This is about my assignment so I would appreciate generic answers with explanation.
I have a for-loop which I have to make for an online ordering system. The for-loop and ordering all works fine but if the same item is put in twice, then it forgets the previous order for the same item and only remembers the latest one. I am required to use simple arrays and for-loops so I would appreciate if the solution/help was also of this basic level.
My code looks something like this (NOTE: The following is just an example of what part of the loop looks like--this is not a complete program):
for (int i = 0; i < 3; i++) { //I changed the loop so there's no confusion about
//what I am actually asking about.
if (order.equalsIgnoreCase(computer) {
int price = quantity[i] + itemcode;
}
}
To explain further, this loop and the if statement work perfectly if a certain item is only ordered once. But if the user enters, say, an order for a computer once and then after 3 more orders, orders another computer, then the output does not add the previous order in the new order but only remembers the latest one.
I would appreciate any work around suggested for this but again, since this is for my studies, I would appreciate explanations rather than direct solutions.
Please ask me questions in case this is not clear.
"Forgets" suggests that you are overwriting something in your code rather than, say, just incrementing. Go through your code, see what parts of it gets reset when you place a new order. For instance, if you are doing
quantity[1] = getNumberFromUser("How many apples?");
then this would obviously erase the old value each time. If you want to merely increment the number of apples, do something like
quantity[1] += getNumberFromUser("How many apples?");
Another general advice is to use print statements to debug your code. That way you can see for yourself what really happens. Learning to use a real debugger would also be of great benefit.
if you have two or more typres of products and want to calculate the price for all the orders together then you can try the following code,, i think thats very simple,,
int price=0;
for (int i = 0; i < 3; i++) { //I changed the loop so there's no confusion about
//what I am actually asking about.
if (order.equalsIgnoreCase(computer) {
price += quantity[i] + itemcode; //use the previous value of price
}
}
or else if you want to have history for each product separately then you have to try the same with a array of price for each product type..
If you cant get the answer then comment here,,
Related
I have an array list with some names inside it (first and last names). What I have to do is go through each "first name" and see how many times a character (which the user specifies) shows up at the end of every first name in the array list, and then print out the number of times that character showed up.
public int countFirstName(char c) {
int i = 0;
for (Name n : list) {
if (n.getFirstName().length() - 1 == c) {
i++;
}
}
return i;
}
That is the code I have. The problem is that the counter (i) doesn't add 1 even if there is a character that matches the end of the first name.
You're comparing the index of last character in the string to the required character, instead of the last character itself, which you can access with charAt:
String firstName = n.getFirstName()
if (firstName.charAt(firstName.length() - 1) == c) {
i++;
}
When you're setting out learning to code, there is a great value in using pencil and paper, or describing your algorithm ahead of time, in the language you think in. Most people that learn a foreign language start out by assembling a sentence in their native language, translating it to foreign, then speaking the foreign. Few, if any, learners of a foreign language are able to think in it natively
Coding is no different; all your life you've been speaking English and thinking in it. Now you're aiming to learn a different pattern of thinking, syntax, key words. This task will go a lot easier if you:
work out in high level natural language what you want to do first
write down the steps in clear and simple language, like a recipe
don't try to do too much at once
Had I been a tutor marking your program, id have been looking for something like this:
//method to count the number of list entries ending with a particular character
public int countFirstNamesEndingWith(char lookFor) {
//declare a variable to hold the count
int cnt = 0;
//iterate the list
for (Name n : list) {
//get the first name
String fn = n.getFirstName();
//get the last char of it
char lc = fn.charAt(fn.length() - 1);
//compare
if (lc == lookFor) {
cnt++;
}
}
return cnt;
}
Taking the bullet points in turn:
The comments serve as a high level description of what must be done. We write them aLL first, before even writing a single line of code. My course penalised uncommented code, and writing them first was a handy way of getting the requirement out of the way (they're a chore, right? Not always, but..) but also it is really easy to write a logic algorithm in high level language, then translate the steps into the language learning. I definitely think if you'd taken this approach you wouldn't have made the error you did, as it would have been clear that the code you wrote didn't implement the algorithm you'd have described earlier
Don't try to do too much in one line. Yes, I'm sure plenty of coders think it looks cool, or trick, or shows off what impressive coding smarts they have to pack a good 10 line algorithm into a single line of code that uses some obscure language features but one day it's highly likely that someone else is going to have to come along to maintain that code, improve it or change part of what it does - at that moment it's no longer cool, and it was never really a smart thing to do
Aominee, in their comment, actually gives us something like an example of this:
return (int)list.stream().filter(e -> e.charAt.length()-1)==c).count();
It's a one line implementation of a solution to your problem. Cool huh? Well, it has a bug* (for a start) but it's not the main thrust of my argument. At a more basic level: have you got any idea what it's doing? can you look at it and in 2 seconds tell me how it works?
