Nested For-loops problem

stguisy · #1 14-Oct-2005, 08:22

I am attempting to create a code where the user inputs an equal number of rows and clumns and the output is the identity matrix. For example, with 3 rows and 3 columns, the output would look like this:

100
010
001

I am required to use nested for-loops. Here is what I thought would work, but won't:

CPP / C++ / C Code:

//identityMatrix.cpp

#include<iostream>
using namespace std;

int main(void)
{
   int rows;
   int columns;
   while(rows != columns)
   {
   cout << "Columns and rows must be equal. \n Please enter number of rows: ";
   cin >> rows;
   cout << endl << "Please enter number of columns: ";
   cin >> columns;
   cout << endl;
   }
   for(int i = 1; i < (rows + 1); i = i + 1)
   {
      for(int j = 1; j < (columns + 1); j = j + 1)
      {
	 if(i = j)
	    cout << "1";
	 else
	   cout << "0";	  
      }
      cout << endl;
   }
}

Any help I could get would be much appreciated. Thank you!

Paramesh · #2 14-Oct-2005, 09:48

Hi,

Just change

CPP / C++ / C Code:

 if(i = j)
    cout << "1";

to this:

CPP / C++ / C Code:

 if(i == j)
    cout << "1";

I hope you understand what error you have committed.

The double equals sign == is the C notation for ``is equal to''. This symbol is used to distinguish the equality test from the single = that C uses for assignment. A word of caution: The result is usually a legal expression, so you will get no warning.

Paramesh.

stguisy · #3 14-Oct-2005, 10:14

Thank you so much, Paramesh! I am so new at this and I never realize when my silly mistakes are there.

Paramesh · #4 14-Oct-2005, 10:28

You are Welcome.

and Read this $ 20 Million Bug story:

Quote:

The $20 Million Bug:
In Spring 1993, in the Operating System development group at SunSoft, we had a "priority one" bug report come in describing a problem in the asynchronous I/O library. The bug was holding up the sale of $20 million worth of hardware to a customer who specifically needed the library functionality, so we were extremely motivated to find it. After some intensive debugging sessions, the problem was
finally traced to a statement that read :

CPP / C++ / C Code:

x==2;

From: Expert C Programming-Deep C Secrets
By Peter van der Linden

Cheers,
Paramesh.

monalin · #5 14-Oct-2005, 13:18

What you have works, but just a couple notes on possibly making it easier to read and easier to program.

instead of useing

CPP / C++ / C Code:

  for(int i = 1; i < (rows + 1); i = i + 1)
   {
      for(int j = 1; j < (columns + 1); j = j + 1)
      {
	 if(i = j)
	    cout << "1";
	 else
	   cout << "0";	  
      }

try this... it does the same thing you can replace
--------------------------------
i < (rows + 1) with i <= rows
--------------------------------
j < (columns + 1) with j <= columns
--------------------------------

Also cpp as a build in operator for adding or subtracting 1 from a number, for example.
--------------------------------
i = i + 1 is the same as i++
--------------------------------
i = i - 1 is the same as i--
--------------------------------

also a couple more handy operators are += and -= there are also *= and /= operators that work in the same way.
--------------------------------
i = i + n is the same as i += n
--------------------------------
i = i - n is the same as i -= n
--------------------------------

CPP / C++ / C Code:

  for(int i = 1; i <= rows; i++)
   {
      for(int j = 1; j <= columns; j++)
      {
	 if(i == j)
	    cout << "1";
	 else
	   cout << "0";	  
      }

Not sure if u will ever check back on this post but those are just a couple of operators in c++ that you might find handy. They make my life much easier, hope it helps you too.

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Nested For-loops problem

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