C++ noob needs help :(

So I'm pretty new to C++, and I'm in need of a little help trying to understand the way my code is being interpreted. I'm not completely clueless, as I've read a few texts at decent length; but my problem is that i've got very little experience in application. I've successfully programmed simple things like a program to change pennies into dollars, quarters, nickels, and dimes.

Anyhow, I read about this Project Euler thing and I thought it was a perfect chance for me to get some practice. But I've failed on the very first problem many times already, and i've approached the problem in many ways, yet nothing i do seems to work. Anyone else fluent in C++ that can point out where I've screwed up?

The problem asks for the sum of all the multiples of 3 and 5 that are less than 1000. The following is my code.

do{
checking++;
     if( (checking % 3) == 0 || (checking % 5) == 0 );
          {
               sum = sum + checking;
          };
    }while(checking < 999);


---edit---
Glitchspot is not agreeing with me today :(

You have a semicolon after the end bracket of the if statement, which causes the if statement to execute a null command and causes the stuff in the brackets to execute unconditionally.

You're also using a do...while when a for loop would be better, but that doesn't affect functionality. :P

You have a semicolon after the end bracket of the if statement, which causes the if statement to execute a null command and causes the stuff in the brackets to execute unconditionally.

You're also using a do...while when a for loop would be better, but that doesn't affect functionality. :P

GabuEx

Aside from that, you both agreed that do-while loops were disagreeable.. why? one of my points of confusion is concerning the different loops and why some are more useful in different situations. It seems to me like a while, do-while, and a for loop can each be coded to do the same things. so where's the distinction?HardQuor

[QUOTE="GabuEx"]

You have a semicolon after the end bracket of the if statement, which causes the if statement to execute a null command and causes the stuff in the brackets to execute unconditionally.

You're also using a do...while when a for loop would be better, but that doesn't affect functionality. :P

HardQuor

Try this instead:

for(checking = 1; checking < 1000; ++checking)
{
   if(checking%3==0||checking%5==0)sum=sum+checking;
}

I guess C++/Java doesn't agree with GS.

Try this instead:

for(checking = 1; checking < 1000; ++checking)
{
   if(checking%3==0||checking%5==0)sum=sum+checking;
}

I guess C++/Java doesn't agree with GS.

OrkHammer007

I definitely agree with Gabu; do...while is one of the most annoying constructs in C/C++, and that's really saying something :) Also, you'll either want a condition of less than 1000 or less than/equal to 999 I would have used actual code examples, but apparently the HTML editor doesn't like less than/greater than characters being used.xaos

Heresy!

I always try to cast all my loops as "do ... while" loops!

Yes, really, and I have a good reason too: A "do ... while" loop has just one conditional jump, whereas a "for loop" or "while loop" compiles down to code with a conditional jump and an unconditional jump. The "do ... while" loop is a more efficient construct.

Yeah, I know you're gonna say "The optimizer takes care of that!". But I don't have enough faith in compiler optimizers to abandon my "do ... while" fixation. :P

Heh, i know it's silly, but a do..while loop reads more humanely than for loops do to me. Again, i'm sure it's just my inexperience speaking.

anywho, i've got another problem with the second project euler problem. (oy. at this rate, i'll NEVER get all 217 :()
The problem asks for the sum of all the even numbers that are less than 4 million that appear in the Fibonacci sequence. The following is my code.

do
{
term3 = term1 + term2;
if( term1 % 2 == 0 )
{
sum = term1 + sum;
}
else if( term2 % 2 == 0 )
{
sum = term2 + sum;
};
term1 = term2;
term2 = term3;
}while( term3 < 4000000 )

I have a feeling that my integers need to be doubles but when i do that, i get a compile-time error telling me that i cant use a binary operator (%) on double integers...

Don't use a double; it'll end up being a decimal instead of the integer you want. Use a long int instead (ex. long int term3 = 0;).

Otherwise, it looks pretty good. :D

Don't use a double; it'll end up being a decimal instead of the integer you want. Use a long int instead (ex. long int term3 = 0;).

Otherwise, it looks pretty good. :D

OrkHammer007

I just copy/pasted what you wrote, with the following initial values:

long int term1 = 0; long int term2 = 1; long int term 3 = 0; long int sum = 0;

It compiled and ran fine. What's the answer supposed to be (I get  5702886)?

Oh... and I added a ; after the "while" part of the loop.

