Operators and reading input

Arithmetic#

+, -, *, / and % work as in C: with two integers, / drops the fraction (7 / 2 is 3) and % is the remainder (7 % 2 is 1). With negatives, % takes the sign of the left side: -7 % 2 is -1.

n % 10 and n / 10 take a number apart one digit at a time, the basis of digit problems.

There is no power operator. pow returns a double, which can be inexact for large results; for whole powers, multiply in a loop.

Shorthand#

i++;            // add one
i--;            // subtract one
total += x;     // total = total + x
total *= 2;

Comparisons and logic#

==, !=, <, >, <=, >= give true or false. Combine with && (and), || (or) and ! (not). Both && and || stop as soon as the answer is known, so put a safety check first:

if (i < (int) v.size() && v[i] == target) { ... }

= assigns, == compares. if (x = 5) is legal and always true; -Wall warns about it.

Reading input with cin#

cin >> reads the next value, skipping spaces and newlines, into a variable of any type:

int a, b;
cin >> a >> b;
cout << a + b << "\n";

string word;
cin >> word;            // one word: stops at a space

Reading until input runs out:

long long total = 0;
int x;
while (cin >> x) {
    total += x;
}

cin >> x is false once there is nothing left to read, which ends the loop.

Reading a whole line#

string line;
getline(cin, line);      // includes spaces

Mixing the two needs care. After cin >> n, the newline you typed is still waiting, and the next getline reads it as an empty line:

int n;
cin >> n;
cin.ignore();            // skip the leftover newline
string line;
getline(cin, line);

Fast input#

For large inputs, these two lines at the start of main make cin and cout much faster:

ios::sync_with_stdio(false);
cin.tie(nullptr);

After this, do not mix cin/cout with scanf/printf in the same program.

Order of operations#

Multiplication, division and remainder before addition and subtraction; arithmetic before comparisons; && before ||. Add brackets whenever a reader might hesitate.

Be careful with << and >> in expressions: they are also shift operators, so cout << a & b does not do what it looks like. Bracket it: cout << (a & b).

Key points#

• Integer / drops the fraction; % keeps the left side's sign.
• cin >> x reads one value and is false when input runs out.
• getline reads a whole line; use cin.ignore() after >> first.
• Two lines at the top of main make large input fast.

Next: strings, which in C++ manage themselves.