Check if a given string is a valid number in C++

Concept

It should be validated if a given string is numeric.

Input − str = "12.5"

Output − true

Input − str = "def"

Output − false

Input − str = "2e5"

Output − true

Input − 10e4.4

Output − false

Method

We have to handle the following cases in the code.

• We have to ignore the leading and trailing white spaces.

• We have to ignore the ‘+’, ‘-‘ and’.’ at the start.

• We have to ensure that the characters in the string belong to {+, -, ., e, [0-9]}

• We have to ensure that no ‘.’ comes after ‘e’.

• A digit should follow a dot character ‘.’.

• We have to ensure that the character ‘e’ should be followed either by ‘+’, ‘-‘, or a digit.

Example

 Live Demo

// C++ program to check if input number // is a valid number #include <bits/stdc++.h> #include <iostream> using namespace std; int valid_number1(string str1){    int i = 0, j = str1.length() - 1;    while (i < str1.length() && str1[i] == ' ')       i++;    while (j >= 0 && str1[j] == ' ')       j--;    if (i > j)       return 0;    if (i == j && !(str1[i] >= '0' && str1[i] <= '9'))       return 0;    if (str1[i] != '.' && str1[i] != '+' && str1[i] != '-' && !(str1[i] >= '0' && str1[i] <= '9'))       return 0;    bool flagDotOrE = false;    for (i; i <= j; i++) {       // If any of the char does not belong to       // {digit, +, -, ., e}       if (str1[i] != 'e' && str1[i] != '.'       && str1[i] != '+' && str1[i] != '-'       && !(str1[i] >= '0' && str1[i] <= '9'))       return 0;       if (str1[i] == '.') {          if (flagDotOrE == true)             return 0;          if (i + 1 > str1.length())             return 0;          if (!(str1[i + 1] >= '0' && str1[i + 1] <= '9'))             return 0;       }       else if (str1[i] == 'e') {          flagDotOrE = true;          if (!(str1[i - 1] >= '0' && str1[i - 1] <= '9'))             return 0;          if (i + 1 > str1.length())             return 0;          if (str1[i + 1] != '+' && str1[i + 1] != '-'             && (str1[i + 1] >= '0' && str1[i] <= '9'))          return 0;       }    }    return 1; } // Driver code int main(){    char str1[] = "0.1e10";    if (valid_number1(str1))       cout << "true";    else       cout << "false";    return 0; }

Output

true