Need help.

[Newbie992]

I was told to make a 2 dimensional array program that get its highest/maximum number.
When I try to run this code it gives me random number.

#include <iostream>

using namespace std;

int main()
{
    int x[4][5];
    int maximum = x[0][0];
    for (int i=0;i<4;i++){
        for (int o=0;o<5;o++){
            cout<<"Enter number: ";
            cin>>x[i][o];
            if (x[i][o] > maximum){
                maximum = x[i][o];
            }
        }
    }
    cout<<"Maximum number is "<<maximum<<endl;
    return 0;
}

[Repeater]

int maximum = x[0][0];

What value will this be? What value does maximum begin with?

[Newbie992]

Supposed to be the first one that the user entered.

I changed the code to:

#include <iostream>

using namespace std;

int main()
{
    int x[4][5];
    int maximum;
    for (int i=0;i<4;i++){
        for (int o=0;o<5;o++){
            maximum = x[0][0];
            cout<<"Enter number: ";
            cin>>x[i][o];

            if (x[i][o] > maximum){
                maximum = x[i][o];
            }
        }
    }
    cout<<"Maximum number is "<<maximum<<endl;
    return 0;
}

Now it takes the last number the user inputted.

[Repeater]

Now you're setting maximum to x[0][0] over and over and over and over again.

One option is to set maximum to the lowest possible value when it is created. What's the lowest possible int value? std::numeric_limits<int>::min() http://www.cplusplus.com/reference/limits/numeric_limits/

Another option is to let the user enter all their values, and THEN set maximum to x[0][0], and then look through the array again to find the maximum.

[lastchance]

#include <iostream>

using namespace std;

int main()
{
    int x[4][5];
    int maximum;
    for (int i=0;i<4;i++){
        for (int o=0;o<5;o++){
            cout<<"Enter element [" << i << "][" << o << "]: ";   // please prompt
            cin>>x[i][o];
            if ( i + o == 0 ) maximum = x[0][0];     // only initialise maximum when you can and should
            if (x[i][o] > maximum)
            {
                maximum = x[i][o];
            }
        }
    }
    cout<<"Maximum number is "<<maximum<<endl;
    return 0;
}

[Newbie992]

@lastchance

Can you explain what does this line do?
Im kinda new to 2d arrays.

 if ( i + o == 0 ) maximum = x[0][0];

[jonnin]

you can collapse a 2d array to a 1-d due to abuse of how c++ internally stores memory for arrays for this kind of iteration. It does not matter if you find the max as you go when loading the structure but if you happened to want to get the max after some process had modified the contents or the like...

int * cheat = &x[0][0];
for(int i = 0; i< maxrow*maxcol; i++)
{
find max as if 1-d here
}

this is one thing I don't think you can do with vectors of vectors.

[lastchance]

Newbie992 wrote:

Can you explain what does this line do?

In this instance, since i and o have minimum values 0, the line is equivalent to
if ( i == 0 && o == 0 ) maximum = x[0][0];

As @Repeater pointed out very early on, you can't set the maximum to x[0][0] until x[0][0] has itself been initialised and that line allows it to be done exactly when necessary and when possible ... IF you choose to do it within the loop.

However, finding a maximum is a separate thing, and I'm inclined to think it would be better to do it AFTER all the data has been input. (Indeed, there's a good argument for doing it in a separate function.)

The following code shows you two ways of getting the maximum after all input: from first principles using nested loops, the second using a standard-library routine (you can look it up). Since arrays like this use contiguous storage (i.e. all elements one after another) you can also do what @jonnin suggested and fake a 1-d array with MAXI*MAXJ elements, so avoiding nesting loops: this is actually what the max_element routine is doing.

Note that I have done a couple of things that make the code a bit easier on the eyes:
- changed variable o (easily confused with 0) to j;
- avoided slightly arbitrary numbers like 4 and 5 and named them as MAXI and MAXJ; then if you want to change them you need only do so in one place. (I've also made them a bit smaller so as to shorten the input during testing).

#include <iostream>
#include <algorithm>

using namespace std;


int main()
{
   const int MAXI = 3, MAXJ = 3;       // try not to use "magic numbers": fix them just once
   int x[MAXI][MAXJ];


   // Enter numbers
   for ( int i = 0; i < MAXI; i++ )
   {
      for ( int j = 0; j < MAXJ; j++)
      {
         cout << "Enter element [" << i << "][" << j << "]: "; 
         cin >> x[i][j];
      }
   }



   // Find maximum
   int maximum = x[0][0];              // initialise to first element
   for ( int i = 0; i < MAXI; i++ )
   {
      for ( int j = 0; j < MAXJ; j++)
      {
         if ( x[i][j] > maximum ) maximum = x[i][j];
      }
   }
   cout << "Maximum number is " << maximum << endl;



   // Find maximum (alternative method)
   int *p = &x[0][0];
   cout << "Maximum number is " << *max_element( p, p + MAXI * MAXJ ) << endl;
}