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| -rw-r--r-- | ch4/4-04_top-swap.c | 201 |
1 files changed, 201 insertions, 0 deletions
diff --git a/ch4/4-04_top-swap.c b/ch4/4-04_top-swap.c new file mode 100644 index 0000000..2f884d4 --- /dev/null +++ b/ch4/4-04_top-swap.c @@ -0,0 +1,201 @@ +#include <stdio.h> +#include <stdlib.h> +#include <ctype.h> + +/* The C Programming Language: 2nd Edition + * + * Exercise 4-4: Add commands to print the top element of the stack without + * popping, to duplicate it, and to swap the top two elements. Add a + * command to clear the stack. + * + * Answer: Achieving this is pretty simple. Make your functions, then assign + * them to "commands", which are basically any special characters that aren't + * being used in the switch statement. Pretty basic manipulation of arrays. + * + * Note that any expression that has subtraction at the end needs either a + * space or an additional newline input after it in order to calculate. This + * is a limitation of the getchar() based approach. + */ + +#define MAXOP 100 +#define NUMBER '0' +#define MAXVAL 100 +#define BUFSIZE 100 + +int getop(char []); +void push(double); +double pop(void); +int getch(void); +void ungetch(int); +void stack_top(void); +double dupe_top(void); +void swap_top_two(void); +void clear_stack(void); + +int sp = 0; // Next free stack position +double val[MAXVAL]; // Value stack +char buf[BUFSIZE]; // buffer for ungetch +int bufp = 0; // next free position in buf + +/* Reverse Polish calculator: + * + * Binary operations (+-*\%) + * operand operand operator + * + * Example: 6 minus 2 in Reverse Polish Notation is "6 2 -" + */ +int main() { + int type; + double op2; + char s[MAXOP]; + + while ((type = getop(s)) != EOF) { + switch (type) { + case NUMBER: + push(atof(s)); + 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: Cannot divide by zero.\n"); + } + break; + /* Yay for modulus! */ + case '%': + op2 = pop(); + if (op2 != 0.0) { + push((int)pop() % (int)op2); + } else { + printf("Error: Cannot modulo by zero.\n"); + } + break; + /* Top of stack */ + case '?': + stack_top(); + break; + /* Dupe the top of the stack */ + case '#': + dupe_top(); + break; + /* Swap the top two */ + case '~': + swap_top_two(); + break; + /* Clear the stack */ + case '!': + clear_stack(); + break; + case '\n': + printf("\t%.8g\n", pop()); + break; + default: + printf("Error: Unknown command %s\n", s); + break; + } + } + return 0; +} + +void push(double f) { + if (sp < MAXVAL) { + val[sp++] = f; + } else { + printf("Error: Stack full. Cannot push %g\n", f); + } +} + +double pop(void) { + if (sp > 0) { + return val[--sp]; + } else { + printf("Error: Stack empty.\n"); + return 0.0; + } +} + +int getop(char s[]) { + int i = 0; + int c, next; + + while ((s[0] = c = getch()) == ' ' || c == '\t') { + } + s[1] = '\0'; + /* The final check is for negative numbers. */ + if (!isdigit(c) && c != '.' && c != '-') { + return c; + } + /* The second half of this if-statement accounts for negatives */ + if (c == '-') { + next = getch(); + if (!isdigit(next) && next != '.') { + return c; + } else { + c = next; + } + } else { + c = getch(); + } + + while (isdigit(s[++i] = c)) { + c = getch(); + } + if (c == '.') { + while (isdigit(s[++i] = c = getch())) { + } + } + s[i] = '\0'; + if (c != EOF) { + ungetch(c); + } + return NUMBER; +} + +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; + } +} + +void stack_top(void) { + if (sp > 0) { + printf("Top of stack is %8g\n", val[sp - 1]); + } else { + printf("Error: Stack empty.\n"); + } +} + +double dupe_top(void) { + double temp = pop(); + push(temp); + push(temp); +} + +void swap_top_two(void) { + double tmp1, tmp2; + tmp1 = pop(); + tmp2 = pop(); + push(tmp1); + push(tmp2); +} + +void clear_stack(void) { + sp = 0; +} |