Bloody hell... the problem just smacked me right in the face:

Why are you checking term1 and term2, when the Fibonacci term is term3? That's the only one you need to check. :P

Sorry I didn't catch that earlier.

I just copy/pasted what you wrote, with the following initial values:

long int term1 = 0; long int term2 = 1; long int term 3 = 0; long int sum = 0;

It compiled and ran fine. What's the answer supposed to be (I get  5702886)?

Oh... and I added a ; after the "while" part of the loop.

OrkHammer007

I just copy/pasted what you wrote, with the following initial values:

long int term1 = 0; long int term2 = 1; long int term 3 = 0; long int sum = 0;

It compiled and ran fine. What's the answer supposed to be (I get  5702886)?

Oh... and I added a ; after the "while" part of the loop.

OrkHammer007

Bloody hell... the problem just smacked me right in the face:

Why are you checking term1 and term2, when the Fibonacci term is term3? That's the only one you need to check. :P

Sorry I didn't catch that earlier.

OrkHammer007

Try this instead:

do{

term3 = term1 + term2;

if(term3%2==0)sum+=term3;

term1=term2;

term2=term3;

}while(term3 "less than symbol"4000000);

Stupid HTML conflict!!!

Well. I shortened the code to evaluate only term3:

do
{
    term3 = term1 + term2;
    if( term3 % 2 == 0 )
    {
        sum = term3 + sum;
     };
     term1 = term2;
     term2 = term3;
}while( term3 (less than or equal to) 4000000 );

and i get 58024584. But that's still wrong. I still don't know what the hell i'm doing wrong. this is pissing me off.
oh, also, term1 and term2 have been reduced to 0 and 1, respectively.

[QUOTE="xaos"]I definitely agree with Gabu; do...while is one of the most annoying constructs in C/C++, and that's really saying something :) Also, you'll either want a condition of less than 1000 or less than/equal to 999 I would have used actual code examples, but apparently the HTML editor doesn't like less than/greater than characters being used.Stesilaus

Heresy!

I always try to cast all my loops as "do ... while" loops!

Yes, really, and I have a good reason too: A "do ... while" loop has just one conditional jump, whereas a "for loop" or "while loop" compiles down to code with a conditional jump and an unconditional jump. The "do ... while" loop is a more efficient construct.

Yeah, I know you're gonna say "The optimizer takes care of that!". But I don't have enough faith in compiler optimizers to abandon my "do ... while" fixation. :P

Getting past those mistakes, setting aside the frustration... that's when the fun really begins...OrkHammer007

Yeah, once you get past those mistakes, you can get on to code more complicated things and have really aggravatingly hard-to-spot mistakes. :P

[QUOTE="OrkHammer007"]Getting past those mistakes, setting aside the frustration... that's when the fun really begins...GabuEx

Yeah, once you get past those mistakes, you can get on to code more complicated things and have really aggravatingly hard-to-spot mistakes. :P

:lol:

So, no one has any clue, 'eh?HardQuor

I just rewrote it to spit out the Fibonacci numbers so I could double check (using Wikipedia as a reference to make sure I got the right calculations), reran the sum, added the numbers on a calculator to triple check my results, and got...

[spoiler] 4613732 [/spoiler] ...which came straight from the last code snippet you posted.

[QUOTE="HardQuor"]So, no one has any clue, 'eh?OrkHammer007

I just rewrote it to spit out the Fibonacci numbers so I could double check (using Wikipedia as a reference to make sure I got the right calculations), reran the sum, added the numbers on a calculator to triple check my results, and got...

4613732 ...which came straight from the last code snippet you posted.

---edit---
aha! i found out what the problem is, by doing what you did, i just had it output term3 at the end of each do..while loop. aaand, it turns out that it's going one step over the 4 million mark for some reason.. gotta figure that out, now.

---re-edit---
nope, I was wrong again, i was actually reading a list of all thefibonacci sequence numbers, not the ones that were being totalled. So now i'm just totally confused, because when i checked the numbers being added together, it was all the rightones, and like you, i added them together manually to get the same sum that you got. But for some reason, my code is coming up with an extra 53410852 somewhere.

---re-re-edit---
OK! I FINALLY figured it out. Turns out that if i don't initialize a long integer, it defines itself as 53410852. So i went ahead and initialized it with a value of 0 and got our 4 million-ish number :lol: I knew from reading some books on C++ that it would be finicky, but JEEZ.

I've never gotten my code to get that output.. I'm guessing that's probably right, but i don't want to put it in until i can get my code to back it up.

