Decisions with if and switch · C++ tutorial

if, else if, else#

int marks = 72;
char grade;

if (marks >= 90) {
    grade = 'A';
} else if (marks >= 75) {
    grade = 'B';
} else if (marks >= 60) {
    grade = 'C';
} else {
    grade = 'F';
}

Conditions are checked in order and only the first true branch runs, which is why the order matters: put >= 60 first and every pass becomes a C.

Always write the braces, even for one line. Without them, a second indented line looks like part of the if but always runs.

Combining conditions#

if (age >= 18 && hasTicket) { ... }
if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') { ... }
if (!found) { ... }

A variable just for the if#

C++17 lets an if declare a variable that exists only inside it, which keeps names from leaking:

if (auto pos = s.find("needle"); pos != string::npos) {
    cout << "found at " << pos << "\n";
}
// pos does not exist here

The ternary operator#

int larger = (a > b) ? a : b;
cout << (n % 2 == 0 ? "even" : "odd") << "\n";

Use it for a short choice between two values, and a full if for anything more.

switch#

One value compared against fixed alternatives:

switch (day) {
    case 1:
        cout << "Monday\n";
        break;
    case 6:
    case 7:
        cout << "Weekend\n";
        break;
    default:
        cout << "Weekday\n";
}

Without break, execution falls through into the next case. That is deliberate for 6 and 7 above and a bug almost everywhere else. switch works on integers, characters and enums, not on strings.

Doubles#

Never compare computed doubles with ==:

double x = 0.1 + 0.2;
if (abs(x - 0.3) < 1e-9) { ... }     // true

Handling special cases first#

Returning early from the odd cases keeps the main logic unindented:

int digits(int n) {
    if (n == 0) return 1;
    int count = 0;
    while (n != 0) { n /= 10; count++; }
    return count;
}

Key points#

Only the first true branch of an if chain runs; order the checks carefully.
Always use braces.
if (init; condition) scopes a variable to the if.
switch needs break, and does not work on strings.

Next: loops, including the range-based for.