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Announcements Homework 2 is assigned at SUCourse About if statements and parametric functions The use of “if” is natural The use of “functions” is enforced • The functions that you have to implement are explained in the homework document There are some other rules • Please read the homework specification very carefully Deadline is Wednesday, October 14, 2015, 19:00 CS201 – Introduction to Computing – Sabancı University 1 Conditional Execution – Sections 4.2, 4.3, 4.4 and 4.7 Up to now, we have seen that instructions are executed unconditionally one after another. What if you want to execute an instruction depending on a condition? Selection: choose from among many options according to a criteria E.g. If response is yes do this, else do that E.g. If year is a leap year, number of days is 366, else 365 You achieve conditional execution with if else statements CS201 – Introduction to Computing – Sabancı University 2 Syntax if (condition) statementtrue; else statementfalse; If condition is TRUE then statementtrue is executed; if condition is FALSE then statementfalse is executed. else and statementfalse are optional if (condition) statementtrue; if condition is FALSE then nothing will be executed and execution continues with the next statement in the program CS201 – Introduction to Computing – Sabancı University 3 More Syntax Condition must be in brackets What happens if you have several statements to execute? write your statements within curly brackets book recommends using this all the time even if you have only one statement (defensive programming) if (condition) { statementtrue 1; statementtrue 2; ... statementtrue N; } else { statementfalse 1; statementfalse 2; ... statementfalse N; } CS201 – Introduction to Computing – Sabancı University Definiton Compound block: Statements written within matching curly brackets 4 Flow diagram of if-else test condition false true false statements true statements next statement CS201 – Introduction to Computing – Sabancı University 5 Example (not in the book) Write a program that inputs two integer numbers and displays the minimum one. Two solutions using if and else together using only if (no else) CS201 – Introduction to Computing – Sabancı University 6 Solution 1 – with_if_else.cpp int main () { int num1, num2, min; cout << "Enter two numbers: "; cin >> num1 >> num2; if (num1 < num2) { min = num1; } else { min = num2; } //check if first number is smaller than the second one //if so minimum is the first number //otherwise minimum is the second number cout << "minimum of these two numbers is: " << min << endl; return 0; } CS201 – Introduction to Computing – Sabancı University 7 Solution 2 – with_if.cpp (no else) int main () { int num1, num2, min; cout << "Enter two numbers: "; cin >> num1 >> num2; min = num1; if (num2 < min) { min = num2; } // default assignment - minimum is the first number //check if second number is smaller than the first one //if so update the minimum, if not do nothing cout << "minimum of these two numbers is: " << min << endl; return 0; } CS201 – Introduction to Computing – Sabancı University 8 Boolean type and expressions The condition in an if statement must be a Boolean expression (named for George Boole) Values are true and false bool is a built-in type like int, double int degrees; bool isHot = false; cout << "enter temperature: "; cin >> degrees; if (degrees > 35) { isHot = true; } Boolean values have numeric equivalents false is 0, true is any nonzero value • prints hello on sreen boolean output yields 0 (for false) or 1 (for true) if (3*4 –8) cout << "hello"; else cout << "bye"; cout << • prints 1 cout << • prints 0 (4 < 5); on screen (5 == 7-2+1); on screen CS201 – Introduction to Computing – Sabancı University 9 Relational Operators Relational operators are used to compare values: Operator Meaning Example use < less than number < 5 <= less than or equal 0 <= number > greater than 48 > 62 >= greater than or equal num1 >= num2 == equality check num1 == 0 != inequality check num1 != num2 They take two operands operands can be literals, variables or expressions Used for many types numeric comparisons string comparisons (lexicographical, i.e. alphabetical) boolean comparisons (false is less than true) CS201 – Introduction to Computing – Sabancı University 10 Examples I used literals in the following numeric examples to see the results quickly. You can compare variables as well. 23 >= 45 false 49.0 == 7*7 true 34-3 != 30+1 false Let’s see some string comparison examples string s1= "elma", s2= "armut", s3= "Elma"; s1 < s2 false s3 < s2 true Why s3 < s2 is true? ‘E’ has a smaller code than ‘a’ Uppercase letters have smaller codes than lowercase letters CS201 – Introduction to Computing – Sabancı University 11 Logical operators Boolean expressions can be combined using logical operators: AND, OR, NOT In C++ we use && || ! respectively A B A || B A && B true true true true true false true false false true true false false false false false CS201 – Introduction to Computing – Sabancı University A !A true false false true 12 Example • Range check: between and including 0 and 100,or not? If so, display a message saying that the number is in the range. If not, the message should say “out of the range”. • Solution 1: using logical AND operator if (num >= 0 && num <= 100) cout << "number in the range"; else cout << "number is out of the range"; • Solution 2: using logical AND and NOT operators if ( !