Control Flow

So far, we have created basic programs that perform a single task without making any decisions. Most (all but the most trivial) programming languages provide decision-making constructs (Conditional Branching).

C++ provides two forms of branching. The "If Statement" and the "Switch Statement" .

Note: Looping is also a form of branching. The looping concept is extensive enough that we will deal with looping separately in its own article.

If Statement

The "If Statement" allows code to be executed when a specific condition is fulfilled and optionally an alternative piece of code otherwise.

ifstatement.cpp

// First version of "If Statement"
// Execute code if <Condition> is true.
//
if (<Condition>)
{
    <code to execute if Condition is true>
}


// Second version of "If Statement"
// Execute code1 if <Condition> is true or code2 if <Condition> is false
//
if (<Condition>)
{
    <code1: execute if Condition is true>
}
else
{
    <code2: execute if Condition is false>
}

The standard comparison operators that you find in most languages can be used. These operators are defined for all the built-in types. User-defined types in the standard library are defined in ways that make their usage obvious. When you define these for your user-defined types, you should also make sure that they behave in the logical manner described below; the language does not enforce this, BUT if you don't follow this suggestion, your types will scare people, and they will not be used, so follow the expected behavior.

Standard Comparison Operators

/*
| Operator  | Usage   | Result Type | Meaning                                                  |
| ----------|---------|-------------|----------------------------------------------------------|
|    !      |  !A     |  bool       | Not A.                                                   |
|           |         |             | If A is true, then false; if A is false, then true       |
|           |         |             | If A is not a bool type, it is converted (see below)     |
|    ==     |  A == B |  bool       | Equality: true if A and B are logically equivalent;      |
|           |         |             | otherwise, false.                                        |
|    !=     |  A != B |  bool       | Should mean !(A == B)                                    |
|    <      |  A <  B |  bool       | true if A is logically less than B.                      |
|    <=     |  A <= B |  bool       | true if A is logically less than or equal to B.          |
|    >      |  A >  B |  bool       | true if A is logically greater than B.                   |
|    >=     |  a >= B |  bool       | true if A is logically greater than or equal to B.       |
|    &&     |  A && B |  bool       | true if A is true **AND** B is true.                     |
|           |         |             | If the expressions A or B are not bool, then             |
|           |         |             | it is converted (see below).                             |
|           |         |             | Also worth noting is that if A is **false** then the     |
|           |         |             | expression for B is not evaluated.                       |
|           |         |             | This is known as a shortcut operator.                    |
|           |         |             | we will describe this later.                             |
|    ||     |  A || B |  bool       | true if A is true **OR** B is true.                      |
|           |         |             | If the expressions A or B are not actually a bool then   |
|           |         |             | it is converted (see below). Also worth noting is that   |
|           |         |             | if A is **true** then the expression for B is not        |
|           |         |             | evaluated.                                               |
|           |         |             | This is known as a shortcut operator.                    |
|           |         |             | we will describe this later.                             |
|-----------|---------|-------------|----------------------------------------------------------|
*/

If the expression you use in <Condition> does not result in a bool value, the compiler will insert a conversion that will result in a bool (true/false) value. If no conversion is possible, it results in a compile-time error.

Type conversion

/*
| Type             | false      | true            | Notes                                      |
|------------------|------------|-----------------|--------------------------------------------|
| bool             | false      | true            | No actual conversion used.                 |
| Integers         | 0          | (anything else) | Integer shorthand for (char/short/int/long)|
| Pointers         | NULL       | (anything else) | Will discuss pointers in detail later.     |
| User Define Type | ?          | ?               | If a cast operator to bool/Integer/pointer |
|                  |            |                 | exists this will be used.                  |
|------------------|------------|-----------------|--------------------------------------------|
*/

An example of using an If Statement:

itest.cpp

#include <iostream>
#include <string>

int main()
{
    std::string    name;
    std::cout << "Please enter your name\n";
    std::cin >> name;

    if (name == "Loki")
    {
        std::cout << "Hello Admin\n";
    }
    else
    {
        std::cout << "Hello Muggle\n";
    }

    int   value;
    std::cin >> value;
    std::cout << "Please enter a non-zero integer value.\n";
    if (value) // integer value converted to bool
    {
        std::cout << "You got it correct. Must use a non-zero value.\n";
    }
}

Switch Statement

The "Switch Statement" is an alternative to the "If Statement". Prefer the switch when you have many options derived from the same expression. Unlike other high-level languages, C++ can only use Integer types in a switch statement; thus, in all case <Value> the <Value> must be an integer literal value.

switch.cpp

switch(<Test Expression>)
{
    case <value1>:
    {
        <code1>
        break;
    }
    case <value2>:
    {
        <code2>
        break;
    }
    case <value3>:
    {
        <code3>
        break;
    }
    default:
    {
        <code Default>
        break
    }
}
//
//
// Equivalent "If Statement"

int test = <Test Expression>;
if (<value1> == test)
{
    <code1>
}
else
{
    if (<value2> == test)
    {
        <code2>
    }
    else
    {
        if (<value3> == test)
        {
            <code3>
        }
        else
        {
            <code Default>
        }
    }
}

If you use a non-integer expression in the switch statement, the compiler will try to convert the value to an integer. If this is not possible, it generates a compile-time error.

switch.cpp

    #include <iostream>

    int main()
    {
        int  value;
        std::cout << "Input a value between 0 and 5\n";
        std::cin >> value;

        switch(value)
        {
            case 0: {std::cout << "You used zero\n";    break;}
            case 1: {std::cout << "You used one\n";     break;}
            case 2: {std::cout << "You used two\n";     break;}
            case 3: {std::cout << "You used three\n";   break;}
            case 4: {std::cout << "You used four\n";    break;}
            case 5: {std::cout << "You used five\n";    break;}
            default: {std::cout << "You failed to follow instructions\n";break;}
        }
    }

Note I: The language does not require you to use a Break Statement in each block. BUT you should; compilers will warn you when you don't.

Note II: You should always use a Default Statement . If the value does not hit a value specified in a Case Statement, then the Default Statement is used; If the Default Statement is not defined in this situation, it results in undefined behavior. To avoid problems, you should always define the Default Statement, even if all this does is generate an error. This will avoid maintenance issues down the road.