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1. Question analysis
Please implement a function to determine whether a string represents a value (including integers and decimals).
For example, the string "+100","5e2","-123","3.1416"and "-1E-16"both represent numeric values.
However, "12e","1a3.14","1.2.3","+-5"and "12e+4.3"not.
The key to this question is right + - . e E, the discussion of these 5 characters
(1) e/E cannot appear twice at the same time, and must be followed by a number.
(2) The positive and negative sign, if it appears for the first time, either the position is at the beginning or immediately after e/E, if it is the second The second occurrence must immediately follow e/E.
(3) The decimal point cannot appear twice and cannot appear after e/E
Second, the code
import java.util.Arrays;
import java.util.Scanner;
/**
* @Auther: Yolo
* @Date: 2020/9/11 16:58
* @Description:
*/
public class Test_11 {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.print("请输入字符串：");
String next = scanner.next();
char[] str = next.toCharArray();
System.out.println(Arrays.toString(str));
boolean result = isNumeric(str);
System.out.println(result);
}
//e的ascii：101，
private static boolean isNumeric(char[] str) {
//标记符号，小数点，e是否出现过
boolean sign = false, decimal = false, hasE = false;
for (int i = 0; i < str.length; i++) {
if (str[i] == 'e' || str[i] == 'E') {
//e 或 E 的后面一定要接数字，不能是末位出现
if (i == str.length - 1) {
return false;
}
//不能同时存在两个 e/E
if (hasE) {
return false;
}
hasE = true;
} else if (str[i] == '+' || str[i] == '-') {
//第二次出现正负号，必须紧接在 e 之后
if (sign && str[i - 1] != 'e' && str[i - 1] != 'E') {
return false;
}
//第一次出现正负号，如果不是在开头，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，小数点也不能出现两次
if (hasE || decimal) {
return false;
}
decimal=true;
} else if (str[i] < '0' || str[i] > '9') {
//其余的不合法字符
return false;
}
}
return true;
}
}
Three, summary
The key is to mark the E/e, decimal point, and sign in advance to determine the number of occurrences