计算错误，使用+代替-

Question

由于培训原因，我想编写一个小计算器。为什么要计算10-6 = 16而不是10-6 = 4？

我得到了错误：

Assertion Failed!
Expression: calc("10-6") == 4 && "could not do substraction"

这是我的代码

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <assert.h>
#include <ctype.h>

double calc(char * input);
double extract_first_integer(char * input);
double extract_last_integer(char * input);
char extract_operand(char * input);

int main()
{
   assert(calc("10-6") == 4 && "could not do substraction");

   return 0;
}

double calc(char * input){
   double num1 = extract_first_integer(input);
   double num2 = extract_last_integer(input);
   char operand = extract_operand(input);
   printf("operand is %c\n", operand);
   switch (operand)
   {
   case '-':
      printf("num1 - num2: %f\n", num1-num2);  // output: 16 instead of 4
      return num1 - num2;
      break;
   }
}

double extract_first_integer(char * input){
   char *str = input, *p = str;
   double val;
   while (*p) { // While there are more characters to process...
      if ( isdigit(*p) || ( (*p=='-'||*p=='+') && isdigit(*(p+1)) )) {
         // Found a number
         val = strtol(p, &p, 10); // Read number
         return val;
      } else {
         // Otherwise, move on to the next character.
         p++;
      }
   }
}

double extract_last_integer(char * input){
   char *str = input, *p = str;
   double val;
   while (*p) { // While there are more characters to process...
      if ( isdigit(*p) || ( (*p=='-'||*p=='+') && isdigit(*(p+1)) )) {
         // Found a number
         val = strtol(p, &p, 10); // Read number
      } else {
         // Otherwise, move on to the next character.
         p++;
      }
   }
   return val;
}

char extract_operand(char * input){
   if (strstr(input, "-")) return '-';
}

Answer 1

在extract_last_integer()你里面

while (*p) { // While there are more characters to process...
  if ( isdigit(*p) || ( (*p=='-'||*p=='+') && isdigit(*(p+1)) )) {
     // Found a number
     val = strtol(p, &p, 10); // Read number
  } else {
     // Otherwise, move on to the next character.
     p++;
  }
}

递增，p直到遇到第一个数字或-/ +后跟一个数字。因此它将与第一个10数字匹配。不过请注意，您并没有像早return val;于那样打破循环extract_first_integer()。当您继续匹配下一位数字时-6，"10-6"将匹配输入。而且10 - (-6)显然是16

您还可能具有未定义的行为

• 将相同的指针传递到strtol。该变量str未使用，应str_end改为在参数中传递
• 将const char*（"10-6"）传递给期望的函数char*