运算表达式求值模板

表达式计算 ##使用方法 输入合法的表达式，加减乘除，可以带括号，用空格分开数字和符号，-1为结束标志，比如： `2 * 5 + 3 -1` 注意：这是用来算具体答案的，不是转化成后缀表达式输出的，当然，思想是递归建立表达式树，然后后序遍历得逆波兰式，然后用栈计算结果 ##模板 ``` #include #include #include #include //#include using namespace std; #define mem(x,num) memset(x,num,sizeof(x)) const int N = 1e3 + 5; const int defaultSize = 100; int lson[N], rson[N];

int nc = 0;
int cut = 0;

struct Num {
char val[defaultSize];
Num() {
val[0] = '0';
}
bool is_op() {
return strlen(val)== 1 && (val[0]<'0' || val[0]>'9');
}
double ToNum() {
return atof(val);
}
};
Num op[N];
Num arra[defaultSize];
void Init() {
mem(lson, 0);
mem(rson, 0);
mem(op, 0);
nc = 0;
cut = 0;
mem(arra, 0);
}
int build_Tree(Num s[], int l, int r) { // Build the expression tree
int tag1 = -1, tag2 = -1, p = 0;// tag1 stand for add&sub is_exist,tag2 for muti&devide
int u;
if (r - l == 1) {// this is only one point in this interval,build it
u = ++nc;
lson[u] = rson[u] = 0;
op[u]=s[l];
return u;// return self num to fa Node
}
for (int i(l); i < r; i++) {
switch (s[i].val[0])// s[i]
{
case '(':p++; break;
case ')':p--; break;
case '+':case '-':if(!p)tag1 = i; break;// if operator in (),continue
case '':case '/':if(!p)tag2 = i; break;
}
}
if (tag1 < 0)tag1 = tag2;// no + or - outside
if (tag1 < 0)return build_Tree(s, l + 1, r - 1); // no * or / outside mean all operator and num in ()
u = ++nc;
lson[u] = build_Tree(s, l, tag1);
rson[u] = build_Tree(s, tag1 + 1, r);
op[u] = s[tag1];
return u;
}
void Post(int rt) {// Postorder
if (lson[rt] != 0)Post(lson[rt]);
if (rson[rt] != 0)Post(rson[rt]);
arra[cut++] = op[rt];// cout << op[rt] << ' ';
}
double query(Num s[],int n) { // cal the value
stackP;
for (int i(0); i < n; i++) {
if (!s[i].is_op()) {
P.push(s[i].ToNum());
}
else {
double x1 = P.top();
P.pop();
double x2 = P.top();
P.pop();
double y;
switch (s[i].val[0])
{
case '+':y = x1 + x2; break;
case '-':y = x2 - x1; break;
case '':y = x1 * x2; break;
case '/':y = x2 / x1; break;
}
P.push(y);
}
}
return P.top();
}
int main() {
Num s[defaultSize];
char str[defaultSize];
while (1) { //cin>>s
Init();
int n=0;
while (cin >> str,strcmp(str,"-1")) { // remove this section
strcpy_s(s[n++].val, 20, str);
}
build_Tree(s, 0,n); // strlen(s)
Post(1);
cout << query(arra,n)<<endl;//
}
return 0;
}