Ch Applet for Network Computing
Table of Contents


Introduction

The Ch language environment can be used for network computing. You can download Ch programs located remotely in a server with the Ch language environment and then execute them in the client, your host computer. Such a network-downloaded program is called an applet. If you have installed the Ch language environment according to its installation instructions, you can run this Ch applet located remotely in our server with the Ch language environment. The program will be downloaded through the Web and executed on your computer. Don't worry, this is a benign program and it will not do any harm to your system. Seeing is believing; take a look at the source code of this Ch applet. You will find out that it is just a simple C program. Indeed, Ch is designed to be a superset of C. When you click the above highlighted word "run", if the applet is not executed to produce the following output

      Hello, world!
      Today is (date)
and instead downloaded as a text file C1.chs, you should download the Ch language environment. Unlike other language environments, the Ch language environment is very simple to setup. Check Ch Installation and System Administration Guide on how to setup the server and client of the Ch language environment.

When comparing different language environments, people often try to construct a simple program to print out "Hello, world!" Here is a Ch applet for such a beauty contest. It can be executed to print out this famous statement.

      Hello, world!
In Ch, what you see is what you get. A Ch applet is executed without compilation. No security error-prone intermediate bytecode will be generated.
Extensions to C for Writing Secure Applets

Many secure features have been built into the Ch language environment to protect your system from hostile and accidental attack. For example, the C pointer declaration is disabled for applets obtained through the network for World-Wide Distributed Computing under a safe shell. If you run an applet with pointer indirection, the execution of the network-downloaded program will fail. The following error message will be produced

      ERROR: lvalue of pointer type is restricted in safe shell
For across-network computing, a string is treated as a first-class object. You can run this applet with string data type to produce the following output
      Hello, world!
      I have just run the Ch language environment.
      It is cool!
However, an applet can invoke other programs with C pointers located in a client computer, which will be described in the next section.

Many people find that computational arrays in Ch are cool. A computational array is a first-class object in Ch. Here is a Ch program with computational arrays, the output

      A = 
      1.000 2.000 3.000 
      4.000 5.000 6.000 
      7.000 8.000 9.000 

      B = 
      1.000 2.000 3.000 

      transpose(A) = 
      1.000 4.000 7.000 
      2.000 5.000 8.000 
      3.000 6.000 9.000 

      A*B = 
      14.000 32.000 50.000 

      transpose(A)+2*inverse(A)*A = 
      3.000 4.000 7.000 
      2.000 22.000 8.000 
      3.000 6.000 11.000 
will be produced by executing it. All nice features in Fortran 90 will be available in the Ch language environment. How can you pass results from a function without pointers in applets? Fortran programmers will say pass-by-reference. Yes! You can pass arguments of function by reference in Ch. References in Ch are C++ compatible. Try to run this C++ style applet with reference data type. The following output will be produced
      x=4,y=5
      x=5,y=4
The equivalent C version of the above C++ style applet can also be executed across the network because declaration of pointers and the indirection operation of a pointer is permitted.
Integration of CGI Scripts and Applets

To demonstrate the difference between the Ch-applet based network computing and the Common Gateway Interface, run this CGI program written in Ch. In Common Gateway Interface, when you click the highlighted word "run", the Ch program is executed on the Web server, not on your client computer. But, in Ch-applet based network computing, when you click the highlighted word "run", the program is executed on your local machine. Take a look at the source code of this CGI program written in Ch to see the difference. This simple example shows how an applet is created in the server and executed in a client machine.

Many interesting and exciting applications can be created using CGI and Ch applets. Here is a Web Calculator. The CGI program dynamically creates a Ch applet on the fly. This applet is then executed in your machine. Such an applet that is created and delivered on demand is called a dynamic applet. The creation of a dynamic applet is easy. In this Web Calculator example, the error message produced by the CGI script is sent as an html file, which is realized by producing the first line of output

     content-type: text/html
followed by a blank line. The first line of output from the CGI script
     content-type: application/x-chs
indicates that the output is the MIME type of a dynamic applet in safe Ch. For example, when you input x with value 3 and expression of x*sin(2*x) into this Web Calculator, the following dynamic applet will be created by the Ch CGI script and then executed in your local machine.
     #include<stdio.h>
     int main() {
       double x = 3;
       printf("x = %f, ", x);
       printf("x*sin(2*x) = %f\n", x*sin(2*x));
     }
The output from the above program is
     x = 3.000000, x*sin(2*x) = -0.838246
A program with similar functionalities may be implemented in C and X-Window alone with thousands of lines of code. That is why Ch is a very high-level language environment. Many applications can be accomplished in the Ch language environment with a fraction of C code.

This Web Matrix Calculator will demonstrate the power of computational arrays in the Ch language environment. For example, if you type in mathematical expression inverse(A)+2*tranpose(A), the Matrix Calculator will compute the sum of inverse of matrix A and product of 2 by transpose of matrix A. Here is the CGI script that generates dynamic applet for this Web Matrix Calculator.

As another example, this Web Plotter can plot 2-dimensional and 3-dimensional graphs according to your input function.


Collection of CGI/CCI Scripts and Applets Presented in this Page

Applets

  1. Hello, world!
  2. Hello, world! and date.
  3. Applet with string data type.
  4. Applet with computational array.
  5. C++-style applet with function pass by reference.
  6. Applet with function pass by value of pointer.
  7. Applet with lvalue of pointer type. (failed).
CGI Scripts
  1. Hello, world!

NOTE: If you cannot run the applets described in this page, please come back to try these demos again after you have downloaded and installed the Ch language environment, it will be much more fun.