It's quite an advanced language feature, it's trick for sure, but it might be a very poor solution because it's hard to understand, hard to maintain as a result, and does a lot while looking like a little- it only really makes sense if you're well versed in the language. This one line bundles up a facility that loops over your list, a feature that effectively has a tiny sub method that is called for every item in the list, and whose job is to calculate if the name ends with the sought char
It p's a brilliant feature, a cute example and it surely has its place in production java, but it's place is probably not here, in your learning exercise
Similarly, I'd go as far to say that this line of yours:
if (n.getFirstName().length() - 1 == c) {
Is approaching "doing too much" - I say this because it's where your logic broke down; you didn't write enough code to effectively implement the algorithm. You'd actually have to write even more code to implement this way:
if (n.getFirstName().charAt(n.getFirstName().length() - 1) == c) {
This is a right eyeful to load into your brain and understand. The accepted answer broke it down a bit by first getting the name into a temporary variable. That's a sensible optimisation. I broke it out another step by getting the last char into a temp variable. In a production system I probably wouldn't go that far, but this is your learning phase - try to minimise the number of operations each of your lines does. It will aid your understanding of your own code a great deal
If you do ever get a penchant for writing as much code as possible in as few chars, look at some code golf games here on the stack exchange network; the game is to abuse as many language features as possible to make really short, trick code.. pretty much every winner stands as a testament to condense that should never, ever be put into a production system maintained by normal coders who value their sanity
*the bug is it doesn't get the first name out of the Name object
Rather than explaining some big problem, I'll skip all that and list the small loop I am struggling with. Anyways, I have to print the key of a map, so I am using a special way to print the key by switching the value and the key around.
for (int i = 0; i < elementData.length; i++){
System.out.print("[" + i + "]");
for (Entry<HashEntry<E>, Integer> entry : foob.entrySet()){
if (entry.getValue().equals(i)){
System.out.print(entry.getKey().toString());
}
}
}
This is my goal: Print [0][1][2][3] like that all the way to 20. Along with that, 9 numbers will go in between those numbers in parens randomly, based on my program.
Here is my result:
[0][1]HashSet$HashEntry#7d4991ad[2][3][4]HashSet$HashEntry#4554617cHashSet$HashEntry#28d93b30[5][6][7][8][9]HashSet$HashEntry#232204a1[10][11]
So there's just some trick to make it not print all this machine language looking stuff. Anyways, what do I have to do? Looks like 1 thing was supposed to come after [1], 2 things after [4], something after [9], and so on.
Thanks!
So there's just some trick to make it not print all this machine language looking stuff?
Yea.
Don't try to print an instance of a class that doesn't override Object.toString(). That "machine language looking stuff" is simply the output of Object.toString().
However, I suspect that your real code is doing this:
if (entry.getValue().equals(i)){
System.out.print(entry.toString());
}
because "HashSet$HashEntry#7d4991ad" looks like the output you would get if you printed a HashSet.HashEntry object. (The other possibility is that you have used HashSet.HashEntry objects as keys in your Map.)
I'm very new to Java and stackoverflow so I'm sorry if I seem ignorant but I wrote this program to multiply two numbers together using the Russian Peasant multiplication algorithm. The complete program includes far more operations and is hundreds of lines of code but I only included what I thought was necessary for this particular method. I have a test harness so I know that all the submethods work correctly. The problem that I'm struggling with though is the 3rd line where I'm adding factor1 to the product. The values add correctly but then when factor1 is multiplied by 2 in the 5th line then the value that was added to product in the 3rd line also gets doubled resulting in an incorrect product value. How can I make sure that when factor 1 is doubled that it doesn't carry backwards to the product term?
while (Long.parseLong(factor2.toString()) >= 1) {
if (factor2.bigIntArray[0] % 2 == 1) {
product = product.add(factor1);
}
factor1 = factor1.multiplyByTwo();
factor2 = factor2.divideByTwo();
}
I think in your method multiplyByTwo you use code
`datamember=datamember*2;`
rather than that try doing this
return new FactorClass(datamember*2);
so it doesnt change the added value.
it would be better if u could show the mulTiplyByTwo method code since that is where your actually are getting the changed value.
