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#1 TroyDowling  Icon User is offline

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Help with Dereference Operators - Project Euler Problem 8

Posted 14 September 2010 - 05:16 PM

Hello,

I'm pretty new to C++ and I have been having trouble getting my head around the idea of (de)reference operators. I've been doing the ProjectEuler.net problems and am stuck on #8 now due to not understanding these operators. Here is my code and output for Problem 8.


/*

Troy Dowling, TroyDowling.org
GNU/GPL

ProjectEuler.net - Problem 3

Find the greatest product of five consecutive digits in the 1000-digit number.

73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450

*/

#include <iostream>

using namespace std;

const char* bigNum = "\
73167176531330624919225119674426574742355349194934\
96983520312774506326239578318016984801869478851843\
85861560789112949495459501737958331952853208805511\
12540698747158523863050715693290963295227443043557\
66896648950445244523161731856403098711121722383113\
62229893423380308135336276614282806444486645238749\
30358907296290491560440772390713810515859307960866\
70172427121883998797908792274921901699720888093776\
65727333001053367881220235421809751254540594752243\
52584907711670556013604839586446706324415722155397\
53697817977846174064955149290862569321978468622482\
83972241375657056057490261407972968652414535100474\
82166370484403199890008895243450658541227588666881\
16427171479924442928230863465674813919123162824586\
17866458359124566529476545682848912883142607690042\
24219022671055626321111109370544217506941658960408\
07198403850962455444362981230987879927244284909188\
84580156166097919133875499200524063689912560717606\
05886116467109405077541002256983155200055935729725\
71636269561882670428252483600823257530420752963450";

int length = 1000;
int product = 0;
int v,w,x,y,z;

int main()
{
    for(int i = 0; i <= length-5; i++)
    {
        v = *(bigNum+i);
        w = *(bigNum+i+1);
        x = *(bigNum+i+2);
        y = *(bigNum+i+3);
        z = *(bigNum+i+4);
        cout << *(bigNum+i) << " ";
        cout << *(bigNum+i+1) << " ";
        cout << *(bigNum+i+2) << " ";
        cout << *(bigNum+i+3) << " ";
        cout << *(bigNum+i+4) << " || ";
        cout << v << " x " << w << " x " << x << " x " << y << " x " << z << " = " << v*w*x*y*z << endl;

        if(v*w*x*y*z > product)
        {
            product = v*w*x*y*z;
        }
    }

    cout << "The largest product of five consecutive digits is: " << product;

    return 0;
}



And the output:

...
4 2 0 7 5 || 52 x 50 x 48 x 55 x 53 = 363792000
2 0 7 5 2 || 50 x 48 x 55 x 53 x 50 = 349800000
0 7 5 2 9 || 48 x 55 x 53 x 50 x 57 = 398772000
7 5 2 9 6 || 55 x 53 x 50 x 57 x 54 = 448618500
5 2 9 6 3 || 53 x 50 x 57 x 54 x 51 = 415991700
2 9 6 3 4 || 50 x 57 x 54 x 51 x 52 = 408142800
9 6 3 4 5 || 57 x 54 x 51 x 52 x 53 = 432631368
6 3 4 5 0 || 54 x 51 x 52 x 53 x 48 = 364321152
The largest product of five consecutive digits is: 570394440



Obviously, the std::cout statements are reading the memory how I want, but the variable assignment statements are not. I have no idea why this is, and no matter how many times I read the pointers tutorial page, or the wiki, I just cannot understand them! Any help, or pokes in the right direction are appreciated.

Troy.

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Replies To: Help with Dereference Operators - Project Euler Problem 8

#2 baavgai  Icon User is offline

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Re: Help with Dereference Operators - Project Euler Problem 8

Posted 14 September 2010 - 05:41 PM

You don't have an array of integers, but an array of characters. Your values are the internal numeric associated with the character, called ASCII.

Consider:
char ch = '1';
cout << ch << endl;
cout << (int)ch << endl;



It's the int cast value you're getting when you add it up for an int result.
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#3 Kilorn  Icon User is offline

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Re: Help with Dereference Operators - Project Euler Problem 8

Posted 14 September 2010 - 05:45 PM

Someone else had an issue with numbers returning the ASCII value earlier too. Seems like a popular problem today. I believe baavgai helped that person as well. Just keep in mind, as the wise baavgai has pointed out, that without casting the value to an int, returning a character that you think is a number will return the ASCII value of that number rather than the actual value of the number.


