Nested loops in c++ and arrays - c++

Question

I am writing this program in c++ and unable to determine what am I doing wrong. This nested loop is supposed to print a matrix with rows and columns and for some reason its stuck at row 0 and column 0 when asking for user input. Thanks in advance.
#include <iostream>
using namespace std;
int main ()
{
//program to print the input values in rows and columns of two dimensional array in revserse roder using loop
//decalre the vraibles and array
int array [3][3],rows,cols,maxrows = 3,maxcols = 3;
//use for loop to intiiate the array and input values from users
for (rows = 0; rows < maxrows;rows = rows++ )
{
for (cols = 0; cols < maxcols;cols = cols++ )
{
cout << " Enter the value for array location : " << " [ " << rows << " , " << cols << " ] " ;
cin >> array [rows][cols] ;
}
}
//display the values entered in array starting with bottom row first and then the rest
for ( rows = 0 ; rows < maxrows ; rows ++ )
{
for ( cols = 0 ; cols < maxcols ; cols ++ )
{
cout << " The values that were entered in this array starting with bottom row are " << array [rows][cols] ;
}
}
}

Answer 1

You already have your answer, but I thought I should point out that the style of variable declaration. Namely, int array [3][3],rows,cols,maxrows = 3,maxcols = 3;
This is bad because it is not very readable. The type of the row and column variables are incorrect in the strictest sense, see why size_t is better when using it as an array index. Additionally, a case could be made for why you should favour pre-increment instead of post-incrementing unless you have a good reason not to (++i instead of i++), although in this particular case it does not matter since the operator is not overloaded. Furthermore, using constants appropriately makes code more readable and eliminates certain types of bugs. Lastly, give your variables a sensible value as soon as possible to eliminate undefined behaviour.
#include <iostream>
using namespace std;
int main ()
{
//program to print the input values in rows and columns of two dimensional array in revserse roder using loop
//decalre the vraibles and array
int array [3][3] = {};
const size_t maxrows = 3,
const size_t maxcols = 3;
//use for loop to intiiate the array and input values from users
for (size_t rows = 0; rows < maxrows; ++rows )
{
for (size_t cols = 0; cols < maxcols; ++cols )
{
cout << " Enter the value for array location : " << " [ " << rows << " , " << cols << " ] " ;
cin >> array [rows][cols] ;
}
}
//display the values entered in array starting with bottom row first and then the rest
for ( size_t rows = 0; rows < maxrows ; ++rows)
{
for ( size_t cols = 0; cols < maxcols ; ++cols)
{
cout << " The values that were entered in this array starting with bottom row are " << array [rows][cols] ;
}
}
}

Answer 2

The comments hinted at the problem, but did not state it explicitly: the statement
rows = rows++
is ambiguous because the right-side is incremented after fetching the value which is then assigned to the left-side. With suitable warnings enabled for g++, it says
foo.c:26: warning: operation on ‘rows’ may be undefined
In effect, it says that some compilers may give differing results, such as making it no different from
rows = rows
If it did that, it would cause an infinite loop. (With g++, by the way, the value does get incremented).