C++实现正整数的四则运算表达式

#include <iostream>
#include <cstdio>

using namespace std;

int main() {
  int i, j, k, len = 0, lenf = 0, lenl = 0, op = 0, z = 0, y = 0, sum = 0;
  char algorithm[100], f[50] = {0}, l[50] = {0};
  cout << "input an expression with one of the four fundamental operations: ";
  gets(algorithm);
  //计算初始字符串长度
  for (i = 0; algorithm[i] != '\0'; ++i) { len += 1; }
  //提取运算符号前的字符串
  for (j = 0; ('0' <= algorithm[j] && algorithm[j] <= '9') || algorithm[j] == ' '; ++j) {
    f[j] = algorithm[j];
  }
  //判断运算符号
  if (algorithm[j] == '+')op = 1;
  if (algorithm[j] == '-')op = 2;
  if (algorithm[j] == '*')op = 3;
  if (algorithm[j] == '/')op = 4;
  while (algorithm[j + 1] == ' ')++j;
  //提取运算符号后的字符串
  for (k = j + 1; algorithm[k] != '\0'; ++k) {
    l[k - j - 1] = algorithm[k];
  }
  //计算两个被提取出来的字符串长度
  for (i = 0; f[i] != '\0'; ++i) { lenf += 1; }
  for (i = 0; l[i] != '\0'; ++i) { lenl += 1; }
  //把字符串转换为数字
  for (i = 0; i < lenf; ++i) {
    if (f[i] == ' ') continue;//遇到空格后跳过
    else z = z * 10 + f[i] - '0';
  }
  for (i = 0; i < lenl; ++i) {
    if (l[i] == ' ')continue;//遇到空格后跳过
    else y = y * 10 + l[i] - '0';
  }
  //判断运算类型
  switch (op) {
    case 4: {
      if (y == 0) {//排除非法除0操作
        cout << z << " / 0: Error !" << endl;
      } else { cout << z << " / " << y << " = " << z / y << endl; }
    }
    default: {
      switch (op) {
        case 1:
          cout << z << " + " << y << " = " << z + y << endl;
          break;
        case 2:
          cout << z << " - " << y << " = " << z - y << endl;
          break;
        case 3:
          cout << z << " * " << y << " = " << z * y << endl;
          break;
        default:
          break;
      }
    }
  }
  return 0;
}