High School Mathematics Extensions/Mathematical Programming/The Complete Program

 //includes
 #include <stdio.h>
 #include <conio.h>

 //function prototypes
 void init();
 void input_message();
 char input();
 void execute_command(char);
 char retrieve_keypress();
 char validate(char check_me);
 double process();
 void output();
 float get_delta();
 
 //type definitions 

 //global variables
 char done;
 double x_val;
 double delta;

 //function definitions
 void init()
 {
     done=0;
     x_val=1.2;
     delta=0.1;
 }
 void input_message()
 {
    cprintf("Key      Action.\n");
    cprintf("=        Display f(x) = result.\n");       
    cprintf("d or D   Change delta.\n");
    cprintf("+        Add delta to x.\n");
    cprintf("-        Subtract delta from x.\n");
    cprintf("x or X   End program.\n");
 }
 char input()
 {
     char read;
     read=retrieve_keypress();
     while (!validate(read))
        {
            cprintf("\nInvalid Command.\n");
            input_message();
            read=retrieve_keypress();
        }
     return read;
 }
 char retrieve_keypress()
 {
    char ret_val;
    ret_val=getche();
    cprintf("\n");
    return ret_val;
 }
 char validate(char check_me)
 {
    char ret_val=0;
    switch (check_me)
    {
        case '=':
        case '+':
        case '-':
        case 'd':
        case 'D':
        case 'x':
        case 'X':   ret_val++;
                    break;
    }
    return ret_val;
 }
 void execute_command(char command)
{
    switch (command)
    {
        case '=':   output();
                    break;
        case '+':   x_val+=delta;
                    output();
                    break;
        case '-':   x_val-=delta;
                    output();
                    break;
        case 'd':
        case 'D':   delta=get_delta();
                    break;
        case 'x':
        case 'X':   done++;
                    break;
    }
}
void output()
{
    double f_val;
    f_val=process();
    cprintf("f(%.3f)=%.6f.\n",x_val,f_val);

}
double process()
{
    return x_val * x_val;
}
float get_delta()
{
    float f_val;
    char lastpress;
    cprintf("Enter delta: ");
    cscanf("%f",&f_val);
    lastpress=getch();
    cprintf("\n");
    if(f_val < 0.001)
       {
           f_val=0.001;
           cprintf("Delta must be floating point number greater than or equalt to 0.001.\nDelta set to %f.\n",f_val);
       }
    else
       {
            cprintf("Delta set to %f.\n",f_val);
       }
    return f_val;
}
 void main()
 {
    char command;
    init();
    input_message();
    while(!done)
       {
        command=input();
        execute_command(command);
       }
 }

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