C++ examples for Data Structure:Stack
Evaluates arithmetic expressions composed of 1-digit numbers
#include <iostream> #include <cstring> using namespace std; const int LEN = 80; //length of expressions, in characters const int MAX = 40; //size of stack class Stack/*w ww . j ava 2 s. co m*/ { private: char st[MAX]; //stack: array of chars int top; //number of top of stack public: Stack() { top = 0; } void push(char var) //put char on stack { st[++top] = var; } char pop() //take char off stack { return st[top--]; } int gettop() //get top of stack { return top; } }; class express //expression class { private: Stack s; //stack for analysis char* pStr; //pointer to input string int len; //length of input string public: express(char* ptr) { pStr = ptr; //set pointer to string len = strlen(pStr); //set length } void parse(); //parse the input string int solve(); //evaluate the stack }; void express::parse() //add items to stack { char ch; char lastval; //last value char lastop; //last operator for(int j=0; j<len; j++) { ch = pStr[j]; //get next character if(ch>='0' && ch<='9') //if it's a digit, save numerical value s.push(ch-'0'); else if(ch=='+' || ch=='-' || ch=='*' || ch=='/') { if(s.gettop()==1) //if it's first operator s.push(ch); //put on stack else //not first operator { lastval = s.pop(); //get previous digit lastop = s.pop(); //get previous operator //if this is * or / AND last operator was + or - if( (ch=='*' || ch=='/') && (lastop=='+' || lastop=='-') ) { s.push(lastop); //restore last two pops s.push(lastval); } else { switch(lastop) //do last operation { //push result on stack case '+': s.push(s.pop() + lastval); break; case '-': s.push(s.pop() - lastval); break; case '*': s.push(s.pop() * lastval); break; case '/': s.push(s.pop() / lastval); break; default: cout << "\nUnknown oper"; exit(1); } } //end else, in all other cases s.push(ch); //put current op on stack } //end else, not first operator } else { cout << "\nUnknown input character"; exit(1); } } } int express::solve() //remove items from stack { char lastval; //previous value while(s.gettop() > 1) { lastval = s.pop(); //get previous value switch( s.pop() ) //get previous operator { //do operation, push answer case '+': s.push(s.pop() + lastval); break; case '-': s.push(s.pop() - lastval); break; case '*': s.push(s.pop() * lastval); break; case '/': s.push(s.pop() / lastval); break; default: cout << "\nUnknown operator"; exit(1); } } return int( s.pop() ); //last item on stack is ans } int main() { char ans; //'y' or 'n' char string[LEN]; //input string from user cout << "\nEnter an arithmetic expression" "\nof the form 2+3*4/3-2." "\nNo number may have more than one digit." "\nDon't use any spaces or parentheses."; do { cout << "\nEnter expresssion: "; cin >> string; //input from user express* eptr = new express(string); //make expression eptr->parse(); //parse it cout << "\nThe numerical value is: " << eptr->solve(); //solve it delete eptr; //delete expression cout << "\nDo another (Enter y or n)? "; cin >> ans; } while(ans == 'y'); return 0; }