BUPT-homework/semester3/pset1/7-2-Infix-to-Postfix-Conversion.c

// 20:05 - 20:50
#include <stdio.h>
#include <stdlib.h>
#define MAX 256

int getPriority(char ch) {
  if (ch == '+' || ch == '-') {
    return 0;
  } else if (ch == '*' || ch == '/') {
    return 1;
  } else {
    return -1;
  }
}

typedef struct stack {
  double *arr;
  int top;
} stack;

stack *newStack() {
  stack *st = malloc(sizeof(stack));
  st->top = 0;
  st->arr = malloc(sizeof(double) * MAX);

  return st;
}

void push(stack *st, double val) {
  // printf("Pushing: %.2lf\n", val);
  st->arr[st->top++] = val;
}

double top(stack *st) { return st->arr[st->top - 1]; }

void pop(stack *st) { st->top--; }

int isEmpty(stack *st) { return (st->top == 0); }

void toPostfix(char *buf, char *out) {
  int count = 0;
  stack *st = newStack();
  for (int i = 0; buf[i] != '\0'; i++) {
    if (buf[i] >= '0' && buf[i] <= '9') {
      // is digit, push to out array
      out[count++] = buf[i];
    } else {
      if (buf[i] == '(') {
        // left paren, don't pop elements in stack.
        push(st, buf[i]);
      } else if (buf[i] == ')') {
        // push everything after the (
        while (top(st) != '(') {
          out[count++] = top(st);
          pop(st);
        }
        pop(st);
      } else {
        // the equal sign is important
        while (!isEmpty(st) && getPriority(top(st)) >= getPriority(buf[i])) {
          out[count++] = top(st);
          pop(st);
        }
        push(st, buf[i]);
      }
    }
  }

  while (!isEmpty(st)) {
    out[count++] = top(st);
    pop(st);
  }

  out[count] = '\0';
}

void calculate(char *postfix) {
  double l, r;
  stack *st = newStack();
  for (int i = 0; postfix[i] != '\0'; i++) {
    if (postfix[i] >= '0' && postfix[i] <= '9') {
      // remember convert from char to int
      push(st, postfix[i] - '0');
    } else {
      r = top(st);
      pop(st);
      l = top(st);
      pop(st);
      if (postfix[i] == '+') {
        push(st, l + r);
      } else if (postfix[i] == '-') {
        push(st, l - r);
      } else if (postfix[i] == '*') {
        push(st, l * r);
      } else if (postfix[i] == '/') {
        push(st, l / r);
      } else {
        printf("ERROR: %c is not recognized\n", postfix[i]);
      }
    }
  }

  printf("%.2lf\n", top(st));
}

void printPostfix(char *postfix) {
  for (int i = 0; postfix[i] != '\0'; i++) {
    printf("%c ", postfix[i]);
  }
}

int main(void) {
  char buf[MAX];
  char out[MAX];

  scanf("%s", buf);

  toPostfix(buf, out);
  calculate(out);
  printPostfix(out);
  return 0;
}