7.4.19

#include <iostream>
#include <string>
using namespace std;

int& replace(string& line, char from, char to) {
    int count = 0, lenght = line.length();
    for (int i = 0; i < lenght; i++) {
        if (line[i] == from) {
            line[i] = to;
            count++;
        }
    }
    return count;
}

int main() {
    string text = "";
    int lenght, i;
    bool isNumber = true, containPoint = false;

    setlocale(LC_ALL, "Russian");
    cout << "Введите вещественное число: ";
    getline(cin, text);

    replace(text, '.', ','); // '.' -> ','
    lenght = text.length();
    if (lenght == 0)
        isNumber = false;
    isNumber = isNumber && (isdigit(text[0]) || text[0] == '+' || text[0] == '-' || text[0] == ',');
    containPoint == isNumber && text[0] == ',';

    for (i = 1; i < lenght && isNumber; i++) {
        isNumber = isdigit(text[i]) || text[i] == ',';
        if (text[i] == ',') {
            isNumber = !containPoint && isNumber;
            containPoint = true;
        }
    }
    cout << (isNumber ? "Является вещ. числом!" : "Не является вещ. числом!") << endl;
}