Bytebook Compiler

From C to C++

Input/Output, Strings & References

Read and write with cin and cout, use the std::string class, format output with iomanip, and meet references — C++'s safer alias for a variable.

4 min read6 runnable examples4-question quiz

cin: reading input

cin (character input) reads values with the extraction operator >>. It works out the type from the variable — no format specifiers and no &:

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

int main() {
    string name;
    int age;
    double cgpa;
    cout << "Name, age and CGPA: ";
    cin >> name >> age >> cgpa;
    cout << name << " is " << age << " with a CGPA of " << cgpa << endl;
    return 0;
}
Output
Name, age and CGPA: Ravi 19 8.6
Ravi is 19 with a CGPA of 8.6
Sample input is includedTry it Yourself

Like scanf("%s"), cin >> name stops at whitespace. To read a whole line, use getline:

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

int main() {
    int roll;
    string fullName;
    cout << "Roll number: ";
    cin >> roll;
    cin.ignore();                       // skip the newline left after the number
    cout << "Full name: ";
    getline(cin, fullName);
    cout << "#" << roll << ": " << fullName << " (" << fullName.length() << " characters)" << endl;
    return 0;
}
Output
Roll number: 21
Full name: Ravi Kumar Sharma
#21: Ravi Kumar Sharma (17 characters)
Sample input is includedTry it Yourself

The string class

std::string grows automatically, knows its own length, and supports +, == and < directly:

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

int main() {
    string a = "Data", b = "Structures";
    string full = a + " " + b;                 // concatenation
    cout << full << " has " << full.size() << " characters" << endl;
    cout << "First letter: " << full[0] << ", last: " << full.back() << endl;
    cout << "Substring: " << full.substr(5, 6) << endl;         // from index 5, length 6
    cout << "\"Struct\" found at " << full.find("Struct") << endl;
    full.replace(0, 4, "Algo");
    cout << "After replace: " << full << endl;
    cout << boolalpha << ("apple" < string("banana")) << endl;   // dictionary order
    for (char c : string("abc")) cout << char(c - 32);           // range-based for
    cout << endl;
    return 0;
}
Output
Data Structures has 15 characters
First letter: D, last: s
Substring: Struct
"Struct" found at 5
After replace: Algo Structures
true
ABC
Try it Yourself

Formatting with iomanip

C++
#include <iostream>
#include <iomanip>
using namespace std;

int main() {
    double pi = 3.14159265;
    cout << fixed << setprecision(2) << pi << endl;          // 2 decimal places
    cout << setw(8) << 42 << "|" << endl;                      // width 8, right-aligned
    cout << left << setw(8) << 42 << "|" << endl;              // left-aligned
    cout << setfill('0') << right << setw(5) << 7 << endl;     // pad with zeros
    cout << hex << 255 << " " << oct << 8 << dec << endl;      // number bases
    return 0;
}
Output
3.14
      42|
42      |
00007
ff 10
Try it Yourself

References

A reference is another name (an alias) for an existing variable. It must be initialised when declared and can never be re-seated to refer to something else.

C++
#include <iostream>
using namespace std;

int main() {
    int marks = 70;
    int &r = marks;          // r is another name for marks
    r += 15;
    cout << "marks = " << marks << ", r = " << r << endl;
    cout << "same address? " << (&marks == &r ? "yes" : "no") << endl;
    return 0;
}
Output
marks = 85, r = 85
same address? yes
Try it Yourself

The real power of references is in function parameters — the subject of the next lesson.

Pointer Reference
can be nullptr always refers to a valid object
can be re-pointed later bound once, forever
needs * to read, & to take an address used like the variable itself
can do arithmetic no arithmetic
has its own memory usually just an alias

bool and auto

C++ has a built-in bool type with true and false. And auto lets the compiler deduce a variable's type from its initialiser:

C++
#include <iostream>
#include <vector>
using namespace std;

int main() {
    bool passed = true;
    auto count = 42;            // int
    auto ratio = 0.75;          // double
    auto name = string("Asha"); // std::string
    cout << boolalpha << passed << " " << count << " " << ratio << " " << name << endl;
    vector<int> v = {3, 1, 4};
    for (auto x : v) cout << x * 10 << " ";
    cout << endl;
    return 0;
}
Output
true 42 0.75 Asha
30 10 40
Try it Yourself

Test yourself

4 questions · pick an answer to check it instantly

  1. 1

    Which reads a full line, including spaces, into string s?

  2. 2

    What is printed by string s = "hello"; cout << s.size();?

  3. 3

    Which statement about references is true?

  4. 4

    What does cout << fixed << setprecision(1) << 2.46; print?

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