#include <stdio.h>
 
#define SIZE 10
double stack[SIZE];
int sp;
 
void push(double value);
double pop(void);
int isFull(void);
int isEmpty(void);
void answer(void);
void rest(void);
 
int main(void)
{
  rest();
 
  while(1){
    int resp;
    double x, a1, a2;
 
    // オペレーションの選択を促すメッセージ
    printf("操作を選んでください (1: +, 2: -, 3: *, 4: /, 5: 入力, 9: 結果表示): ");
 
    // scanf の戻り値を確認
    if (scanf("%d", &resp) != 1) {
      printf("入力エラー。再度入力してください。\n");
      // 入力に失敗した場合、バッファをクリア
      while (getchar() != '\n');
      continue;
    }
 
    switch(resp){
      case 1:  // 足し算
        if (!isEmpty()) {
          x = pop() + pop();
          push(x);
        }
        break;
      case 2:  // 引き算
        if (!isEmpty()) {
          a1 = pop(); a2 = pop();
          x = a2 - a1;
          push(x);
        }
        break;
      case 3:  // 掛け算
        if (!isEmpty()) {
          x = pop() * pop();
          push(x);
        }
        break;
      case 4:  // 割り算
        if (!isEmpty()) {
          a1 = pop(); a2 = pop();
          if (a1 != 0) {
            x = a2 / a1;
            push(x);
          } else {
            printf("ゼロ除算エラーです。\n");
            push(a2);  // エラー時は元に戻す
            push(a1);
          }
        }
        break;
      case 5:  // 値の入力
        printf("値を入力してください: ");
        if (scanf("%lf", &x) != 1) {  // 入力の確認
          printf("入力エラー。再度入力してください。\n");
          while (getchar() != '\n');  // バッファクリア
          continue;
        }
        push(x);
        break;
      case 9:  // 結果表示して終了
        answer();
        return 0;  // プログラム終了
      default:
        printf("不正な操作です。\n");
    }
  }
 
  return 0;
}
 
void push(double value){
  if( isFull() ){
    printf("スタックが満杯で入りませんでした\n");
  }else{
    stack[sp++] = value;
  }
}
 
double pop(void){
  if( isEmpty() ){
    printf("スタックが空で取り出せませんでした\n");
    return 0;
  }else{
    return stack[--sp];
  }
}
 
int isFull(void){
  return sp >= SIZE;
}
 
int isEmpty(void){
  return sp <= 0;
}
 
void answer(void){
  if (!isEmpty()) {
    printf("answer: %.2lf\n", pop());  // 小数点以下2桁まで表示
  } else {
    printf("スタックが空です。\n");
  }
}
 
void rest(void){
  sp = 0;
}
 

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