This lesson is a continuation of lesson 8.2 -- If statements and blocks. In this lesson, we’ll take a look at some common problems that occur when using if-statements.
Nested if-statements and the dangling else problem
It is possible to nest if-statements within other if-statements:
#include <iostream>
int main()
{
std::cout << "Enter a number: ";
int x{};
std::cin >> x;
if (x >= 0) // outer if statement
// it is bad coding style to nest if statements this way
if (x <= 20) // inner if statement
std::cout << x << " is between 0 and 20\n";
return 0;
}Now consider the following program:
#include <iostream>
int main()
{
std::cout << "Enter a number: ";
int x{};
std::cin >> x;
if (x >= 0) // outer if-statement
// it is bad coding style to nest if statements this way
if (x <= 20) // inner if-statement
std::cout << x << " is between 0 and 20\n";
// which if statement does this else belong to?
else
std::cout << x << " is negative\n";
return 0;
}The above program introduces a source of potential ambiguity called a dangling else problem. Is the else-statement in the above program matched up with the outer or inner if-statement?
The answer is that an else-statement is paired up with the last unmatched if-statement in the same block. Thus, in the program above, the else-statement is matched up with the inner if-statement, as if the program had been written like this:
#include <iostream>
int main()
{
std::cout << "Enter a number: ";
int x{};
std::cin >> x;
if (x >= 0) // outer if statement
{
if (x <= 20) // inner if statement
std::cout << x << " is between 0 and 20\n";
else // attached to inner if statement
std::cout << x << " is negative\n";
}
return 0;
}This causes the above program to produce incorrect output:
Enter a number: 21 21 is negative
To avoid such ambiguities when nesting if-statements, it is a good idea to explicitly enclose the inner if-statement within a block. This allows us to attach an else-statement to either the inner or the outer if-statement without ambiguity:
#include <iostream>
int main()
{
std::cout << "Enter a number: ";
int x{};
std::cin >> x;
if (x >= 0)
{
if (x <= 20)
std::cout << x << " is between 0 and 20\n";
else // attached to inner if statement
std::cout << x << " is greater than 20\n";
}
else // attached to outer if statement
std::cout << x << " is negative\n";
return 0;
}The else-statement within the block attaches to the inner if-statement, and the else-statement outside of the block attaches to the outer if-statement.
Flattening nested if-statements
Nested if-statements can often be flattened by either restructuring the logic or by using logical operators (covered in lesson 6.8 -- Logical operators). Code that is less nested is less error prone.
For example, the above example can be flattened as follows:
#include <iostream>
int main()
{
std::cout << "Enter a number: ";
int x{};
std::cin >> x;
if (x < 0)
std::cout << x << " is negative\n";
else if (x <= 20) // only executes if x >= 0
std::cout << x << " is between 0 and 20\n";
else // only executes if x > 20
std::cout << x << " is greater than 20\n";
return 0;
}Here’s another example that uses logical operators to check multiple conditions within a single if-statement:
#include <iostream>
int main()
{
std::cout << "Enter an integer: ";
int x{};
std::cin >> x;
std::cout << "Enter another integer: ";
int y{};
std::cin >> y;
if (x > 0 && y > 0) // && is logical and -- checks if both conditions are true
std::cout << "Both numbers are positive\n";
else if (x > 0 || y > 0) // || is logical or -- checks if either condition is true
std::cout << "One of the numbers is positive\n";
else
std::cout << "Neither number is positive\n";
return 0;
}Null statements
A null statement is an expression statement that consists of just a semicolon:
if (x > 10)
; // this is a null statementNull statements do nothing. They are typically used when the language requires a statement to exist but the programmer doesn’t need one. For readability, null statements are typically placed on their own lines. We’ll see examples of intentional null statements later in this chapter, when we cover loops.
Null statements are rarely intentionally used with if-statements. However, they can unintentionally cause problems for new (or careless) programmers. Consider the following snippet:
if (nuclearCodesActivated()); // note the semicolon at the end of this line
blowUpTheWorld();In the above snippet, the programmer accidentally put a semicolon on the end of the if-statement (a common mistake since semicolons end many statements). This unassuming error compiles fine, and causes the snippet to execute as if it had been written like this:
if (nuclearCodesActivated())
; // the semicolon acts as a null statement
blowUpTheWorld(); // and this line always gets executed!Warning
Be careful not to “terminate” your if-statement with a semicolon, otherwise the statement(s) you wanted to execute conditionally will execute unconditionally instead (even if they are inside a block).
Tip
In Python, the pass keyword serves as a null statement. It is often used as a placeholder for code that will be implemented later. Because it is a word rather than a symbol, pass is less prone to inadvertent misuse and more searchable (allowing you to easily find those placeholders later).
for x in [0, 1, 2]: pass # To be completed in the future
In C++, we can mimic pass by using the preprocessor:
#define PASS
void foo(int x, int y)
{
if (x > y)
PASS;
else
PASS;
}
int main()
{
foo(4, 7);
return 0;
}For consistency with other C++ statements, our PASS requires a trailing semicolon. PASS gets stripped out by the preprocessor, and the trailing semicolon is interpreted by the compiler as a null statement.
Operator== vs Operator= inside the conditional
Inside your conditional, you should be using operator== when testing for equality, not operator= (which is assignment). Consider the following program:
#include <iostream>
int main()
{
std::cout << "Enter 0 or 1: ";
int x{};
std::cin >> x;
if (x = 0) // oops, we used an assignment here instead of a test for equality
std::cout << "You entered 0\n";
else
std::cout << "You entered 1\n";
return 0;
}This program will compile and run, but will produce the wrong result in some cases:
Enter 0 or 1: 0 You entered 1
In fact, this program will always produce the result You entered 1. This happens because x = 0 first assigns the value 0 to x, then evaluates to the value of x, which is now 0, which is Boolean false. Since the conditional is always false, the else-statement always executes.