Statements and flow control

A compound statement is a group of statements (each of them terminated by its own semicolon), but all grouped together in a block, enclosed in curly braces: {}:

{ statement1; statement2; statement3; }

The entire block is considered a single statement (composed itself of multiple substatements). Whenever a generic statement is part of the syntax of a flow control statement, this can either be a simple statement or a compound statement.

Selection statements: if and else

The if keyword is used to execute a statement or block, if, and only if, a condition is fulfilled. Its syntax is:

if (condition) statement

If this condition is true, statement is executed. If it is false, statement is not executed (it is simply ignored), and the program continues right after the entire selection statement.

Selection statements with if can also specify what happens when the condition is not fulfilled, by using the else keyword to introduce an alternative statement. Its syntax is:

if (condition) statement1 else statement2

where statement1 is executed in case condition is true, and in case it is not, statement2 is executed.

For example:

if (x == 100)
  cout << "x is 100";
else
  cout << "x is not 100";

if (x > 0)
  cout << "x is positive";
else if (x < 0)
  cout << "x is negative";
else
  cout << "x is 0";

Iteration statements (loops)
Loops repeat a statement a certain number of times, or while a condition is fulfilled. They are introduced by the keywords while, do, and for.

The while loop
The simplest kind of loop is the while-loop. Its syntax is:

while (expression) statement

The while-loop simply repeats statement while expression is true. If, statement is not true then the loop ends, and the program continues right after the loop.

// custom countdown using while
#include <iostream>
using namespace std;

int main ()
{
  int n = 10;

  while (n>0) {
    cout << n << ", ";
    --n;
  }

  cout << "liftoff!\n";
}

10, 9, 8, 7, 6, 5, 4, 3, 2, 1, liftoff!

The whole process of the previous program can be interpreted according to the following script (beginning in main):

n is assigned a value
The while condition is checked (n>0). At this point there are two possibilities:
condition is true: the statement is executed (to step 3)
condition is false: ignore statement and continue after it (to step 5)
Execute statement:
cout << n << ", ";
--n;
(prints the value of n and decreases n by 1)
End of block. Return automatically to step 2.
Continue the program right after the block:
print liftoff! and end the program.
Note that the complexity of this loop is trivial for a computer, and so the whole countdown is performed instantly, without any practical delay between elements of the count (if interested, see sleep_for for a countdown example with delays).

The do-while loop
A very similar loop is the do-while loop, whose syntax is:

do statement while (condition);

It behaves like a while-loop, except that condition is evaluated after the execution of statement instead of before, guaranteeing at least one execution of statement, even if condition is never fulfilled. For example, the following example program echoes any text the user introduces until the user enters goodbye:

// echo machine
#include <iostream>
#include <string>
using namespace std;

int main ()
{
  string str;
  do {
    cout << "Enter text: ";
    getline (cin,str);
    cout << "You entered: " << str << '\n';
  } while (str != "goodbye");
}

Enter text: hello
You entered: hello
Enter text: who's there?
You entered: who's there?
Enter text: goodbye
You entered: goodbye

The do-while loop is usually preferred over a while-loop when the statement needs to be executed at least once, such as when the condition that is checked to end of the loop is determined within the loop statement itself. In the previous example, the user input within the block is what will determine if the loop ends. And thus, even if the user wants to end the loop as soon as possible by entering goodbye, the block in the loop needs to be executed at least once to prompt for input, and the condition can, in fact, only be determined after it is executed.

The for loop
The for loop is designed to iterate a number of times. Its syntax is:

for (initialization; condition; increase) statement;

Like the while-loop, this loop repeats statement while condition is true. But, in addition, the for loop provides specific locations to contain an initialization and an increase expression, executed before the loop begins the first time, and after each iteration, respectively. Therefore, it is especially useful to use counter variables as condition.

It works in the following way:
initialization is executed. Generally, this declares a counter variable, and sets it to some initial value. This is executed a single time, at the beginning of the loop.
condition is checked. If it is true, the loop continues; otherwise, the loop ends, and statement is skipped, going directly to step 5.
statement is executed. As usual, it can be either a single statement or a block enclosed in curly braces { }.
increase is executed, and the loop gets back to step 2.
the loop ends: execution continues by the next statement after it.
Here is the countdown example using a for loop:

// countdown using a for loop
#include <iostream>
using namespace std;

int main ()
{
  for (int n=10; n>0; n--) {
    cout << n << ", ";
  }
  cout << "liftoff!\n";
}

10, 9, 8, 7, 6, 5, 4, 3, 2, 1, liftoff!

The three fields in a for-loop are optional. They can be left empty, but in all cases the semicolon signs between them are required. For example, for (;n<10;) is a loop without initialization or increase (equivalent to a while-loop); and for (;n<10;++n) is a loop with increase, but no initialization (maybe because the variable was already initialized before the loop). A loop with no condition is equivalent to a loop with true as condition (i.e., an infinite loop).

Because each of the fields is executed in a particular time in the life cycle of a loop, it may be useful to execute more than a single expression as any of initialization, condition, or statement. Unfortunately, these are not statements, but rather, simple expressions, and thus cannot be replaced by a block. As expressions, they can, however, make use of the comma operator (,): This operator is an expression separator, and can separate multiple expressions where only one is generally expected. For example, using it, it would be possible for a for loop to handle two counter variables, initializing and increasing both:

for ( n=0, i=100 ; n!=i ; ++n, --i )
{
   // whatever here...
}

This loop will execute 50 times if neither n or i are modified within the loop:



n starts with a value of 0, and i with 100, the condition is n!=i (i.e., that n is not equal to i). Because n is increased by one, and i decreased by one on each iteration, the loop's condition will become false after the 50th iteration, when both n and i are equal to 50.

Range-based for loop
The for-loop has another syntax, which is used exclusively with ranges:

for ( declaration : range ) statement;

This kind of for loop iterates over all the elements in range, where declaration declares some variable able to take the value of an element in this range. Ranges are sequences of elements, including arrays, containers, and any other type supporting the functions begin and end; Most of these types have not yet been introduced in this tutorial, but we are already acquainted with at least one kind of range: strings, which are sequences of characters.

An example of range-based for loop using strings:

// range-based for loop
#include <iostream>
#include <string>
using namespace std;

int main ()
{
  string str {"Hello!"};
  for (char c : str)
  {
    cout << "[" << c << "]";
  }
  cout << '\n';
}

[H][e][l][l][o][!]

Note how what precedes the colon (:) in the for loop is the declaration of a char variable (the elements in a string are of type char). We then use this variable, c, in the statement block to represent the value of each of the elements in the range.

This loop is automatic and does not require the explicit declaration of any counter variable.

Range based loops usually also make use of type deduction for the type of the elements with auto. Typically, the range-based loop above can also be written as:

for (auto c : str)
  cout << "[" << c << "]";

Here, the type of c is automatically deduced as the type of the elements in str.

Jump statements
Jump statements allow altering the flow of a program by performing jumps to specific locations.