Author Topic: Java Calculation Help Please! (Read 49 times)

Dr. Tox · « **on:** January 17, 2012, 08:47:20 AM »

I get the error that * operator is not defined but maths operators are supposed to be native, are they not? This program runs fine up until I try to multiply velocity by time to get distance.... In other simple calculator progs I've done none of the maths operators needed to be defined so WTF!??

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class strings {

   /**
    * @param args
    */
   public static void main(String[] args) {

System.out.print("Enter velocity value in Mph and press Enter: ");
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
String velocity = null;
try {
velocity = br.readLine();
} catch (IOException e) {
System.out.println("Error!");
System.exit(1);
}
System.out.println("velocity is " + velocity);

System.out.print("Enter time value in hours and press Enter: ");
BufferedReader br2 = new BufferedReader(new InputStreamReader(System.in));
String time = null;
try {
time = br.readLine();
} catch (IOException e) {
System.out.println("Error!");
System.exit(1);
}
System.out.println("time is " + time);
String distance = (velocity * time);
}
}

Dr. Tox · « **Reply #1 on:** January 17, 2012, 08:51:46 AM »

I get this message at: String distance = (velocity * time);

Multiple markers at this line
   - The operator * is undefined for the argument
    type(s) java.lang.String, java.lang.String
   - distance cannot be resolved to a variable
   - The operator * is undefined for the argument
    type(s) java.io.BufferedReader, java.io.BufferedReader

I get that distance can't be resolved because I can't tell the fucking thing what distance means until I can use the fucking * operator!

Vesp · « **Reply #2 on:** January 17, 2012, 03:13:30 PM »

I don't know much about Java.. just started to learn it... but shouldn't velocity be defined as something other than a string? like an int?

Also... when doing multiplication like that, does it need to have (these) around it?

No idea, literally only have had my java class like, 3 times so far. But I figured I would at least attempt to try to understand this and try to solve it.

Dr. Tox · « **Reply #3 on:** January 17, 2012, 07:41:00 PM »

Yeah, I've tried defining the values and integers and it laughs at me when I do. So I do it this way and it seems to work...

http://ocw.csail.mit.edu/f/1

MIT opencourseware is where I'm practicing from and i'm using eclipse just like they say to and so far so good.

The thing is, I was under the impression that mathematical operators aren't supposed to have to be defined like in C++. I thought they were natively understood by java. I mean, they have been so far. I've done stuff like:

double score = 2.0 * 5.0
System.out.println(score);

<-Run

10.0

There's no definition needed. And as far as the (parentheses) go, well, I was just fucking around with all kinds of combination to get it to work and that's where I gave up. No matter what I do it's not recognizing * as a math operator. Besides, it still recognizes the + operator so what the fuck?

This is not logical!

Now I just tried it in NetBeans 7.1
It says "Bad operand types for binary operator "*".

Dr. Tox · « **Reply #4 on:** January 17, 2012, 11:02:55 PM »

Yeah, so, "velocity" & "time" do have to be converted from strings to integers but I'm not sure how to do that with the code here that takes a user inputs and utilizes them to do perform further mathematics functions....

I see this page
http://docs.oracle.com/javase/tutorial/java/data/converting.html

Which doesn't really seem to help me in this instance...

Amending with

System.out.println("time is " + time);
double a = velocity;
double b = time;
System.out.println(a * b);

Gives the errors: Cannot convert from strings to double

Double.parseDouble(velocity);
Double.parseDouble(time);

Gives the error "Type mismatch: Cannot convert from strings to double"

int velocity = (int) ((Double.parseDouble(velocity.getText)));

Error: -The primitive type int of velocity does not have a field getText
- Duplicate local variable velocity

Vesp · « **Reply #5 on:** January 18, 2012, 12:10:27 AM »

You'll have to cast them I believe? I don't remember how to do casting.. however. Not totally sure... I know casting can be used for

Wouldn't you do something like this?

int velocity = 0;
int time = 0;

int distance = velocity * time;

System.out.println("distance traveled" + distance);

I'm not sure... I honestly do not even understand what it is you are doing? Is there a link to an online homework sort of thing for me to look at as well? I'm not sure... but it doesn't look like you've linked it.

Dr. Tox · « **Reply #6 on:** January 18, 2012, 02:55:13 AM »

Oh, this is just something I wanted to try on my own. I'm not following any guide.

This is just supposed to be a simple distance-velocity-time calculator. I just happened to choose to solve for distance.

Evidently I'm supposed to use something like this and adapt it.

public class StringToNumber {
public static void main(String[] args) {
// a string - could have come from user input
// or from any other source
String numStr = "3.14159";

// the method used in the tutorial
double num = (Double.valueOf(numStr)).doubleValue();
System.out.println("2pi is " + (2 * num)); // multiplication is good

// an alternative (which I linked to)
double pi = Double.parseDouble(numStr);
System.out.println("2pi is " + (2 * pi));

It's evidently doing three things according to pbrockway2 over at javaforums: (1) declaring a double variable pi, (2) parsing numStr to get a number from the string and (3) assigning the number to the double variable.

So I'm guessing I should define numStr in my code as the input from the user and then I can perform multiplication on it?

Dr. Tox · « **Reply #7 on:** January 18, 2012, 03:05:40 AM »

Success!

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class strings {

   /**
    * @param args
    */
   public static void main(String[] args) {

System.out.print("Enter velocity value in Mph and press Enter: ");
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
String numStr = null;
try {
numStr = br.readLine();
} catch (IOException e) {
System.out.println("Error!");
System.exit(1);
}
double num = Double.parseDouble(numStr);
System.out.println("velocity is " + (1 * num) + " Miles per hour");

System.out.print("Enter time value in hours and press Enter: ");
BufferedReader br2 = new BufferedReader(new InputStreamReader(System.in));
String numStr2 = null;
try {
numStr2 = br.readLine();
} catch (IOException e) {
System.out.println("Error!");
System.exit(1);
}
System.out.println("time is " + numStr2 + " Hours");
double num2 = Double.parseDouble(numStr2);
System.out.println("distance is " + (num * num2) + " Miles");
}
}

OUTPUT:
Enter velocity value in Mph and press Enter: 2
velocity is 2.0 Miles per hour
Enter time value in hours and press Enter: 4
time is 4 Hours
distance is 8.0 Miles

n.snostorm · « **Reply #8 on:** January 18, 2012, 02:10:07 PM »

Java is strongly typed language. All conversions between types are done through functions.
And java has two types of types - primitive and object oriented representation of said types. They are not interchangeable or
assignable directly. For example you can't assign value of double to variable of type(actually it's not a type but an object) Double.

When learning java, you really should read into typing system and how to do conversions between types to avoid problems
and misunderstandings later on.

Author Topic: Java Calculation Help Please! (Read 49 times)

Dr. Tox

Java Calculation Help Please!

Dr. Tox

Re: Java Calculation Help Please!

Vesp

Re: Java Calculation Help Please!

Dr. Tox

Re: Java Calculation Help Please!

Dr. Tox

Re: Java Calculation Help Please!

Vesp

Re: Java Calculation Help Please!

Dr. Tox

Re: Java Calculation Help Please!

Dr. Tox

Re: Java Calculation Help Please!

n.snostorm

Re: Java Calculation Help Please!