My assignment asks for me to add items to a shopping cart, asking the user for input. We are to use the add method but I keep getting an error. I have tried this for days, I have also searched the web and can't find an answer I understand.
List<Double> shoppingCart = new ArrayList<Double>();
shoppingCart.add(" ");
double shoppingCartItem;
shoppingCartItem = numberReader.nextDouble();
for (int counter = 0; counter <shoppingCart.size(); counter++) {
System.out.print("Item");
}
Any help would be greatly appreciated. Also, I am extremely new to java.
I realized what I was trying to do may not have been clear. This is how the completed program should look. I have been working on this for days. Any direction would be so appreciated. I am at a lost, everything I've done has made this worst.
![enter image1
The add(obj) function will add the given object to the list. So you should add your double variable to it. You don't need to add an 'empty' element beforehand (which is what I assume you were tying to do with the empty string?)
E.g
ArrayList<Double> shoppingCart = new ArrayList<Double>();
double shoppingCartItem;
shoppingCartItem = numberReader.nextDouble();
shoppingCart.add(shoppingCartItem);
for (int counter = 0; counter <shoppingCart.size(); counter++) {
System.out.print("Item");
}
Although it might be intimidating at first, its always a good idea to have a read of the docs when working with Classes and functions you arn't familiar with. For example here is information about the add() method.
And I'm guessing you'll probably want to use the retrieve the doubles in your list at some point, so have a read of the get() method.
The following is part of the code for my college assignment.
else if (!codeList.contains(userCode)) {
i--; // i is the counter for the for-loop
}
else if (userQuantity[i]==0) {
i--;
}
The first part makes sure that if the user enters the wrong code, the counter i does not increment 1, or rather, it subtracts 1 from the recently incremented counter. This part works fine.
The second part however is what I seem to be having problems with. userQuantity[] is an int array and it has to be an array. This does not seem to do anything to the code. Even if 0 is entered for the quantity, it still incrememnts the counter which is not desireable.
I should explain, to avoid confusion, that this is an infinite for-loop (with a break statement). The reason I am doing a for-loop is because I am required to. Is it because of my for-loop that the condition isn't working or am I doing something completely wrong with it?
This is for my college assignment so I would appreciate answers with explanation and not just quick-fixes. If you need me to explain, let me know please.
Although it's not strictly illegal in Java, it's not a good idea to change the value of the for loop control variable from within the loop. (Such modification is illegal in some other languages.)
By changing the loop iteration variable within the loop, you're messing with the implicit assumptions offered by your use of a for loop. For example, if a reader sees:
for (int i = 0; i < 10; i++) {
// ...
}
the reader will rightfully assume that the loop is intended to execute exactly 10 times (or, no more than 10 if there is a break in there). However, if you go changing the value of i within the loop, this assumption is no longer valid.
If you must change the value of the counter, I would suggest writing this as a while loop instead:
int i = 0;
while (i < 10) {
// ...
i++;
}
along with a comment that explains why you are changing i within the loop and what it means to do so.
This is very bad practice. Change the for loop to a while loop and only increment if
codeList.contains(userCode)==true or userQuantity[i]!=0.
I should explain, to avoid confusion, that this is an infinite for-loop (with a break statement). The reason I am doing a for-loop is because I am required to. Is it because of my for-loop that the condition isn't working or am I doing something completely wrong with it?
I have a feeling that you are misunderstanding the requirements (e.g. you are not required to use a for loop), or that there is a mistake in your thinking; i.e. there is a simpler solution that doesn't involve the counter going backwards.
(It is surprising that a programming exercise would require you to write code that most experienced Java programmers would agree is bad code. The simple explanation is that it is not.)
Either way:
Changing the loop variable in a for loop is bad practice, for the reasons described by Greg.
The idea of an "infinite for loop" is really strange. The following is legal Java ...
for (int i = 0; true; i++) {
...
}
but the idiomatic way to write it is:
int i = 0;
while (true) {
...
i++; // ... at the appropriate point / points
}
... which in most cases means that you don't need to make the variable go backwards at all.