---edit---
aha! i found out what the problem is, by doing what you did, i just had it output term3 at the end of each do..while loop. aaand, it turns out that it's going one step over the 4 million mark for some reason.. gotta figure that out, now.

---re-edit---
nope, I was wrong again, i was actually reading a list of all thefibonacci sequence numbers, not the ones that were being totalled. So now i'm just totally confused, because when i checked the numbers being added together, it was all the rightones, and like you, i added them together manually to get the same sum that you got. But for some reason, my code is coming up with an extra 53410852 somewhere.

---re-re-edit---
OK! I FINALLY figured it out. Turns out that if i don't initialize a long integer, it defines itself as 53410852. So i went ahead and initialized it with a value of 0 and got our 4 million-ish number :lol: I knew from reading some books on C++ that it would be finicky, but JEEZ.

HardQuor

Edit #3: One of the first rules we learned in programming (whether it was C++ or Java or Visual Basic) was to always initialize a variable. There are some instances where a "null" value is needed, but even then, we were told to intialize it as a "null."

Did the initialization solve your problem?

[QUOTE="HardQuor"]I've never gotten my code to get that output.. I'm guessing that's probably right, but i don't want to put it in until i can get my code to back it up.

---edit---
aha! i found out what the problem is, by doing what you did, i just had it output term3 at the end of each do..while loop. aaand, it turns out that it's going one step over the 4 million mark for some reason.. gotta figure that out, now.

---re-edit---
nope, I was wrong again, i was actually reading a list of all thefibonacci sequence numbers, not the ones that were being totalled. So now i'm just totally confused, because when i checked the numbers being added together, it was all the rightones, and like you, i added them together manually to get the same sum that you got. But for some reason, my code is coming up with an extra 53410852 somewhere.

---re-re-edit---
OK! I FINALLY figured it out. Turns out that if i don't initialize a long integer, it defines itself as 53410852. So i went ahead and initialized it with a value of 0 and got our 4 million-ish number :lol: I knew from reading some books on C++ that it would be finicky, but JEEZ.

OrkHammer007

Edit #3: One of the first rules we learned in programming (whether it was C++ or Java or Visual Basic) was to always initialize a variable. There are some instances where a "null" value is needed, but even then, we were told to intialize it as a "null."

Did the initialization solve your problem?

It's zombie thread time. sorry.
Anyhow, this time i'm more confuzzled than ever before. I got through problem 6 just fine, but the next step up the difficulty ladder (problem 5), is proving much more complicated. Here's the problem.

2520 is the smallest number that can be divided by each of the numbers from 1 to 10 without any remainder.
What is the smallest number that is evenly divisible by all of the numbers from 1 to 20?

and my code so far

int x=0;
for( int n=0; n (less than or equal to) 10; n++ )
{
     if( (x % n) != 0 )
     {
         x++;
     };
};

I'm only doing the first 10 as a proof of concept, since they give me the answer for that, so i can tell when i've done something wrong. As far as i can tell, i think i'm on the right path. "For each number up to ten, if x is not divisible by it, then increase x by 1.." it compiles fine, but it crashes at runtime for some reason. Anyone know what's going on? :?

It's zombie thread time. sorry.
Anyhow, this time i'm more confuzzled than ever before. I got through problem 6 just fine, but the next step up the difficulty ladder (problem 5), is proving much more complicated. Here's the problem.

2520 is the smallest number that can be divided by each of the numbers from 1 to 10 without any remainder.
What is the smallest number that is evenly divisible by all of the numbers from 1 to 20?

and my code so far

int x=0;
for( int n=0; n (less than or equal to) 10; n++ )
{
     if( (x % n) != 0 )
     {
         x++;
     };
};

I'm only doing the first 10 as a proof of concept, since they give me the answer for that, so i can tell when i've done something wrong. As far as i can tell, i think i'm on the right path. "For each number up to ten, if x is not divisible by it, then increase x by 1.." it compiles fine, but it crashes at runtime for some reason. Anyone know what's going on? :?

HardQuor

(There are other problems with the program, but they aren't what's making it crash.)

You're going to need 2 loops for this one: one to increment your target number, and one to loop over your divisions.

Try:

bool check=false;

int x=1;

int target=1;

while(!check){ *Note: a do...while will work, too, if it makes you more comfortable.

*second loop goes here, much like the one you put up earlier*

I used a "break" condition if (target%x!=0) here, and an else-if to check if x==20 and change the boolean to "true."

