Exercise 4. Logical Operators and Branching. Daniel Bogado Duffner - n.ethz.ch/~bodaniel. Informatik I für D-MAVT

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1 Exercise 4 Logical Operators and Branching Daniel Bogado Duffner - bodaniel@student.ethz.ch n.ethz.ch/~bodaniel Informatik I für D-MAVT

2 Feedback/Quiz Branching (Bedingte Anweisung und Verzweigung) Relational operators (vergleichende Operatoren) Logical expressions (logische Ausdrücke) Conditionals (if/else und switch) Arrays Introduction Argument array Agenda 2

3 Overflow: Feedback - Typenumwandlung b) long int --> short int: Short in hat deutlich weniger Bits als long int, bits werden abgeschnitten -> Overflow ; Exemplarisches Beispiel: +/ Result = 107 +/ Result = -3 g) long double short int: Gleicher Grund wie bei b) es wird jedoch zuerst abgerundet (Nachkommastelle abgeschnitten) -> dann den Bits zugeordnet 3

4 Overflow: Feedback - Typenumwandlung d) long int --> float: Float ist an sich gleich gross oder grösser als long int, aber float speichert immer Gleitkommazahl (Andere Präzision) Bsp: 1.6 E4 statt ist hier die Mantisse und float hat zum Beispiel nur 16 bits Speicherplatz für die Mantisse -> Fehler wenn zu grosse Zahl Ergibt: 4

5 Feedback - Typenumwandlung Ascii Umwandlung: Char <-> double/float/int Char -> double/float/int: Es wird der nach ASCII Code entsprechende Wert gespeichert Double/float/int -> Char: Von 0 bis 255 wird das entsprechende ASCII Code Zeichen gespeichert Bool umwandlung: Bool ->double/float/int: true: 1; false: 0 Double/float/int -> bool: 0: false; alles andere bedeutet true (egal wie gross die Zahl ist) 5

6 Feedback - Typenumwandlung Aufgabe 3: d1 = ; i1 = ; // int (9.99) wandelt den Datentypen in int um (type cast) i2 = int (9.99) + int (19.99); Es wird immer zuerst die Klammer, dann die Rechnung und dann die Zuordnung (=) vorgenommen 6

7 #include <iostream> using namespace std; Feedback - Typenumwandlung float mean (float a, float b, float c) { return (a + b + c) / 3.0f; } Funktion (Variablen haben nichts zu tun mit den Variablen aus main) int main (int argc, char ** argv) { float f1 = 0; float f2 = 0; float f3 = 0; cout << " 1. Zahl : "; cin >> f1 ; cout << " 2. Zahl : "; cin >> f2 ; cout << " 3. Zahl : "; cin >> f3 ; cout << " Der Durchschnitt ist " << mean(f1, f2, f3) << endl; } main-funktion 7

8 Data Types and Specifiers unsigned char b; unsigned bool h; float k; long signed int t; signed long int p; short double b; long double p; long float g; singed short int c; // Valid // Not Valid, bool cannot be unsigned // Valid // Valid // Valid // Not valid, double can t be short // Valid // Not valid, float can t be long // Not valid, typo in signed 8

9 Naming Rules int hallo_2; int if; int _2Hallo; int gruezi-5; int Moin 3; int olá; int Hey; // Valid // Not valid, is called if // Valid, but shouldn t be used (starts with _) // Not valid, contains - // Valid // Not valid, contains special letter // Valid, but shouldn t be used (starts with _) 9

10 Naming Rules Erlaubt: Buchstaben, Nummern, Underlines(_) Nicht erlaubt: Erstes Zeichen eine Ziffer, Sonderzeichen, C++ reservierte Wörter (main, int, for, ) Nicht gern gesehen: Underlines(_) zu Beginn Gross- und Kleinschreibung beachten 10

11 Examples: Arithmetic Operations 5 * (13 % 4) + 1; // 6 5 * (4-1); // 15 2 * 3 % / 2; // 2 9 % (5 * 4) + 1; // 10 9 % (4 + 1); // 4 5 * (3 % 4) 6 * 2; // 3 5 * 3 % 4 * (2+4); // 18 11

12 Type Conversion Even more examples: 13.0f * 4 = / (10 / 6) = * (3 / 7) = / (10.0f / 6) = f*10.0 / 6 = / 5f = 0.8 Float Int Double Float Double NOT Vaild 12

13 Compare two values Relational Operators The whole term is called an expression (German: Ausdruck) The result is either true (1) or false (0) Relational operators are evaluated after other arithmetic operations. E.g. (1+2 < 1+3) is true. 13

14 Relational Operators int a=3, b=5; bool res; res = a < b; // less res = a <= b; // less or equal res = a > b; // greater res = a >= b; // greater or equal res = a == b; // equal res = a!= b; // not equal 14

15 = is not == = is the assignment operator. It changes the value of the variable on the left to the value on the right. The value of the entire expression is equal to the assigned value (which is true if non-zero!) == is the equal to operator. It evaluates whether the expressions on either side are equal. The result is a Boolean and can be true or false. 15

16 = is not == Example: int a=3, b=5; bool res; res = (a == b); // res is FALSE, because 3!=5 res = (a = b); // res is TRUE, even though 3!=5. // (Value of expression (a=b) is 5, // which as a bool is TRUE.) // Also, a is set to 5! 16