(num >= 0 && num <= 100) ) cout << "number is out of the range"; else cout << "number is in the range"; • Solution 3: using logical OR operator if (num < 0 || num > 100) cout << "number is out of the range"; else cout << "number is in the range"; CS201 – Introduction to Computing – Sabancı University 13 De Morgan’s Rules (Section 4.7) Compare solution 2 and 3 two conditions are equivalent (!(num >= 0 && num <= 100)) (num < 0 || num > 100) De Morgan’s Rules (assume a and b are two boolean expressions) !(a && b) = !a || !b !(a || b) = !a && !b De Morgan’a Rules can be generalized to several expressions (e.g. 4 boolean expressions case) ! (a && b && c && d) = !a || !b || !c || !d ! (a || b || c || d) = !a && !b && !c && !d CS201 – Introduction to Computing – Sabancı University 14 Operator Precedence - Revisited Upper operator groups have precedence Operator Explanation Associativity plus and minus signs, logical NOT right-to-left multiplication, division and modulus left-to-right addition, subtraction left-to-right stream insertion and extraction left-to-right Inequality comparison operators left-to-right equal, not equal comparison left-to-right && logical and left-to-right || logical or left-to-right assignment operators right-to-left + - * + / == = *= % - << < ! >> <= > >= != += /= -= %= CS201 – Introduction to Computing – Sabancı University 15 Operator Precedence Examples cout << num1 < year; syntax error (very cryptic) the problem is that << has precedence over < • does not compile as intended Solution: cout << (num1 < year); Advice: use parenthesized expressions in cout What about (0 <= num <= 100) for range check? not a syntax error but that expression does not make a range check. It is always true. Why? What is the value of !12+5&&32/35 ? result is 0 CS201 – Introduction to Computing – Sabancı University 16 Nested if statements if/else statements are inside other if/else statements Method to select from multiple choices Example: input a numeric grade and display messages according to its value 0 .. 50 low 51 .. 70 average 71 .. 100 good otherwise invalid grade Several solutions exist (we’ll see three) – not in the book first two solution: ifs are after elses • see if_after_else.cpp and if_after_else2.cpp third solution: ifs are after ifs • see if_after_if.cpp CS201 – Introduction to Computing – Sabancı University 17 Nested if-else Syntax (else after if) if condition_1 is TRUE then if (condition_1) statements_1true are executed. { All subsequent elses are skipped. statements_1true; if condition_1 is FALSE then check } condition_2. If condition_2 is TRUE else if (condition_2) then { statements_2true are executed. statements_2true; All subsequent elses are skipped. } if both condition_1 and condition_2 else if (condition_3) are FALSE then check condition_3. If { condition_3 is TRUE then statements_3true; statement_3true are executed. } All subsequent elses are skipped. . . . . . . else if all of the conditions are FALSE then statement_allfalse are executed. { statements_allfalse; } CS201 – Introduction to Computing – Sabancı University 18 Nested if-else Syntax (if after if) For three conditions, use analogy for more condition cases if condition_1 is TRUE then check if (condition_1) condition_2 { if (condition_2) if condition_2 is also TRUE then { check condition_3 if (condition_3) if condition_3 is also TRUE { statements_alltrue are executed, statements_alltrue; } if condition_1 and 2 are TRUE else but condition_3 is FALSE { statements_true_1and2 statements_true_1and2; are executed, } } else if condition_1 is TRUE but { condition_2 is FALSE statements_true_1; statements_true_1 } are executed, } else if condition_1 is FALSE { statements_1_false are executed. statements_1_false; CS201 – Introduction to Computing – Sabancı University 19 } Short-circuit Evaluation Some subexpressions in Boolean expressions are not evaluated if the entire expression’s value is already known using the subexpression evaluated so far Rule: Evaluate the first (leftmost) boolean subexpression. If its value is sufficient to judge about the value of the entire expression, then stop there. Otherwise continue evaluation towards right. if (count != 0 && scores/count < 60) { cout << "low average" << endl; } In this example, if the value of count is zero, then first subexpression becomes false and the second one is not evaluated. In this way, we avoid “division by zero” error (that would cause to crash the execution of the program) Alternative method to avoid division by zero without using shortcircuit evaluation: if (count != 0) { if (scores/count < 60) { cout << "low average warning" << endl; } } CS201 – Introduction to Computing – Sabancı University 20 Dangling Else Problem if ( x % 2 == 0) if ( x < 0 ) cout << x << " is an even, negative number" << endl; else cout << x << " is an odd number << endl; Do the above messages make sense? The problem is that it displays “odd number” for positive even numbers and zero. Reason is that, although indentation says the reverse, else belongs to second (inner) if Solution: use braces (see next slide) CS201 – Introduction to Computing – Sabancı University 21 Solution to Dangling Else Problem if ( x % 2 == 0) { if ( x < 0 ) cout << x << " is an even, negative number"<< endl; } else { cout << x << " is an odd number << endl; } Now else belongs to first if CS201 – Introduction to Computing – Sabancı University 22 if – else matching rule Each else belongs to the nearest if for which there is no else and in the same compound block CS201 – Introduction to Computing – Sabancı University 23