专栏主页：计算机专业基础知识总结（适用于期末复习考研刷题求职面试）系列文章
https://blog.csdn.net/seeker1994/category_12585732.html
题目描述
请实现一个函数用来判断字符串是否表示数值（包括整数和小数）。例如，字符串"+100","5e2","-123","3.1416"和"-1E-16"都表示数值。 但是"12e","1a3.14","1.2.3","+-5"和"12e+4.3"都不是。
题解报告
编译原理状态机
# include <iostream>
# include <string>
using namespace std;
bool IsNum(char ch){if(ch<'0'||ch>'9') return false;else return true;
}
bool isNumeric(char* str){int i = 0;if(!(str[i] == '+' || str[i] == '-' || IsNum(str[i])) )return false;else{i++;while(IsNum(str[i]) && str[i] != '\0')i++; //如果是数字一直往后找if(str[i] == '\0'){return true;} else if(str[i] == '.'){i++;if(str[i] == '\0')return true;else if(IsNum(str[i])){while(str[++i]!='\0' && IsNum(str[i]));if(str[i]=='e'||str[i]=='E'){i++;if(str[i]=='+' || str[i]=='-' || IsNum(str[i])){while(str[++i]!='\0' && IsNum(str[i]));if(str[i]=='\0')return true;elsereturn false;}elsereturn false;}else if(str[i]=='\0'){return true;}else{return false;} }else{return false;}}else if(str[i]=='e'||str[i]=='E'){i++;if(str[i]=='+' || str[i]=='-' || IsNum(str[i])){while(str[++i]!='\0' && IsNum(str[i]));if(str[i]=='\0') return true;else return false;}else return false;} else{return false; } }
}
int main(){char str[] = "1.25";if(isNumeric(str)){cout<<"YES"<<endl; }else{cout<<"NO"<<endl;}return 0;
}
优化后的代码，使用状态表:
# include <iostream>
# include <string>
using namespace std;
char arr[10] = "+-n.ne+-n";
int turn[10][9] = {//+ - n . n e + - n{1, 1, 1, 0, 0, 0, 0, 0, 0}, // # start{0, 0, 1, 1, 0, 0, 0, 0, 0}, // +{0, 0, 1, 1, 0, 0, 0, 0, 0}, // -{0, 0, 1, 1, 0, 1, 0, 0, 0}, // n{0, 0, 0, 0, 1, 0, 0, 0, 0}, // .{0, 0, 0, 0, 1, 1, 0, 0, 0}, // n{0, 0, 0, 0, 0, 0, 1, 1, 1}, // e{0, 0, 0, 0, 0, 0, 0, 0, 1}, // +{0, 0, 0, 0, 0, 0, 0, 0, 1}, // -{0, 0, 0, 0, 0, 0, 0, 0, 1} // n
};
bool isNumeric(char* string) {int cur = 0,i,j;for(i = 0; string[i]; i++) {for(j = 0; j < 9; j++) {if(turn[cur][j]) {//状态转移 //状态转移 if(('0' <= string[i] && string[i] <= '9' && arr[j] == 'n') ||(string[i] == 'E' && arr[j] == 'e') ||string[i] == arr[j]) {cur = j + 1;break;}}}if(j == 9)return false;}if(cur == 3 || cur == 4 || cur == 5 || cur == 9)return true;return false;
}
int main(){char str[] = "1.25";if(isNumeric(str)){cout<<"YES"<<endl; }else{cout<<"NO"<<endl;}return 0;
}#1正则
public boolean isNumeric(char[] str) {String string = String.valueOf(str);return string.matches("[\\+-]?[0-9]*(\\.[0-9]*)?([eE][\\+-]?[0-9]+)?");
}
//标记法
bool isNumeric(char* str) {// 标记符号、小数点、e是否出现过bool sign = false, decimal = false, hasE = false;for (int i = 0; i < strlen(str); i++) {if (str[i] == 'e' || str[i] == 'E') {if (i == strlen(str)-1) return false; // e后面一定要接数字if (hasE) return false; // 不能同时存在两个ehasE = true;} else if (str[i] == '+' || str[i] == '-') {// 第二次出现+-符号，则必须紧接在e之后if (sign && str[i-1] != 'e' && str[i-1] != 'E') return false;// 第一次出现+-符号，且不是在字符串开头，则也必须紧接在e之后if (!sign && i > 0 && str[i-1] != 'e' && str[i-1] != 'E') return false;sign = true;} else if (str[i] == '.') {// e后面不能接小数点，小数点不能出现两次if (hasE || decimal) return false;decimal = true;} else if (str[i] < '0' || str[i] > '9') //不合法字符return false;}return true;
}
