Find first positive elemnt in 2d array [closed] - c++

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Maybe someone can help. The task is to find first positive element in 2D array and in which row is is located. But my program doesn't stop at the first one and keep searching while outputting the last one.
And second question, why my 2D array output not in rows and columns, but in one line.
#include<iostream>
#include<iomanip>
using namespace std;
int main()
{
int n, m;
cout << "Enter row count ";
cin >> n;
cout << "Enter column count ";
cin >> m;
float **arr = new float*[n];
for (int i = 0; i < n; i++)
arr[i] = new float[m];
srand(time(NULL));
for (int i = 0; i < n; i++)
for (int j = 0; j < m; j++)
{
cout << "Enter elements [" << i << "][" << j << "]: ";
cin >> arr[i][j];
}
for (int i = 0; i < n; i++)
for (int j = 0; j < m; j++)
{
cout << setw(4) << arr[i][j] << " ";
}
cout << endl;
int rowNumber;
for (int i = 0; i < n; i++)
for (int j = 0; j < m; j++)
{
if (arr[i][j] >= 0)
{
rowNumber = i;
break;
}
}
cout << "First positive elemnt is in row " << rowNumber << endl;
return 0;
}

Answer 1

Find first positive element
Remember that break only stops the loop it is part of. Any outside loops continue on their merry way.
There are two three ways to deal with this: goto, flags, and return.
for (...rows...)
{
for (...columns...)
{
if (...found it...)
goto done; // break from the inner ‘columns’ loop
} // AND from the outer ‘rows’ loop
}
done:
JSYK, people will burn you as a witch if you use goto.
You could use a little boolean flag in the outer loop(s) to quit early:
bool done = false;
for (int i = 0; (i < n) and !done; i++)
{
for (int j = 0; (i < m) and !done; j++)
{
if (...found it...)
{
done = true;
}
}
}
A “pro” to using a flag is that it will tell you whether or not you found what you were looking for: if done is still false after the loops then no positive value was found.
I personally think that flags add unnecessary complexity to the code: the computer has to do more work and it is harder to visually separate out the mechanism of the algorithm over the extra bookkeeping.
My recommendation: create yourself a little function instead, and use return:
int get_row_of_first_positive_value( float** arr, int nrows, int ncolumns )
{
for (int row = 0; row < nrows; row++)
for (int col = 0; col < ncolumns; col++)
{
if (arr[row][col] >= 0)
{
return row;
}
}
}
return -1; // no positive element found
}
You don’t need a sentinel value (like -1) to use a function. In this case it works, since -1 is not a valid row number, but you can easily return a std::optional value or use any number of other ways to return a success/failure flag.
As an aside: names matter. Most people will recognize things like i, j, n, m, etc, but even so it makes life much more readable to use better names. Here it is abundantly obvious that we are returning the row where the positive element was found.
Back in main you can say:
int rowNumber = get_row_of_first_positive_value( arr, n, m );
Print newlines between rows
Again this is a nested loop problem. The inner loop prints elements of the row. These are separated by spaces (or tabs or whatever).
for (int col = 0; col < ncolumns; col++)
{
std::cout << std::setw(4) << arr[row][col];
std::cout << " "; // printed after every element in the row
} // in order to separate columns
But the elements of the outer loop (the rows) are separated by newlines.
for (int row = 0; row < nrows; row++)
{
...print the row here...
std::cout << "\n"; // printed after every row in the matrix
} // in order to separate rows
See how the two structures are similar. Recommendation, write yourself a helper function just to print the array:
void print_matrix( float** arr, int nrows, int ncolumns )
{
...
}
Then you can use it in main easily:
print_matrix( arr, n, m );
int rowNumber = get_row_of_first_positive_value( arr, n, m );
if (rowNumber < 0)
std::cout << "Alas, ...\n";
else
std::cout << "First positive elemnt is in row " << rowNumber << "\n";
}