SA119 – Agnel Kurian's blog

FORFILES command

There is such a command on windows. Cool.

Seeing this message when trying to exit bash means some interactive processes launched from this terminal are still running in the background. Use the jobs command to list them. Use fg <n> to bring job number n to the foreground and exit the process properly.

As an example, if a command being piped to less exits and less is left hanging without anything to show, it goes to the background. Example:

agnel@Carmen:~$ non-existent-command | less






non-existent-command: command not found
(END)
[1]+  Stopped                 non-existent-command | less
agnel@Carmen:~$

agnel@Carmen:~$ non-existent-command | less

non-existent-command: command not found

(END)

[1]+ Stopped non-existent-command | less

agnel@Carmen:~$

This means that less is still running in the background waiting for input. Here’s how to bring it to the foreground and exit it.

agnel@Carmen:~$ jobs
[1]+  Stopped                 non-existent-command | less
agnel@Carmen:~$ fg 1
non-existent-command | less

agnel@Carmen:~$ jobs

[1]+ Stopped non-existent-command | less

agnel@Carmen:~$ fg 1

non-existent-command | less

In this case, after fg 1, it is not immediately clear what to do. But since I know that it is less that’s running now, I hit q (since q is the less keystroke to quit).

Object Relationships using smart pointers and “Modern C++”

Don’t use raw pointers as class members; that requires managing object life time and manually avoiding dangling pointers
Use smart pointers instead of raw pointers
Don’t use object references as class members as that makes objects of the class non-assignable (which means you cannot store said objects in standard C++ containers)
Using raw pointers and object references to objects stored in standard C++ containers is problematic or impossible. Objects in containers can be relocated (addresses can change) rendering raw pointers and object references invalid.
Best (only feasible?) option is to store std::shared_ptr of the dynamically allocated object in the container.
std::shared_ptr is reference counted std::weak_ptr is not
Use std::make_shared to create an std::shared_ptr to a dynamically created object instance or use std::shared_ptr(new T())
Only way to access members via std::weak_ptr is by calling std::weak_ptr::lock which returns an std::shared_ptr or 0
To create an std::weak_ptr from the this pointer, the class must first derive from std::enable_shared_from_this and then call the shared_from_this function to get an std::shared_ptr pointing to the object. The object in question must be “owned” by an std::shared_ptr or else undefined behaviour results. Also, shared_from_this must not be called from a constructor since the object is yet to be reference counted (i.e. object is not yet “owned” by an std::shared_ptr)
As a result of the previous point, it helps to ensure that objects can only be created via std::shared_ptr by using a public static create function (which returns objects “owned” by an std::shared_ptr) and making constructors private. Note that when constructors are made private, you can no longer use std::make_shared to create a shared pointer. You must use std::shared_ptr(new T()) in this case.

Modern C++ Object Relationships

Sage Quick Reference

Run a file:

%runfile filename.sage

1	%runfile filename.sage

Create matrices:

matrix(RR, nrows, ncols)  #  real ring
matrix(SR, nrows, ncols)  #  symbolic ring
matrix([[a,b,c],[d,e,f],[g,h,i]])  #  real
vector([a,b,c])  #  single column matrix

matrix(RR, nrows, ncols) # real ring

matrix(SR, nrows, ncols) # symbolic ring

matrix([[a,b,c],[d,e,f],[g,h,i]]) # real

vector([a,b,c]) # single column matrix

Solve M * X = Y (where M – co-efficients; Y – constants) :

M.solve_right(Y)

1	M.solve_right(Y)

Create variables (symbolic math):

x,y = var('x,y')
tan_theta = y / x;

1 2	x,y = var('x,y') tan_theta = y / x;

Algebra using Sage

Here are some examples of Sage used as a Computer algebra system.

Define the symbols x and y using var. Then define r2 in terms of x and y as per the Pythagorean theorem.

sage: x, y = var('x, y')
sage: r2 = x * x + y * y
sage: r2

sage: x, y = var('x, y')

sage: r2 = x * x + y * y

sage: r2

The output remains in terms of x and y:

x^2 + y^2

x^2 + y^2

This works with trigonometry…

sage: r = sqrt(r2)
sage: sin0 = y/r
sage: cos0 = x/r
sage: theta = atan(sin0/cos0)
sage: tan(theta)

sage: r = sqrt(r2)

sage: sin0 = y/r

sage: cos0 = x/r

sage: theta = atan(sin0/cos0)

sage: tan(theta)

Output:

y/x

y/x

…and matrices too. Below is an example of a matrix to translate and rotate a point in two dimensions.

sage: dx, dy, r = var('dx, dy, r')
sage: s = sin(r)
sage: c = cos(r)
sage: tx = matrix([[c, s, dx],
....: [-s, c, dy],
....: [0, 0, 1]])
sage: v = vector([x, y, 1])
sage: tx * v

sage: dx, dy, r = var('dx, dy, r')

sage: s = sin(r)

sage: c = cos(r)

sage: tx = matrix([[c, s, dx],

....: [-s, c, dy],

....: [0, 0, 1]])

sage: v = vector([x, y, 1])

sage: tx * v

Output:

(x*cos(r) + y*sin(r) + dx, y*cos(r) - x*sin(r) + dy, 1)

1	(xcos(r) + ysin(r) + dx, ycos(r) - xsin(r) + dy, 1)

Sage also does differentiation and integration but I am still trying to wrap my head around those two concepts.

