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    /**
    *
    * Exercise 4.6 of The C Programming Language by Brian Kernighan and Dennis
    * Ritchie.
    *
    * Add commands for handling variables. (It's easy to provide twenty-six
    * variables with single-letter names.)
    *
    * Add a variable for the most recently printed value.
    */

    #include <stdio.h>
    #include <stdlib.h>

    #define MAXOP 100
    #define NUMBER '0'

    int getop(char[]);
    void push(double);
    double pop(void);

    int main(int argc, char *argv[]) {
    int type;
    int var = 0;

    double op2;
    double v = 0.0;
    double variable[26];

    while (--argc > 0) {
    type = getop(*++argv);
    switch (type) {
    case NUMBER:
    push(atof(*argv));
    break;
    case '+':
    push(pop() + pop());
    break;
    case '*':
    push(pop() * pop());
    break;
    case '-':
    op2 = pop();
    push(pop() - op2);
    break;
    case '/':
    op2 = pop();
    if (op2 != 0.0)
    push(pop() / op2);
    else
    printf("error: zero divisor\n");
    break;
    case '%':
    op2 = pop();
    if (op2 != 0.0)
    push((int)pop() % (int)op2);
    else
    printf("error: zero divisor\n");
    break;
    case '=':
    pop();
    if (var >= 'A' && var <= 'Z')
    variable[var - 'A'] = pop();
    else
    printf("error: no variable name\n");
    break;
    case 'v':
    push(v);
    break;
    case 'V':
    v = pop();
    push(v);
    break;
    case 's':
    op2 = pop();
    push(pop());
    push(op2);
    break;
    case 'c':
    while (pop() != 0.0)
    ;
    break;
    case 'd':
    op2 = pop();
    push(op2);
    push(op2);
    break;
    case 'p':
    op2 = pop();
    printf("\t%.8g\n", op2);
    push(op2);
    break;
    default:
    if (type >= 'A' && type <= 'Z')
    push(variable[type - 'A']);
    else if (type == 'v')
    push(v);
    else
    printf("error: unknown command %s\n", *argv);
    break;
    }
    var = type;
    }
    return 0;
    }

    #define MAXVAL 100

    int sp = 0;
    double val[MAXVAL];

    void push(double f) {
    if (sp < MAXVAL)
    val[sp++] = f;
    else
    printf("error: stack full, can't push %g\n", f);
    }

    double pop(void) {
    if (sp > 0)
    return val[--sp];
    else {
    printf("error: stack empty\n");
    return 0.0;
    }
    }

    #include <ctype.h>

    int getch(void);
    void ungetch(int);

    int getop(char s[]) {
    int i, c;

    while ((s[0] = c = getch()) == ' ' || c == '\t')
    ;
    s[1] = '\0';
    if (!isdigit(c) && c != '.')
    return c;
    i = 0;
    if (isdigit(c))
    while (isdigit(s[++i] = c = getch()))
    ;
    if (c == '.')
    while (isdigit(s[++i] = c = getch()))
    ;
    s[i] = '\0';
    if (c != EOF)
    ungetch(c);
    return NUMBER;
    }

    #define BUFSIZE 100

    char buf[BUFSIZE];
    int bufp = 0;

    int getch(void) { return (bufp > 0) ? buf[--bufp] : getchar(); }

    void ungetch(int c) {
    if (bufp >= BUFSIZE)
    printf("ungetch: too many characters\n");
    else
    buf[bufp++] = c;
    }