Here's a table with ASCII values on it, just in case anyone finds it useful or interesting.

http://www.asciitable.com/

This post has been edited by Kilorn: 14 September 2010 - 05:47 PM

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#4 TroyDowling  Icon User is offline

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Re: Help with Dereference Operators - Project Euler Problem 8

Posted 14 September 2010 - 06:07 PM

Ohhhh... that makes complete sense now! So I just need to be more careful with my types from now on. Alright, I'll tackle the problem after dinner then. Thanks to both; I'll let you know how it goes. =)
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#5 TroyDowling  Icon User is offline

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Re: Help with Dereference Operators - Project Euler Problem 8

Posted 14 September 2010 - 10:21 PM

Alright, awesome, the code works wonders now. I found some good information on casting char to int over at this website and used it in the code. So, the only thing I have left that bugs me is that I have to manually enter the value of 'length'. I suppose because of the dereference operators, my usual method doesn't work. I normally would have used
int length = sizeof(bigNum)/sizeof(char);
but that yields incorrect output. So, if I can solve that, it would be awesome. The only other thing I wanted to ask, was for some general insight. Do you think this code is acceptable for the task? Is there anywhere I could improve the simplicity of the program? Also, since I struggle with the use of reference and dereference operators, can you recommend a good tutorial on them? I'd like to be able to use them well. Again, thanks for the help.


/*

Troy Dowling, TroyDowling.org
GNU/GPL

ProjectEuler.net - Problem 3

Find the greatest product of five consecutive digits in the 1000-digit number.

73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450

*/

#include <iostream>

using namespace std;

const char* bigNum = "\
73167176531330624919225119674426574742355349194934\
96983520312774506326239578318016984801869478851843\
85861560789112949495459501737958331952853208805511\
12540698747158523863050715693290963295227443043557\
66896648950445244523161731856403098711121722383113\
62229893423380308135336276614282806444486645238749\
30358907296290491560440772390713810515859307960866\
70172427121883998797908792274921901699720888093776\
65727333001053367881220235421809751254540594752243\
52584907711670556013604839586446706324415722155397\
53697817977846174064955149290862569321978468622482\
83972241375657056057490261407972968652414535100474\
82166370484403199890008895243450658541227588666881\
16427171479924442928230863465674813919123162824586\
17866458359124566529476545682848912883142607690042\
24219022671055626321111109370544217506941658960408\
07198403850962455444362981230987879927244284909188\
84580156166097919133875499200524063689912560717606\
05886116467109405077541002256983155200055935729725\
71636269561882670428252483600823257530420752963450";

// Length, in number of place values, of bigNum
int length = 1000;
// These variables "crawl" along bigNum in sequence. I.e., abcde, bcdef, cdefg...
int v,w,x,y,z;
// The product of v,w,x,y,z and our answer
int product = 0;

int main()
{
    for(int i = 0; i <= length-5; i++)
    {
        // Assign v though z the value pointed by bigNum+i+(0 through 4)
        v = *(bigNum+i);
        w = *(bigNum+i+1);
        x = *(bigNum+i+2);
        y = *(bigNum+i+3);
        z = *(bigNum+i+4);
        // Since bigNum is actually a char array, we need to typecast these to their ASCII literals (the actual digits 0-9). Substracting '0' from the variable does exactly this!
        v -= '0';
        w -= '0';
        x -= '0';
        y -= '0';
        z -= '0';

        if(v*w*x*y*z > product)
        {
            product = v*w*x*y*z;
        }
    }

    cout << "The largest product of five consecutive digits is: " << product;

    return 0;
}


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#6 baavgai  Icon User is offline

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Re: Help with Dereference Operators - Project Euler Problem 8

Posted 15 September 2010 - 04:23 AM

Good, you figured the one part out. Did you ever actually do this:
cout << bigNum << endl;



If you did, you might have spotted your problem. Here's a sanity check:
const int length = 1000;
// the +1 is to account for the zero
const char bigNum[length+1] = "\



If you don't get a compile fail and the output looks like a continuous number, you're probably good.

Globals are bad, btw. Also, int v,w,x,y,z; seems much. Perhaps you could use a loop?
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