1. © M. Gross, ETH Zürich, 2014 Informatik I für D-MAVT (FS 2014) Exercise 4 – Logical Operators & Branching

2. © M. Gross, ETH Zürich, 2014 Agenda 2  Branching  relational operators  logical expressions: and, or, not  if, else if, else  switch  “? : “ operator  Arrays  introduction  argc and argv[] in main()

3. © M. Gross, ETH Zürich, 2014 Relational operators  Used to compare two values.  The whole term is called an expression (german Ausdruck).  The result of the comparison is either TRUE (1) or FALSE (0).  Relational operators are evaluated after other arithmetic operations. E.g. (1+2 < 1+3) is TRUE. 3 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

4. © M. Gross, ETH Zürich, 2014 Difference between „=“ and „==“  „=“ is the assignment operator. It changes the value of the variable on the left to the value on the right. The result (meaning the value of the expression) is equal to the assignment value (which is TRUE for all values not zero)!  „==“ is the equality operator. It evaluates whether the 2 values on the left and the right are equal. The result can be either TRUE or FALSE.  Example: 4 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. //Also, a is set to 5!

5. © M. Gross, ETH Zürich, 2014 Boolean Algebra: 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. (x OR y is TRUE if either x, y or both are TRUE.)  NOT x is TRUE if x is FALSE, and vice versa. 5 AND01 000 101 OR01 001 111 NOT 01 10

6. © M. Gross, ETH Zürich, 2014 Logical Operators in C++: &&, || and !  Works on Boolean values ( bool type) 6 bool a = true; bool b = false; logical AND a && b== false a && a== true logical OR a || b== true b || b== false logical NOT !a== false !b== true

7. © M. Gross, ETH Zürich, 2014 „if“ statement  If „condition“ is TRUE, the instruction(s) inside the code block { } are executed.  In contrary, if „condition“ is FALSE, the block is jumped over. 7 if (condition) { DoSomething(); } if (a==5) { cout << "a is equal to 5!\n“; }

8. © M. Gross, ETH Zürich, 2014 „else if“ statement  The „else if“-condition is only checked if the preceding condition(s) are FALSE. It states an alternative which is evaluated when the other conditions are not met.  Otherwise, it behaves the same as an „if“ statement.  Many „if - else if – else if – else if...“ statements can be chained to form a complex program flow. 8 if(firstCondition) { DoSomething(); } else if(secondCondition) { DoSomethingElse(); } if(a==5) { cout << "a is equal to 5!\n"; } else if(a==10) { cout << "a is equal to 10!\n"; }

9. © M. Gross, ETH Zürich, 2014 „else“ statement  The „else“ code block is executed if all other conditions are FALSE.  Remember: The program runs from top to bottom, line after line. After one if/else if/else block { } is executed, the program jumps to the bottom of the whole „if – else if – else“ statement.  e.g. if „firstCondition“ is FALSE but „secondCondition“ is TRUE, „DoSomethingElse()“ is executed, then the program jumps to the bottom. 9 if(firstCondition) { DoSomething(); } else if(secondCondition) { DoSomethingElse(); } else { IfEverythingElseFails(); } //”the bottom”

10. © M. Gross, ETH Zürich, 2014 switch statement  Switch statement replaces long chains of „if – else if -...“.  Depending on the value of the variable, the program jumps to the according „case“.  The program jumps to default if the variable doesn‘t match any cases.  The default case is optional. 10 switch (variable) { case 0: cout<<”variable is 0!\n"; break; case 7: cout<<”variable is 7!\n"; break; default: cout<<”variable is ???\n"; }

11. © M. Gross, ETH Zürich, 2014 switch statement 2  In contrary to if/else if statements, the program does not jump to the bottom after a case has been handled. This is why we need the „break;“ command after every case that we want to treat individually.  Without break, everything that follows in the same „switch“ statement is executed until we reach another break.  We don‘t need a break for the default case because it is placed at the bottom anyway. 11 switch (variable) { case 1: case 3: //var. is either 1 or 3 DoSomething(); break; default: DefaultCase(); }

12. © M. Gross, ETH Zürich, 2014 comparison: if/else and switch 12 if (x == 1) { cout << “x is 1” << endl; } else if (x == 2) { cout << “x is 2” << endl; } else { cout << “x is something else” << endl; } switch (x) { case 1: cout << “x is 1” << endl; break; case 2: cout << “x is 2” << endl; break; default: cout << “x is something else” << endl; } if/else statement switch statement

13. © M. Gross, ETH Zürich, 2014 „? : “ operator  The „? : “ operator returns the first value if the condition is TRUE, and the second value if the condition is FALSE.  It can be used with any types of values. 13 value = condition ? valTrue : valFalse; int bearDmg = isStrong ? 100 : 45; string name = firstName ? "Hansulrich" : "Hubschmid"; any type (int, float, …). type bool. same type as “value”.

14. © M. Gross, ETH Zürich, 2014 Introduction to arrays  This topic will be covered in more detail the next week!  This week: Explanation for the „argv“ variable used in the main() function. 14

15. © M. Gross, ETH Zürich, 2014 Introduction to arrays 2  An array is a variable that holds many values of the same type. „vectorA“ thus holds 3 floats.  The individual elements can be accessed using square brackets [ ]: 15 float vectorA[3]; vectorA[0] = 3.1f; float f = vectorA[1];  Arrays are zero-based: Their indices go from 0 to size-1! E.g. the first element of vectorA is „vectorA[0]“ and the last element is „vectorA[2]“!

16. © M. Gross, ETH Zürich, 2014 argc and argv in main()  „argc“: argument count. „argv“: argument vector.  argv is an array of „char *“ (C strings, meaning „text“), as indicated by the brackets [ ].  Thus each element of argv, namely argv[0], argv[1],..., argv[argc-1] holds one program argument in text form. For subsequent use, they are often parsed using atof() or similar functions (see Ex. 2).  C strings will be looked at in more detail next week! 16 int main(int argc, char *argv[]) { //... }

17. © M. Gross, ETH Zürich, 2014 argc and argv in main() 2  When running a program from the terminal, arguments can be passed to it like shown in the example command above.  In Eclipse, use Run -> Run Configurations -> Arguments.  argv[0] (first element) always contains the program path and name and has no further meaning. 17./MyProgram 47 11 Cologne argc = 4;