If the loop exited and check was still false, I incremented the target.

}

Then I spat it to the command line, and done.

It took forever to run: the answer is HUGE.

Hope this helps. :D

Ask yourself this question: what math is the program attempting when n = 0?

 

(There are other problems with the program, but they aren't what's making it crash.)

GabuEx

You're going to need 2 loops for this one: one to increment your target number, and one to loop over your divisions.

Try:

bool check=false;

int x=1;

int target=1;

while(!check){ *Note: a do...while will work, too, if it makes you more comfortable.

*second loop goes here, much like the one you put up earlier*

I used a "break" condition if (target%x!=0) here, and an else-if to check if x==20 and change the boolean to "true."

If the loop exited and check was still false, I incremented the target.

}

Then I spat it to the command line, and done.

It took forever to run: the answer is HUGE.

Hope this helps. :D

OrkHammer007

Just a quick rule of thumb when it comes to loops:

do-whiles should only be used when you want your program to execute at least one time.

while loops should be used if there is the possibility of your program never executing a certain method. (if the first boolean argument fails, the method will not execute).

for loops should only be used if you know the exact number of times you want to iterate through an algorithm.

I'm sure most of you know this, but just in case =]

Hopefully, you can read the code from the screen shot; since the forum code doesn't like C++/Java, it'll probably work a bit better like this.

It's written in Java (I'm a bit more comfortable with it), but it should be simple to convert it to C++.

It took about 25 seconds to run, and it came up with a figure in 9 digits... :shock:

Hopefully, this helps. :D

Hopefully, you can read the code from the screen shot; since the forum code doesn't like C++/Java, it'll probably work a bit better like this.

It's written in Java (I'm a bit more comfortable with it), but it should be simple to convert it to C++.

It took about 25 seconds to run, and it came up with a figure in 9 digits... :shock:

Hopefully, this helps. :D

OrkHammer007

    int x=20;
    int y=0;
    for( int n=20; n > 1; n-- )
    {
         while( (x % n) != 0 )
         {
                x = x + y;
         };
         y = x;
    };

I figured that my mistake would be easily corrected by just incrementing x by it's last known value instead of by 1. And even better, the code is simpler and it rendered instantly :) thanks for your help and sorry i wasn't able to consider your contribution :?

By the by, I hope i'm not annoying you guys with my constant questions? :?

since (0 % 0) == 0HardQuor

Nope. x % 0 for any x is asking "what is the remainder when x is divided by 0"? Division by 0 is obviously not allowed, which thus causes your program to crash.

oops, i didn't get to see your post before i decided to see my methodology to the end :?

but i do have good news, i got it!

 

    int x=20;
    int y=0;
    for( int n=20; n > 1; n-- )
    {
         while( (x % n) != 0 )
         {
                x = x + y;
         };
         y = x;
    };

I figured that my mistake would be easily corrected by just incrementing x by it's last known value instead of by 1. And even better, the code is simpler and it rendered instantly :) thanks for your help and sorry i wasn't able to consider your contribution :?

By the by, I hope i'm not annoying you guys with my constant questions? :?

HardQuor

Actually, there's an even easier way to do this that doesn't need two loops or extra variables:

    int x = 1;

    for (int n = 2; n (less than or equal to) 20; n++)
    {
         if (x % n != 0)
         {
                x++;
                n = 1;
         }
    }

This basically loops through all values of n, and if it finds that x is not divisible by one of the values, then it increments x by 1 and starts over.

Of course, this is very brute force, and there is a very elegant way of figuring it out if you add in a bit of smart math, but this nonetheless works.

I figured that my mistake would be easily corrected by just incrementing x by it's last known value instead of by 1. And even better, the code is simpler and it rendered instantly :) thanks for your help and sorry i wasn't able to consider your contribution :?

By the by, I hope i'm not annoying you guys with my constant questions? :?

HardQuor

Don't worry about annoying me: I tutored programming (VB, C++, and Java) to pay part of my tuition, and I love a good coding challenge (even if it aggravates my insomnia :lol: ). Ask away!

Oh... and I think GS is running part of your code: somehow, you quadruple-posted. :shock:

Actually, there's an even easier way to do this that doesn't need two loops or extra variables:GabuEx

[QUOTE="HardQuor"]since (0 % 0) == 0GabuEx

Nope. x % 0 for any x is asking "what is the remainder when x is divided by 0"? Division by 0 is obviously not allowed, which thus causes your program to crash.

