Example 1: From a C-style string
This program takes a C-style string from the user and calculates the number of vowels, consonants, digits and white-spaces.
#include <iostream>
using namespace std;
int main()
{
    char line[150];
    int vowels, consonants, digits, spaces;
    vowels =  consonants = digits = spaces = 0;
    cout << "Enter a line of string: ";
    cin.getline(line, 150);
    for(int i = 0; line[i]!='\0'; ++i)
    {
        if(line[i]=='a' || line[i]=='e' || line[i]=='i' ||
           line[i]=='o' || line[i]=='u' || line[i]=='A' ||
           line[i]=='E' || line[i]=='I' || line[i]=='O' ||
           line[i]=='U')
        {
            ++vowels;
        }
        else if((line[i]>='a'&& line[i]<='z') || (line[i]>='A'&& line[i]<='Z'))
        {
            ++consonants;
        }
        else if(line[i]>='0' && line[i]<='9')
        {
            ++digits;
        }
        else if (line[i]==' ')
        {
            ++spaces;
        }
    }
    cout << "Vowels: " << vowels << endl;
    cout << "Consonants: " << consonants << endl;
    cout << "Digits: " << digits << endl;
    cout << "White spaces: " << spaces << endl;
    return 0;
}
Output
Enter a line of string: This is 1 hell of a book. Vowels: 7 Consonants: 10 Digits: 1 White spaces: 6
Example 2: From a String Object
This program takes a string object from the user and calculates the number of vowels, consonants, digits and white-spaces.
#include <iostream>
using namespace std;
int main()
{
    string line;
    int vowels, consonants, digits, spaces;
    vowels =  consonants = digits = spaces = 0;
    cout << "Enter a line of string: ";
    getline(cin, line);
    for(int i = 0; i < line.length(); ++i)
    {
        if(line[i]=='a' || line[i]=='e' || line[i]=='i' ||
           line[i]=='o' || line[i]=='u' || line[i]=='A' ||
           line[i]=='E' || line[i]=='I' || line[i]=='O' ||
           line[i]=='U')
        {
            ++vowels;
        }
        else if((line[i]>='a'&& line[i]<='z') || (line[i]>='A'&& line[i]<='Z'))
        {
            ++consonants;
        }
        else if(line[i]>='0' && line[i]<='9')
        {
            ++digits;
        }
        else if (line[i]==' ')
        {
            ++spaces;
        }
    }
    cout << "Vowels: " << vowels << endl;
    cout << "Consonants: " << consonants << endl;
    cout << "Digits: " << digits << endl;
    cout << "White spaces: " << spaces << endl;
    return 0;
}
Output
Enter a line of string: I have 2 C++ programming books. Vowels: 8 Consonants: 14 Digits: 1 White spaces: 5
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