Common Mistakes with Conditions in C++: Understanding if-else and else-if Statements

1. Selection Structures Continued… The Ohio State University

2. Common Mistakes with Conditions(1) • Consider the following code: int age(26); if (age = 18) // Line 1 { cout << “You are 18.” } else { cout << “You are not 18.”; } • The output will output “You are 18.” Why? The Ohio State University

3. Common Mistakes with Conditions(2) • On Line 1: The equality operator is ==, and not = • = is the assignment operator, so what we are actually doing on line 1 is assigning age to the value of 18. • So the condition becomes: if (18) { cout << “You are 18.” } • Now remember the Boolean data type. 0 evaluates to false, and any value other than 0 evaluates to true. Since 18 is not 0, this condition will be satisfied!! The Ohio State University

4. else-if Statements (1) • The if-then-else statements that we have seen thus far allows us to select from at most two alternatives. • The code to execute if the condition succeeds • The code to execute if the condition fails • What if there are many alternatives to consider? The Ohio State University

5. else-if Statements (2) • Consider this example: • We have a variable, age, that contains a person’s age. • If 1 ≤ age ≤ 12, print “You are very young” • If 13 ≤ age ≤ 19, print “You are a teenager” • If 20 ≤ age ≤ 39, print “You are getting old” • If 40 ≤ age, print “You are over the hill” The Ohio State University

6. Solve It with What We Know (1) if (1 <= age && age <= 12) // Line 1 { cout << “You are a child” << endl; } if (13 <= age && age <= 19) // Line 2 { cout << “You are a teenager” << endl; } if (age <= 20 && age <= 39) // Line 3 { cout << “You are getting old” << endl; } if (40 <= age) // Line 4 { cout << “You are over the hill” << endl; } The Ohio State University

7. This works, but why is it undesirable? • If the condition at line 1 succeeds, do we need to continue checking the conditions at Lines 2, 3, and 4? Not in this case. • So this is a waste of time that the program could be using doing something more constructive! Here’s a better solution: The Ohio State University

8. else-if Statements if (1 <= age && age <= 12) // Line 1 { cout << “You are a child” << endl; } else if (13 <= age && age <= 19) // Line 2 { cout << “You are a teenager” << endl; } else if (20 <= age && age <= 39) // Line 3 { cout << “You are getting old” << endl; } else if (40 <= age) // Line 4 { cout << “You are over the hill” << endl; } The Ohio State University

9. A much better solution: only check other conditions upon failure! • Anytime a condition triggers true, run the corresponding statements and continue out of the selection structure. • The use of else-if statements are perfect for multiple “exclusive selections” such as this. The Ohio State University

10. Notes about else-ifs • Like else statements, else-if statements are optional, but ALWAYS sandwiched in between the if- and the else statement. • There is no limit to the number of else-if statements succeeding an if-statement. • It is important to realize the difference between a chain of else-ifs and a chain of ifs. The Ohio State University

11. else-if Error if (1 <= age && age <= 12) // Line 1 { cout << “You are a child” << endl; } else if (13 <= age && age <= 29) // Line 2 ERROR { cout << “You are a teenager” << endl; } else if (20 <= age && age <= 39) // Line 3 { cout << “You are getting old” << endl; } else if (40 <= age) // Line 4 { cout << “You are over the hill” << endl; } The Ohio State University

12. Another Example of else-if if (a > 0) { //do something if condition succeeds } else if (b < 3) { //here, we know a ≤ 0 because the first condition //failed, now we also test for b < 3 do something //if condition succeeds } else { //here, we know neither conditions were met //do something else, if necessary } The Ohio State University

13. ifExample.cpp • What is the output on input: • 1 2 3 • 3 2 1 • 1 3 2 • 2 1 3 • 3 1 2 • 2 3 1 ... int main() { int a(0), b(0), c(0); cout << "Enter a, b, c: "; cin >> a >> b >> c; if (a < b) { if (b < c) { cout << "b < c" << endl; } else { cout << "b >= c" << endl; } } else if (a < c) { cout << "a < c" << endl; } else { cout << "a >= c" << endl; } return 0; } The Ohio State University

14. Exercise • Write if-else-if statements which print: • “You are too young to drive.” if age  14; • “You can get a learners permit.” if age = 15; • “You pay more for insurance.” if 16  age  25; • “You can drive.” if age > 25; The Ohio State University

15. The switch Statement • An alternative to the if-else chain, but not as general. switch (expression) { case value1: ... case value2: ... ... } (See text.) The Ohio State University

16. cerr and exit() The Ohio State University

17. Error Handling • if-then statements are often used to detect and handle errors. • Use cerr() instead of cout() for error output messages. • Use exit() instead of return to quit a program on detecting an error. The Ohio State University

18. cerrExample.cpp #include <cstdlib> // File cstdlib contains exit() ... int main() { double x(0.0); cout << "Enter non-negative value: "; cin >> x; if (x < 0) { // Use cerr instead of cout. Use exit instead of return. cerr << "Error: Illegal negative value: " << x << endl; exit(20); } cout << "sqrt(" << x << ") = " << sqrt(x) << endl; return 0; } The Ohio State University

19. exit() • To use exit(), we need: #include <cstdlib> • To help in debugging, use a different number with each exit statement: exit(10); exit(20); exit(30); The Ohio State University

20. cerrExample2.cpp ... int main() { double x(0.0); cout << "Enter non-negative value: "; cin >> x; if (x < 0) { // Use cerr instead of cout. cerr << "Warning: Illegal negative value: " << x << endl; cerr << "Changing " << x << " to " << -x << endl; x = -x; } cout << "sqrt(" << x << ") = " << sqrt(x) << endl; return 0; } The Ohio State University

21. Error Handling • cerr() instead of cout(): cerr() messages can be sent to a different place than cout(). cerr() also forces messages to be printed immediately. • exit() instead of return: exit() quits the program and returns control to the operating system. exit() frees up resources associated with the program. “return” returns control to any calling program/function. The Ohio State University