Actually, there's an even easier way to do this that doesn't need two loops or extra variables:

    int x = 1;

    for (int n = 2; n (less than or equal to) 20; n++)
    {
         if (x % n != 0)
         {
                x++;
                n = 1;
         }
    }

GabuEx

This basically loops through all values of n, and if it finds that x is not divisible by one of the values, then it increments x by 1 and starts over.

Of course, this is very brute force, and there is a very elegant way of figuring it out if you add in a bit of smart math, but this nonetheless works.

GabuEx

...I'm still trying to figure out why that worked... OrkHammer007

I'm a bit dense at the moment: programming challenges tend to cost me sleep (one time, while I was taking C++ 1 in college, I was at the PC for 12 straight hours because I literally couldn't sleep until I figured out the method that output a pyramid on the screen).

 

OrkHammer007

Don't worry about annoying me: I tutored programming (VB, C++, and Java) to pay part of my tuition, and I love a good coding challenge (even if it aggravates my insomnia :lol: ). Ask away!

OrkHammer007

Oh... and I think GS is running part of your code: somehow, you quadruple-posted. :shock:

OrkHammer007

was that? could it be? did you complement my code, or am i delusional? :lol:HardQuor

Well, yours was a bit better, but there's actually a very much more streamlined way of doing this.  If you're curious about what that is, it comes from considering the prime numbers that make up each number from 2 to 20 (we'll ignore 1 since everything is divisible by 1).  For example:

2 = 2
3 = 3
4 = 2 x 2
5 = 5
6 = 2 x 3

...and so on.

The smallest number divisible by all the numbers from 2 to 20 is the number that includes as its factors the prime numbers that make up each number, and no more.  So, for example, the smallest number divisible by the numbers 2 to 6 would be:

2 x 3 x 2 x 5 = 60

To make it divisible by 2, you add 2; for 3, you add 3; for 4, you add 2 (since one 2 was already present); for 5, you add 5; and for 6, you add nothing (since both 2 and 3 were already present).

So, basically, what you need to do is just start a list of prime factors as an empty set, and then find the prime factors of all of the numbers from 2 to 20 and then test to see whether or not they're already in the list.  Once you've done this for all numbers, you then just multiply all of the prime numbers together and you have your answer.

This requires code that's rather more complicated than what we've got here, though.  Just for fun, though, I implemented it as a proof of concept; it pretty much instantly finds the answer, even if you make it much larger and have it find the smallest number divisible by 1 to 500.

I thought about trying the Project Euler thing when you brought it up.

On the one hand, I like the programming part. If I didn't, then I put a lot of effort into getting a degree in something I hate (and got near-straight A's in it, too... bloody interface design :evil: )... :lol:

On the other hand, I don't do well with numbers (something that might baffle other programmers, and my math teachers in college). It took me several tries to finally understand algebra (high school, two math courses in the Navy, then it finally clicked in college... where I tutored math, too... :? ), and arithmetic can get me in trouble (9 * 7 != 16... I know it, I feel it, but it's even money if I actually write it correctly).

I'll ponder some more... I'm leaning towards "go for it," though. ;)

Well, yours was a bit better, but there's actually a very much more streamlined way of doing this.  If you're curious about what that is,

...

This requires code that's rather more complicated than what we've got here, though.  Just for fun, though, I implemented it as a proof of concept; it pretty much instantly finds the answer.

 

GabuEx

Yeah, that's way over my head :( I'll just consider my solution a good compromise between conciseness and elegance :PHardQuor

Well, it's actually not too conceptually difficult.  I probably just explained it badly.  There are two main concepts that need to be understood with what I was talking about:

1. First, there are prime numbers (numbers which are divisible by only 1 and itself), and there are composite numbers (numbers which are divisible by more than that).  Any composite number can be represented as a product of prime numbers (for example, 12 = 2 * 2 * 3).

2. Second, to say that x is divisible by y is to say that all prime factors of y are also prime factors of x.  For example, 12 is divisible by 4 because 12 = 2 * 2 * 3, while 4 = 2 * 2 - all prime factors of 4 are also prime factors of 12.

See if what I said makes any more sense in light of those two things (or tell me if the above also makes no sense).

Try this instead:

for(checking = 1; checking < 1000; ++checking)
{
   if(checking%3==0||checking%5==0)sum=sum+checking;
}

I guess C++/Java doesn't agree with GS.

OrkHammer007

No type of formatting text agrees with GS, it would seem.KOTORkicker