Now consider the following program:
#include int main() < std::cout ; std::cin >> x; if (x >= 0) // outer if statement // it is bad coding style to nest if statements this way if (xThe 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 is matched up with the inner if statement , as if the program had been written like this:
#include int main() < std::cout ; std::cin >> x; if (x >= 0) // outer if statement < if (x return 0; >This causes the above program to produce incorrect output:
Enter a number: 21 21 is negativeTo 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 to either if statement without ambiguity:
#include int main() < std::cout ; std::cin >> x; if (x >= 0) < if (x else // attached to outer if statement std::coutThe 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 6.7 -- Logical operators). Code that is less nested is less error prone.
For example, the above example can be flattened as follows:
#include int main() < std::cout ; std::cin >> x; if (x < 0) std::cout = 0 std::cout 20 std::coutHere’s another example that uses logical operators to check multiple conditions within a single if statement :
#include int main() < std::cout ; std::cin >> x; std::cout ; std::cin >> y; if (x > 0 && y > 0) // && is logical and -- checks if both conditions are true std::cout 0 || y > 0) // || is logical or -- checks if either condition is true std::coutA 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()); 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!Be careful not to “terminate” your if statement with a semicolon, otherwise your conditional statement(s) will execute unconditionally (even if they are inside a block).
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 int main() < std::cout ; std::cin >> x; if (x = 0) // oops, we used an assignment here instead of a test for equality std::coutThis program will compile and run, but will produce the wrong result in some cases:
Enter 0 or 1: 0 You entered 1In 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.
