.

Quote

06-10-2016 , 07:12 AM

#1263

Europa

adept

Join Date: Sep 2012 Posts: 727

Doesn't % give you the remainder in Java, which would be -1, as also seen in the error message?

http://stackoverflow.com/questions/5...gative-numbers

Quote

06-10-2016 , 07:16 AM

#1264

ChrisV

Carpal \'Tunnel

Join Date: Jul 2004 Posts: 40,336

Modulus of negative numbers in programming is just a matter of convention. In Java a negative number mod a positive number is negative and -1 % 5 is -1.

Quote

06-10-2016 , 07:20 AM

#1265

ChrisV

Carpal \'Tunnel

Join Date: Jul 2004 Posts: 40,336

The Wikipedia article on modulo has a table of how modulo is calculated in various languages. There are plusses and minuses, no pun intended, to different approaches.

Quote

06-10-2016 , 07:23 AM

#1266

Loki

binary duo

Join Date: Nov 2015 Posts: 12,772

>.<

i must not be using java 8 because i don't have Math.floorMod(). :sad:

Quote

06-10-2016 , 09:11 AM

#1267

ChrisV

Carpal \'Tunnel

Join Date: Jul 2004 Posts: 40,336

Code:

int x = y % z;
if(x < 0) x = y - x;

Quote

06-15-2016 , 12:14 PM

#1268

RV-

grinder

Join Date: Aug 2009 Posts: 412

This is what I'm supposed to build:

Code:

puts "Text to search through: "
text = gets.chomp
puts "Word to redact: "
redact = gets.chomp

words = text.split(" ")

words.each do |word|
  if word != redact
    print word + " "
  else
    print "REDACTED "
  end
end

This is what I'm trying to do (I really want to understand how it works, so I'm trying different ways of achieving the goal):

Code:

puts "Say something!"
text = gets.chomp

puts "Wanna take it back?"
redact = gets.chomp

words = text.split (" ")

words.each do |everyword|
    if everyword = redact
    print "REDACTED "
  else
    print everyword + " "
  end
end

With my code it just prints REDACTED REDACTED REDACTED (REDACTED for every word). Even if "redact" variable is completely different from the "text". Why?

Besides that, do you think it's a good approach I'm taking, messing around like this? It's like my 4th hour into Ruby and, damn, it's fun, but in cases like this I feel silly.

Last edited by RV-; 06-15-2016 at 12:25 PM.

Quote

06-15-2016 , 12:25 PM

#1269

Loki

binary duo

Join Date: Nov 2015 Posts: 12,772

Is your second snippet a copy and paste version of the code you're running?

It looks like "if everyword = redact" might be an assignment statement, whereas "if everyword == redact" would compare the values. An assignment statement must return a truthy value, so that's why it's printing REDACTED for every word.

Also, Ruby tip: I think .split defaults to .split(" ") if you don't give it the (" ") parameter. I could be wrong, but worth a shot.

Quote

06-15-2016 , 12:32 PM

#1270

RV-

grinder

Join Date: Aug 2009 Posts: 412

Quote:

Originally Posted by Noodle Wazlib

Second is what I'm running. Had a space after .split - mistake. And YES! YES! YES! YES! It's that "double =" thing. And I should have known that by now, it was in the first couple of lessons. Shame on me.

Well at least my initial logic was ok... Thanks again! On to the next lesson!

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06-15-2016 , 12:44 PM

#1271

Loki

binary duo

Join Date: Nov 2015 Posts: 12,772

Good luck! I know some of codecademy has issues, so if you work on something and can't figure it out, definitely check out their Q&A forum. Some people have figured out specific ways of loading the problems or answering in non-obvious ways to move on, even when your code should work.

But mostly, it's going to be stuff like typos or logic errors that get you. Happens to the best of us.

Quote

06-15-2016 , 09:40 PM

#1272

gaming_mouse

Carpal \'Tunnel

Join Date: Oct 2004 Posts: 13,786

Quote:

Originally Posted by RV-

if you want to have even more fun with ruby, write ruby instead of C

Code:

words = text.split.map {|w| w == redact ? 'REDACTED' : w }
print words

also separate data and presentation

Quote

06-15-2016 , 10:36 PM

#1273

Loki

binary duo

Join Date: Nov 2015 Posts: 12,772

Let him get to method chaining before throwing all that at him, gawd!

Quote

06-15-2016 , 11:06 PM

#1274

gaming_mouse

Carpal \'Tunnel

Join Date: Oct 2004 Posts: 13,786

Quote:

Originally Posted by Noodle Wazlib

Let him get to method chaining before throwing all that at him, gawd!

but the man needs something to look forward to...

Quote

06-16-2016 , 09:38 AM

#1275

RV-

grinder

Join Date: Aug 2009 Posts: 412

Is there an easy way to explain why:

Code:

strings.each do |convert|
  symbols.push(convert.to_sym)
end

Works, but this one doesn't:

Code:

strings.each do |convert|
  convert.to_sym
  symbols.push(convert)
end

In second code it doesn't save new values(after .to_sym) and when we try to push we push restored version of |convert|? If so - why? If it's something else - what?

Last edited by RV-; 06-16-2016 at 09:45 AM.