The following program is not designed to be pretty - but it does a job. (We
certainly don't expect you to follow how it works at this stage although you
are free to inspect it of course.) However, if you save this program as
ademo.c and compiile it as ademo then you
can use it as an experimental workbench for working with simple expressions. In
particular, it will allow you to try out various C operators and see
how they behave.
The program sets up four integer variables
(a, b, c, and d) initialised
to zero. It then enters an interactive (command-line) mode in which you can type
in a C statement to update the variables and then observe the results. Please
note, this program has only been designed to work for straightforward assignment
statements to the given variables -- anything it can't understand will simply
result in an error which it will ignore. The commands it recognises are
set(p,q,r,s)
- Sets (
a,b,c,d) directly
to the values (p,q,r,s) respectively. For example:
set(0,0,0,0)
to initialise all four variables to zero.
assignment statement(s)
- Updates the variables. For example:
a = b + c
b = c % b + (a - b)
c = a < b
d = c++ * (b && a)
a++; b = c++
etc.
undo
- Undoes the last experiment - returning the variables to their previous
state.
quit
- Stops the program.
Note: Don't spend too long playing with this demo program now. Try some examples
and then keep it somewhere safe so that you can experiment with it whenever
you want.
#include <stdio.h>
#include <string.h>
/* Author: drs
* C program to allow interactive experimentation with assignment statements
* Note: this is only a demo program - it does not catch division by zero errors
* and it does not parse arbitrarily complex C expressions. Its purpose is to
* allow the entering and editing of simple C assignment expressions based on
* four integer variables: a, b, c and d.
* Usage:
* Save this program as: ademo.c
* Compilie: gcc ademo.c -o ademo
* Run: ./ademo
* This will use the local files:
* __default__program.c (which it will create)
* a.out
* You can change the name of the C program source by providing a
* command line argument (e.g. to use a file called "foo.c":
* Run: ./ademo foo.c
*
* You can enter expressions such as:
* set(1,4,7,9)
* quit
* a = b + c
* d++
* c = a++ - b
* undo
*/
#define MAX_LINES 100
#define MAX_CHARS 40
#define CMD_WIDTH 80
#define DEFAULT_PROGRAM "__default__program.c"
#define BEFORE "#include <stdio.h>\n\
int a=0,b=0,c=0,d=0;\n\
void show(int n, char* s)\
{printf(\"%3d. %-24s%10d%10d%10d%10d\\n\",n,s,a,b,c,d);}\n\
void init(){a=0,b=0,c=0,d=0;}\n\
void set(int w, int x, int y, int z){a=w;b=x;c=y;d=z;}\n\
int main(void)\n{\nint n = 0;\n\
printf(\" No. %-24s%10s%10s%10s%10s\\n\",\"Expression\",\"a\",\"b\",\"c\",\"d\");\n"
#define AFTER "return 0;\n}\n"
int main(int argc, char** argv)
{
char a[MAX_LINES][MAX_CHARS];
int i, n = 0, undo = 0;
FILE * f;
char compile[CMD_WIDTH];
char program[CMD_WIDTH];
char command[MAX_CHARS];
if (argc>1)
strcpy(program,argv[1]);
else
strcpy(program,DEFAULT_PROGRAM);
printf("Using program file %s\n", program);
compile[0] = '\0';
strcpy(compile,strcat(strcat(compile,"gcc "),program));
while (n<MAX_LINES) {
f = fopen(program,"w");
fprintf(f,"%s",BEFORE);
for (i=0; i<n; i++)
fprintf(f,"%s; show(++n,\"%s\");\n",a[i],a[i]);
fprintf(f,"%s",AFTER);
fclose(f);
if(system(compile) == 0)
system("./a.out");
else {
if (n>0) n--;
}
printf("(%2i)Expr? ",n+1);
fgets(command,MAX_CHARS,stdin);
command[strlen(command)-1] = 0;
if (strncmp(command,"undo",4) == 0) {
undo = 1;
}
else if (strncmp(command,"quit",4) == 0) {
printf("stopping...\n");break;
}
if (undo) {
if (n>0) n--;
undo = 0;
printf("Undone\n");
}
else {
strcpy(a[n],command);
n++;
}
}
return 0;
}