第41行中的'0'是什么意思？

Question

#include <iostream>
#include <cctype> // isdigit
using namespace std;

// Global buffer
const int LINE_LENGTH = 128;
char line[LINE_LENGTH];
int lineIndex;

void getLine () {
// Get a line of characters.
// Install a newline character as sentinel.
   cin.getline (line, LINE_LENGTH);
   line[cin.gcount () - 1] = '\n';
   lineIndex = 0;
}

enum State {eI, eF, eM, eSTOP};

void parseNum (bool& v, int& n) {
   int sign;
   State state;
   char nextChar;
   v = true;
   state = eI;

   do {
      nextChar = line[lineIndex++];
      switch (state) {
         case eI:
            if (nextChar == '+') {
               sign = +1;
               state = eF;
            }
            else if (nextChar == '-') {
               sign = -1;
               state = eF;
            }
            else if (isdigit (nextChar)) {
               sign = +1;
               n = nextChar - '0'; // line 41
               state = eM;
            }
            else {
               v = false;
            }
            break;
         case eF:
            if (isdigit (nextChar)) {
               n = nextChar - '0';
               state = eM;
            }
            else {
               v = false;
            }
            break;
         case eM:
            if (isdigit (nextChar)) {
               n = 10 * n + nextChar - '0';
            }
            else if (nextChar == '\n') {
               n = sign * n;
               state = eSTOP;
            }
            else {
               v = false;
            }
            break;
      }
   }
   while ((state != eSTOP) && v);
}

int main () {
   bool valid;
   int num;
   cout << "Enter number: ";
   getLine();
   parseNum (valid, num);
   if (valid) {
      cout << "Number = " << num << endl;
   }
   else {
      cout << "Invalid entry." << endl;
   }
   return 0;
}

第41行中的'0'是什么意思？这行是否指定下一个字符减去nextChar的第一个字符？

Answer 1

nextChar - '0'返回字符的整数值.例如,如果字符值为'1'(即ASCII 0x31),那么如果减去'0'(即0x30),则得到值1.由于ASCII值'0' - '9'是连续的,因此技术会起作用.

Answer 2

它将ne​​xtChar中引用的数字(ASCII)转换为实际值.因此,如果ASCII值为'5',则从'5'减去'0'会得到数值5.

此转换允许您对通过键盘输入的值执行数学运算.