C - Write the program expr which evaluates a reverse Polish expression from the command line

客官°小女子只卖身不卖艺 2022-10-22 09:08 45阅读 0赞

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/*
     * Write the program expr, which evaluates a reverse Polish expression from the
     * command line, where each operator or operand is a separate argument. For example,
     * expr 2 3 4 + *
     * evaluates 2 * (3+4).
     *
     * Expr.c - by FreeMan
     */
    
    #include <stdio.h>
    #include <stdlib.h>
    #include <math.h>
    #include <string.h>
    
    #define NUMBER 0
    
    void push(double f);
    double pop(void);
    
    main(int argc, char *argv[])
    {
    	int type;
    	int c;
    	double op1, op2, latest;
    	while (--argc > 0)
    	{
    		*++argv;
    		if (!isdigit(c = **argv) && strlen(*argv) == 1)
    		{
    			type = c;
    		}
    		else
    		{
    			type = NUMBER;
    		}
    		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(fmod(pop(), op2));
    			}
    			else
    			{
    				printf("error: zero divisor\n");
    			}
    			break;
    		case '^':
    			op2 = pop();
    			op1 = pop();
    			if (op1 == 0.0 && op2 <= 0)
    			{
    				printf("if x = 0.0, y must be greater than 0\n");
    			}
    			else
    			{
    				push(pow(op1, op2));
    			}
    			break;
    		case 'e':
    			push(exp(pop()));
    			break;
    		case '~':
    			push(sin(pop()));
    			break;
    		default:
    			printf("error: unknown command: %c\n", type);
    			break;
    		}
    	}
    	latest = pop();
    	printf("\t%.8g\n", latest);
    	return 0;
    }
    
    #define MAXVAL 100
    
    int sp = 0;
    double val[MAXVAL];
    
    /* push: push f onto value stack */
    void push(double f)
    {
    	if (sp < MAXVAL)
    		val[sp++] = f;
    	else
    		printf("error: stack full, can't push %g\n", f);
    }
    
    /* pop: pop and return top value from stack */
    double pop(void)
    {
    	if (sp > 0)
    		return val[--sp];
    	else {
    		printf("error: stack empty\n");
    		return 0.0;
    	}
    }

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