Download presentation

Presentation is loading. Please wait.

Published byJaclyn Cowan Modified over 5 years ago

1
CS201 – Introduction to Computing – Sabancı University 1 Conditional Execution – Sections 4.2, 4.3, 4.4 and 4.7 l Up to now, we have seen that instructions are executed unconditionally one after another. l What if you want to execute an instruction depending on a condition? l Selection: choose from among many options according to a criteria ä E.g. If response is yes do this, else do that ä E.g. ä If year is a leap year, number of days is 366, else 365 if else statements

2
CS201 – Introduction to Computing – Sabancı University 2 Syntax if ( condition ) statement true ; else statement false ; l If condition is TRUE then statement true is executed, if FALSE statement false is executed. l else and statement false are optional if ( condition ) statement true ; ä if condition is FALSE then nothing will be executed and execution continues with the next statement in the program

3
CS201 – Introduction to Computing – Sabancı University 3 More Syntax l Condition must be in brackets l What happens if you have several statements to execute? ä write your statements within curly brackets ä book recommends using this all the time even if you have only one statement (defensive programming) if ( condition ) {... statement true list ;... } else {... statement false list ;... } Definiton Compound block: Statements written within matching curly brackets

4
CS201 – Introduction to Computing – Sabancı University 4 Example (not in the book) l Write a program that inputs two integer numbers and displays the minimum one. l Two solutions using if and else together using only if (no else )

5
CS201 – Introduction to Computing – Sabancı University 5 Solution 1 – with_if_else.cpp int main () { int num1, num2, min; cout << "Enter two numbers: "; cin >> num1 >> num2; if (num1 < num2) //check if first number is smaller than the second one { min = num1; //if so minimum is the first number } else { min = num2; //otherwise minimum is the second number } cout << "minimum of these two numbers is: " << min << endl; return 0; }

6
CS201 – Introduction to Computing – Sabancı University 6 Solution 2 – with_if.cpp (no else) int main () { int num1, num2, min; cout << "Enter two numbers: "; cin >> num1 >> num2; min = num1; // default assignment - minimum is the first number if (num2 < min) //check if second number is smaller than the first one { min = num2; //if so update the minimum, if not do nothing } cout << "minimum of these two numbers is: " << min << endl; return 0; }

7
CS201 – Introduction to Computing – Sabancı University 7 Boolean type and expressions l 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; } l Boolean values have numeric equivalents ä false is 0, true is any nonzero value if (3*4 –8) cout << "hello"; else cout << "bye"; prints hello on sreen ä boolean output yields 0 (for false) or 1 (for true) cout << (4 < 5); prints 1 on screen cout << (5 == 7-2+1); prints 0 on screen

8
CS201 – Introduction to Computing – Sabancı University 8 Relational Operators l Relational operators are used to compare values: < less than <= less than or equal > greater than >= greater than or equal == equality check != inequality check l They take two operands ä operands can be literals, variables or expressions l Used for many types ä numeric comparisons ä string comparisons (lexicographical, i.e. alphabetical) ä boolean comparisons (false is less than true)

9
CS201 – Introduction to Computing – Sabancı University 9 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 l 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? ä ‘E’ has a smaller code than ‘a’ ä Uppercase letters have smaller codes than lowercase letters

10
CS201 – Introduction to Computing – Sabancı University 10 Logical operators l Boolean expressions can be combined using logical operators: AND, OR, NOT In C++ we use && || ! respectively ABA || BA && B true falsetruefalse true false A!A truefalse true

11
CS201 – Introduction to Computing – Sabancı University 11 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 100) cout << "number is out of the range"; else cout << "number is in the range";

12
CS201 – Introduction to Computing – Sabancı University 12 De Morgan’s Rules (Section 4.7) l Compare solution 2 and 3 ä two conditions are equivalent (!(num >= 0 && num <= 100)) (num 100) l De Morgan’s Rules (assume a and b are two boolean expressions) ! (a && b) = !a || !b ! (a || b) = !a && !b l 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

13
CS201 – Introduction to Computing – Sabancı University 13 Operator Precedence - Revisited l Upper operator groups have precedence OperatorExplanationAssociativity + - ! plus and minus signs, logical NOTright-to-left * / % multiplication, division and modulus left-to-right + - addition, subtractionleft-to-right > stream insertion and extractionleft-to-right >= Inequality comparison operatorsleft-to-right == != equal, not equal comparisonleft-to-right && logical andleft-to-right || logical orleft-to-right = += -= *= /= %= assignment operatorsright-to-left

14
CS201 – Introduction to Computing – Sabancı University 14 Operator Precedence Examples cout << num1 < year; ä syntax error (very cryptic) the problem is that << has precedence over < does not compile as intended Solution: cout << (num1 < year); ä Advice: use parenthesized expressions in cout What about (0 <= num <= 100) for range check? ä not a syntax error ä 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

15
CS201 – Introduction to Computing – Sabancı University 15 Nested if statements l if/else statements are inside other if/else statements l Method to select from multiple choices l Example: input a numeric grade and display messages according to its value 0.. 50 low 51.. 70 average 71.. 100 good otherwise invalid grade l Several solutions exist (we’ll see three) – not in the book ä first two solution: ifs are after elses see if_after_else.cpp and if_after_else2.cpp ä third solution: ifs are after ifs see if_after_if.cpp

16
CS201 – Introduction to Computing – Sabancı University 16 Short-circuit Evaluation l Some subexpressions in Boolean expressions are not evaluated if the entire expression’s value is already known using the subexpression evaluated so far l 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; } ä In this example, if the value of count is zero, then first subexpression becomes false and the second one is not evaluated. ä In this way, we avoid “division by zero” error (that would cause to crash the execution of the program) ä Alternative method to avoid division by zero without using short- circuit evaluation: if (count != 0) { if (scores/count < 60) { cout << "low average warning" << endl; }

17
CS201 – Introduction to Computing – Sabancı University 17 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; l Do the above messages make sense? l The problem is that it displays “odd number” for positive even numbers and zero. l Reason is that, although indentation says the reverse, else belongs to second (inner) if l Solution: use braces (see next slide)

18
CS201 – Introduction to Computing – Sabancı University 18 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; } l Now else belongs to first if

19
CS201 – Introduction to Computing – Sabancı University 19 if – else matching rule l Each else belongs to the nearest if for which there is no else and in the same compound block

Similar presentations

© 2019 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google