《C程序设计语言》练习题 5-10
编写程序 expr，计算从命令行输入的逆波兰表达式的值，其中每个运算符或操作数用一个单独的参数表示。例如，命令
expr 2 3 4 + *
计算表达式2x（3+4）的值
算法实现
getfloat.c:
// getfloat.c
#include<stdio.h>
#include<ctype.h>
int getfloat(char* str, double* store)
{
while (isspace(*str))
str++;
int ch, sign;
double power = 1.0;
ch = *str++;
sign = ch == '-' ? -1 : 1;
if (ch == '+' || ch == '-') {
ch == *str++;
}
for (*store == 0.0; isdigit(ch); ch = *str++) {
*store = *store*10.0 + ch - '0';
}
if (ch == '.') {
ch=*str++;
}
for (; isdigit(ch); ch = *str++) {
*store = *store*10.0 + ch - '0';
power *= 10.0;
}
if (ch == 0 || isspace(ch) || ch == EOF) {
//如果不能继续读取数字，那么跳出的原因必须是 ch==0 || isspace(ch) || ch==EOF
*store /= power;
*store *= sign;
return 1;
}
else {
return -1;
}
}
expr.c:
//expr.c
#include<stdio.h>
#include<ctype.h>
#define MAX_NUM 100
double stack[MAX_NUM];
extern int getfloat(char* str,double*);
int main(int argc, char* argv[])
{
if (argc < 2) {
printf("usage: expr num1 num2 operator ...\n");
return 1;
}
else {
int stackp = 0;
int ch = 0;
double num1, num2, ans;
while (--argc >= 1) {
ch = (*++argv)[0];
if (ch == '-'){
if (!isdigit(ch = (*argv)[1])) {
//是负号
if (stackp >= 2) {
num2 = stack[--stackp];
num1 = stack[--stackp];
stack[stackp++] = num1 - num2;
continue;
}
else {
printf("Error: at least two numbers are needed before operator '-'\n");
return -1;
}
}
}
if (isdigit(ch)) {
if (stackp < 0) {
printf("stack is underflowed!\n");
return -2;
}
else if (stackp >= MAX_NUM) {
printf("stack is overflowed!\n");
return -3;
}
else if (getfloat(*argv, &stack[stackp++]) == -1) {
printf("wrong argument to get number: %s\n", *argv);
return -4;
}
}
else if (ch == '+'||ch=='*'||ch=='/'||ch=='%') {
if (stackp >= 2) {
num2 = stack[--stackp];
num1 = stack[--stackp];
switch (ch) {
case '+':
ans = num1 + num2;
break;
case '*':
ans = num1*num2;
break;
case '/':
if (num2 == 0.0) {
printf("error zero divisor: %f / %f\n", num1, num2);
return -5;
}
ans = num1 / num2;
break;
case '%':
ans = (int)num1%(int)num2;
break;
}
stack[stackp++] = ans;
}
else {
printf("Error: at least two numbers are needed before operator '%c'\n",ch);
return -6;
}
}
else {
printf("wrong arguments for %c,usage: expr num1 num2 operator ...\n", ch);
return -7;
}
}
if (stackp == 1) {
printf("%.4f\n", stack[0]);
}
else {
printf("the format of input is wrong!\n");
}
}
return 0;
}