17 Boolean Algebra: AND, OR, NOT AND OR NOT x AND y is true if and only if both x and y are true. x OR y is false if and only if both x and y are false. (or: x OR y is true if either x, y or both are true.) NOT x is true if x is false, and vice versa. 17

18 Logical Operators: &&,,! Work on Boolean values (type bool) bool a = true; bool b = false; a && b a && a a b b b == false == true == true == false!a == false!b == true 18

19 if Statement if (condition) { DoSomething(); } if (a==5) { cout << "a is equal to 5!\n ; } If condition is true, the instruction(s) inside the code block {} are executed. Conversely, if condition is false, the block is skipped. 19

20 else if Statement else if is only checked if the preceding condition(s) were false. It states an alternative which is evaluated when the other conditions are not met. Otherwise, it behaves the same as an if statement. if (firstcondition) { DoSomething(); } else if (secondcondition) { DoSomethingElse(); } 20

21 else if Statement if (a==5) { cout << "a is equal to 5!\n"; } else if (a==10) { cout << "a is equal to 10!\n"; } Many if else if else if statements can be chained to form a complex program flow. 21

22 The else code block is executed if all other conditions are false. else Statement if (firstcondition) { DoSomething(); } else if (secondcondition) { DoSomethingElse(); } else { IfEverythingElseFails(); } // the bottom 22

23 else Statement Remember: the program runs from top to bottom. After one of the if/else if/else conditions evaluates to true and its block {} is executed, the program jumps to the bottom of the whole if statement. No other conditions will be checked. 23

24 switch Statement Replaces long chains of if and else if Program jumps to a case according to a variable s value Program jumps to default if no cases are a match default is optional switch (variable) { case 0: cout<< variable is 0!\n"; break; case 7: cout<< variable is 7!\n"; break; default: cout<< No idea!\n"; } 24

25 switch Statement Unlike if statements, switch does not jump to the bottom after a matching case! Without an explicit break, program falls through to the next case. No break needed for default case (it s at the end anyway). switch (variable) { case 0: cout<< variable is 0!\n"; break; case 7: cout<< variable is 7!\n"; break; default: cout<< No idea!\n"; } 25

26 if/else vs. switch if (x == 1) { } cout << x is 1 << endl; else if (x == 2) { } else { } cout << x is 2 << endl; cout << x is something else << endl; switch (x) { case 1: cout << x is 1 << endl; case 2: break; cout << x is 2 << endl; break; default: cout << x is something else << endl; } 26

27 Conditional Operator value = condition? valtrue : valfalse; any type (int, float, ) type bool same type as value The? : operator returns the value before : if the condition is true. the value after : if the condition is false. It can be used with any type of value. 27

28 Conditional Operator value = condition? valtrue : valfalse; any type (int, float, ) type bool same type as value Examples: int beardamage = isstrong? 100 : 45; string welcome = timeisam? "Guten Morgen" : "Guten Abend"; 28

29 Arrays: Introduction Will be covered in more detail next week This week: Enough information to use argv argument of main() 29

30 Arrays: Introduction Array: Variable holding several values of same type float vectora[3]; vectora holds 3 floats Individual elements accessed using square brackets vectora[0] = 3.1f; float f = vectora[1]; 30

31 Arrays: Introduction Array indexing is zero-based Indices (subscripts) go from 0 to size-1 (where size is the number of elements) The first element is vectora[0] The third and last element is vectora[2] C++ doesn t check whether your index is valid! Bad things happen if you go beyond the end 31

32 Arrays: argc and argv in main() argc: Argument count int main(int argc, char *argv[]) { //... } argv: Argument vector argv is an array of char * (C strings, text ) [] indicates that it is an array 32

33 Arrays: argc and argv in main() int main(int argc, char *argv[]) { //... } Each element of argv (argv[0], argv[1], argv[argc-1]) holds one program argument. For subsequent use, they are often parsed using atof() or similar functions (see Exercise 2). C strings also looked at in more detail next exercise! 33

34 Arrays: argc and argv in main() From terminal:./myprogram Cologne When running a program, arguments can be passed to it like shown in the example above. Eclipse: Run Run Configurations Arguments argv[0] (first element) always contains program path and name. 34

35 Arrays: argc and argv in main() From terminal:./myprogram Cologne Variable Content argv[0]./myprogram argv[1] 47 argv[2] 11 argv[3] Cologne argc = 4; 35

36 Practice logical expressions Use conditionals and branching in programs Parsing command line arguments Read the tasks careful and do as asked in the task! Hand in: Solutions to task 1 Source code for tasks 2-4 Exercise 4 36

37 Exercise 4 Task 1 and Task 2 Task 1: Wie funktionieren logische Operatoren und welche Bedeutung haben sie? Überlegen welche Operatoren Priorität geniessen (ZSF Tabelle) Dementsprechend formulieren, entweder von Text zu Code bei a) c) oder von Code zu Text bei d) f) Task 2: Anwendung von Funktionen und if/else/switch 37

38 Exercise 4 Task 3 and Task 4 Task 3: Genau wie Task 2 plus Zusätzliche Frage. Die dritte Eingabe soll entscheiden, ob das Maximum oder das Minimum ausgegeben wird. Wenn der Nutzer das Zeichen < eingibt, dann das Minimum, wenn > dann das Maximum bzw die Maximumsfunktion anwenden Task 4: Kann man sich zum Beispiel mit der ASCII Tabelle veranschaulichen. Solange eingelesene Variable c grösser gleich a und kleiner gleich z und das ganze noch für die Grossbuchstaben 38