Conditional Expressions
Conditional expressions use the concepts of equality and ordering to allow programs to branch. This means that different parts of a program will execute depending on the current state of a program. This allows for programs to be more adaptive and flexible to create.
Scope
One important concept in programming is the idea of scope. In C++, scope is very important as it can have an important impact on the design, performance in the safety of a program. Currently we have been running a program purely in the scope of the main() function. What denotes a scope in C++ is a pair of braces {}. Anything introduced within the braces is now in a new scope separate from the outside program. Objects from outside the new scope can be captured but anything created in a new scope is dropped at the end of the scope.
#include <iostream>
auto main () -> int
{
auto a {4};
{
auto b {6};
std::cout << a << std::endl;
std::cout << b << std::endl;
}
std::cout << a << std::endl;
std::cout << b << std::endl; ///< Will fail here, comment out to run
return 0;
}
Scope blocks (or sometimes code blocks) allow us to encapsulate expressions.
if-expressions
Control flow allows for programs to branch and follow non-linear execution patterns. In C++, this is done through structured conditional expressions. The primary one being an if expression. if expressions are followed by a code block surrounded in {}. If the condition is met, then the code block executes.
if (/* Boolean condition */) { /* code */ }
Note: One-liner execution blocks don't have to be in
{}.
#include <iostream>
auto main () -> int
{
auto a {1};
auto b {2};
if (a < b)
{
std::cout << "a is less then b" << std::endl;
}
if (a == b)
std::cout << "a is equal to b" << std::endl;
if (a > b)
std::cout << "a is greater then b" << std::endl;
return 0;
}
We can also use an else clause at the end. This indicates that if and if expression fails, the else clause will execute instead.
#include <iostream>
auto main () -> int
{
auto a {1};
auto b {2};
if (a == b)
std::cout << "a is equal to b" << std::endl;
else
std::cout << "a is not equal to b" << std::endl;
return 0;
}
else-expression
We can combine the else clause with an if expression to create and else if expression. This allows for you to create multiple branches in a single if statement.
#include <iostream>
auto main () -> int
{
auto a {1};
auto b {2};
if (a < b)
std::cout << "a is less then b" << std::endl;
else if (a == b)
std::cout << "a is equal to b" << std::endl;
else if (a > b)
std::cout << "a is greater then b" << std::endl;
else
std::cout << "a is unordered to b" << std::endl;
return 0;
}
Ternary Operator
if statements are the first step to building much more complex programs however, they are quite bulky in syntax for short conditional expressions. Instead we can use the ternary operator ?: to build short and concise conditional expressions. A ternary expression has the following syntax.
auto /* result */ = /* Boolean expression */ ? /* expression is true */ : /* expression is false */;
#include <iostream>
auto main () -> int
{
auto a {1};
auto b {2};
auto msg = a < b ? "a is less then b" : "b is less then a";
std::cout << msg << std::endl;
return 0;
}
Switch Statements
Another useful construct in C++ is a switch statement. switch statements encapsulate the idea of a jump table however, it is limited to integral types, thus the switch condition must be an integral type and have integral case labels.
#include <iostream>
auto main () -> int
{
auto a {1};
switch (a)
{
case 1:
std::cout << "a + 1 = " << a + 1 << std::endl;
break;
case 2:
std::cout << "a * 2 = " << a * 2 << std::endl;
break;
case 3:
std::cout << "a - 3 = " << a - 3 << std::endl;
break;
default:
std::cout << "a / 5 = " << a / 5 << std::endl;
break;
}
return 0;
}
Fallthroughs
You will note that at the end of each of the case blocks there are break statements. These are used to exit the switch statement entirely. If this weren't there, each case would run in sequence. This is called fallthrough. most compilers will warn you of this.
#include <iostream>
auto main () -> int
{
auto a {1};
switch (a)
{
case 1:
std::cout << "a + 1 = " << a + 1 << std::endl;
case 2:
std::cout << "a * 2 = " << a * 2 << std::endl;
case 3:
std::cout << "a - 3 = " << a - 3 << std::endl;
default:
std::cout << "a / 5 = " << a / 5 << std::endl;
}
return 0;
}