C++ string confusion

Question

I would like to say thanks for your patience to before hand. I am just starting to learn how to code with some background knowledge from a 101 cs course I took beforehand, so I may be making some fundamental mistakes. Here is the question I am trying to solve:

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

I will write the error I am receiving first: I get the following error when I call the findmax function that I've created:

error: could not conver '(std::string*)(&arr)' from 'std::string* {aka
  std::basic_st...}

And here is my code I've written so far:

#include <iostream>
#include <string>

using namespace std;
void findmax(string test);
int main()
{
  int assignNum = 0;
  int n2;
  string p =     "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450";
  string arr[995];
  string plast = "";

  //FOR LOOP TO LIST ALL CONSECUTIVE INTEGERS OF 5 DIGITS
  for (assignNum = 0; assignNum < 995; assignNum++) {  //Sliding up initial digit
    plast = "";

    for (int j =0; j<5; j++) {

      plast = plast + p[assignNum + j];

    }
    arr[assignNum]= plast;

  }
  cout << sizeof(arr)/sizeof(arr[0]) << endl;
  findmax(arr); // THE ERROR REFERS TO HERE ******!!!!!!
}

//FIND MAX FUNCTION USING RECURSION
void findmax(string test) {
  string right="";
  int k;
  int stringSize = sizeof(test)/sizeof(test[0]);
  int middle = (sizeof(test)/sizeof(test[0]))/2;
  for (k=0; k<=stringSize; k++){
    if (test[k] >= test[middle]){
      right = right + test[k];
    }
    else {
      cout << "do nothing " << endl;
    }
    if (stringSize != 2){
      findmax(right);
    }
    else {
      if (test[0] >= test[1]){
        cout << test[0]<< endl;
        break;
      } 
      else {
        cout <<  test[1] << endl;
        break;
      }
    }
  } 
}

Any help would be greatly appreciated, I've been working on this for quite a while and really want it to work.

For anyone interested, I solved the problem, which I actually misread at the start. Here is my code (without trying to use recursion):

#include <iostream>
#include <string>
#include <stdio.h> /* printf, fgets */
#include <stdlib.h> /* atoi */
#include <math.h>

using namespace std;
void findmax(string test[]);
int main()
{
int assignNum = 0;
int n2;
int ntp;
int ltp = 0;
string p = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450";

string pnew = "";

//FOR LOOP TO LIST ALL CONSECUTIVE INTEGERS OF 5 DIGITS
for (assignNum = 0; assignNum < 995; assignNum++){ //Sliding up initial digit

pnew = "";

for (int j =0; j<5; j++){

pnew = pnew + p[assignNum + j];
}

string myString = pnew;
int value = atoi(myString.c_str()); //value = 45

ntp = ((value / 10000) % 10) *((value / 1000) % 10) * ((value /100) % 10) * ((value / 10)% 10) * ((value / 1) % 10);
cout << (value / 10000) % 10 << endl;
cout << (value / 1000) % 10 << endl;
cout << (value / 100) % 10 << endl;
cout << (value / 10) % 10 << endl;
cout << (value / 1) % 10 << endl;

cout << "this is ntp: " << ntp << endl;

if (ltp>=ntp){
cout << "ltp: " << ltp << "is >= ntp: " << ntp << endl;

}
else {
cout << "ntp: " << ntp << "is > ltp: " << ltp << endl;
ltp=ntp;
}
cout << ltp << endl;
}

cout << "THE LARGEST CONSECUTIVE 5 DIGIT PRODUC IS: " << ltp << endl;

}



Answer: 40824 
execution time: .986s

Answer 1

p is one std::string variable that is instantiated using the (const char *) value. arr is an array of std::string that can hold up to 995 independent std::string values. findmax is a function returning void that accepts a copy of std::string value. So when you try passing arr to findmax, the compiler which only knows about a findmax that accepts std::string gets confused on being invoked with an array of std::string.

You are probably getting confused with the std::string being initialized with a char [] or char *. 73167176531330624919225 ... can either be treated as a char arr[995] or a char *arr with the appropriate code modifications so that when findmax is invoked, a std::string object gets constructed.

PS: Also, when passing around non-pod values to functions you are better off passing them as a reference as to avoid unnecessary copying:

void findmax(std::string & test)   // If you want to manipulate value in function

OR

void findmax(const std::string & test)    // To avoid surprises in client code

Answer 2

findmax expects a string, but you're feeding it a string [].

You can fix the prototype and the signature:

void findmax(string test[]);
void findmax(string test[]) {

But you do it in another location:

string right="";
// ...
findmax(right); // right is not an array

I suspect your intention is something like this:

std::string res = ""; // build string out of array
for (int i = 0; i < sizeof(arr)/sizeof(arr[0]); i++)
    res += arr[i];
findmax(res);

But I get a segmentation fault, so there is something wrong with your algorithm regardless.