© m. gross, eth zürich, 2014 informatik i für d-mavt (fs 2014) exercise 4 – logical operators...

© M. Gross, ETH Zürich, 2014

Informatik I für D-MAVT (FS 2014)

Exercise 4 – Logical Operators & Branching


Agenda

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Branching relational operators logical expressions: and, or, not if, else if, else switch “? : “ operator

Arrays introduction argc and argv[] in main()


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.

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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


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:

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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!


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

AND

0 1

0 0 0

1 0 1

OR 0 1

0 0 1

1 1 1

NOT

0 1

1 0


Logical Operators in C++: &&, || and !

Works on Boolean values (bool type)

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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


„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.

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if (condition) {DoSomething();

}

if (a==5) {cout << "a is equal to 5!\n“;

}


„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.

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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";}


„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.

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if(firstCondition) {DoSomething();

}else if(secondCondition) {

DoSomethingElse();}else {

IfEverythingElseFails();}//”the bottom”


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.

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switch (variable) {

case 0:

cout<<”variable is 0!\n";

break;

case 7:

cout<<”variable is 7!\n";

break;

default:

cout<<”variable is ???\n";

}


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.

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switch (variable) {

case 1:

case 3: //var. is either 1 or 3

DoSomething();

break;

default:

DefaultCase();

}


comparison: if/else and switch

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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


„? : “ 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.

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value = condition ? valTrue : valFalse;

int bearDmg = isStrong ? 100 : 45;

string name = firstName ? "Hansulrich" : "Hubschmid";

any type (int, float, …). type bool. same type as “value”.


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.

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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 [ ]:

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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]“!


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!

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int main(int argc, char *argv[]) {

//...}


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.

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./MyProgram 47 11 Cologne

argc = 4;

© m. gross, eth zürich, 2014 informatik i für d-mavt (fs 2014) exercise 4 – logical operators...

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