Solve set of equations using Sage

Given the set of equations:

$ \begin{bmatrix} 5 & 0 & 0 \\ 0 & 10 & 0 \\ 0 & 0 & 15 \end{bmatrix} \times \left[ \begin{array}{c} x_1 \\ x_2 \\ x_3 \end{array} \right] = \left[ \begin{array}{c} 1 \\ 2 \\ 3 \end{array} \right] $

Below is how to solve them using Sage:

sage: A = matrix([[5,0,0],[0,10,0],[0,0,15]])
sage: Y = vector([1,2,3])
sage: X = A.solve_right(Y)
sage: X

sage: A = matrix([[5,0,0],[0,10,0],[0,0,15]])

sage: Y = vector([1,2,3])

sage: X = A.solve_right(Y)

sage: X

The output:

(1/5, 1/5, 1/5)

1	(1/5, 1/5, 1/5)

In order to get the output as floating point numbers use the numerical_approx function:

sage: n(X, digits=4)

1	sage: n(X, digits=4)

The output:

(0.2000, 0.2000, 0.2000)

1	(0.2000, 0.2000, 0.2000)

C++ Surprise 1 – const reference

“There are just two kinds of languages: the ones everybody complains about and the ones nobody uses.” — Bjarne

This doesn’t compile:

int main(){
  int x = 10;
  double &y = x; // error: invalid initialization of reference of 
                 // type 'double&' from expression of type 'int'
}

int main(){

int x = 10;

double &y = x; // error: invalid initialization of reference of

// type 'double&' from expression of type 'int'

}

This does:

int main(){
  int x = 10;
  const double &y = x;
}

int main(){

int x = 10;

const double &y = x;

}

In the second case, y is initialized by a temporary double initialized from x. Not sure what the rationale is but read up TCPPPL 5.5.

Here is where the inconsistency becomes very difficult to ignore:

#include <iostream>
using std::cout;
using std::endl;

int main(){
  int x = 10;
  const int &iy = x;
  const double &dy = x;
  x = 20;
  cout << x << endl;
  cout << iy << endl;
  cout << dy << endl;

  return 0;
}

#include

using std::cout;

using std::endl;

int main(){

int x = 10;

const int &iy = x;

const double &dy = x;

x = 20;

cout << x << endl;

cout << iy << endl;

cout << dy << endl;

return 0;

}

If anyone knows the reasoning behind this, let me know.

Setting RDLC Subreport Parameters via Code

To set parameters on an RDLC report on Windows Forms, you can say viewer.LocalReport.SetParameters and send in an array of ReportParameter objects. However, what if you need to set parameter values on a subreport object? There is no way to call SetParameters on the subreport object as far as I can tell. Google turned up several seemingly promising results for “rdlc subreport parameter” but none of them provided an answer.

I discovered a solution by trial and error. Here it is:

Open the subreport and define parameters (say SR_P1, SR_P2)
Open main report, insert a subreport object on it
Define the corresponding parameters on the main report (MR_P1 and MR_P2)
Right click on the subreport, select “Subreport Properties…”
Select “Parameters” and add one row for each parameter
Set each subreport parameter to the value of the corresponding main report parameter like this:

Name Value

SR_P1 [@MR_P1]
In code, set the values of the main report parameters MR_P1 and MR_P2 as usual using viewer.LocalReport.SetParameters

Name	Value
SR_P1	[@MR_P1]

You are welcome.

“The procedure entry xxx could not be located in the dynamic link library msvcrt.dll.”

This is the error I was getting when trying to install an MSI I created for a C++/CLI project. Turns out that the MSI included glut.dll and opengll.dll. When building the MSI files, warnings are displayed to the effect that those two files should be excluded.

Click on the file name under “Application Folder” and set the “Exclude” property to false in the property grid. This fixes the problem.

Qt4 Development on Ubuntu – 1

Searching Google for [qt development ubuntu] took me to this article which details downloading and installing binaries from qt-project.org. I found from other sources that simply installing the qt4-dev-tools package (using apt-get or synaptic) is the easier option.

“Getting Started Programming with Qt” is a very good introduction to programming Qt. One quirk I came across in the code in that article is the qApp pointer. What looked like a typo turned out to be a global pointer to the QApplication instance (similar to MFC’s CWinApp instance).

I did not like the “private slots:” syntax. That is not standard C++. I think I liked almost everything else about Qt that I have seen so far.