1. Exercise 1 – Datentypen & Variablen
Informatik I / D-ITET
Informatik I / D-ITET
Exercise 1 – Datentypen & Variablen 1

2. Agenda Informatik I / D-ITET Variables Declaration & Assignment
Allowed names for variables
Type Conversion
Operators & Arithmetic operations
cin & cout
Functions
Scope 2 2

3. Data-types Informatik I / D-ITET C++ has 5 built-in basic Datatypes
Function
Memory
int
Saves integer numbers
2 Byte
float
Saves real numbers
4 Byte
double
Saves real numbers with higher precision than the float data-type
8 Byte
bool
Saves logical expressions (True/False or respectively 0/1)
1 Byte
char
Saves characters
Memory size may vary, because they depend on the implementation and Hardware. 3 3

4. Data-types Informatik I / D-ITET The memory is binary:
Larger Numbers need many bits:
Overflow: int i = MAX_INT; //highest possible value i++; // overflow! Lowest negative value!
5 -> > > 1100
32,767 -> s
16 bits = 2 bytes 4 4

5. Declaration & Assignment
Informatik I / D-ITET
Informatik I / D-ITET
Declaration & Assignment
Declaration of a variable
The syntax for declaring a variable is as follows:
<type> <name>;
<type> <name> = <Number/variable>;
For example:
int My_first_Variable;
Declares a integer Variable with the Names „My_first_Variable“. 5 5

6. Declaration & Assignment
Informatik I / D-ITET
Declaration & Assignment
More Examples:
double My_first_Variable;
My_first_Variable = 5.994;
This declares a variable and assignes a value to it one line later. This can also be done like this:
double My_first_Variable = 5.994;
Thus declaring and assigning the Variable in one step 6

7. Specifiers Informatik I / D-ITET
Specify a modifier to the variable type
Optional
<specifier> <type> <name>;
Example: unsigned int ui;
Specifier
Function
long
Variable gets double the memory
short
Variable gets half the memory
unsigned
Variable will NOT be able to have a sign (only positives can be stored), thus is able to save numbers with more accuracy/ more range
const
Variable value cannot be changed 7 7

8. Allowed names for variables
Informatik I / D-ITET
Informatik I / D-ITET
Allowed names for variables
Not all names are allowed for variables!
Variable names may only contain alphabetic characters, numeric digits and the underscore character (_)
The first character cannot be a numeric digit
You cannot use C++ reserved words as names (such as main, int or for)
Names should not begin with underscore Characters
Uppercase characters are considered to be different from lowercase characters 8 8

9. Allowed names for variables
Informatik I / D-ITET
Informatik I / D-ITET
Allowed names for variables
Examples:
int firstname;
int Firstname;
int leet1337;
int 1337leet;
int what_a_great_name;
int var!;
int _thisvar;
\\Valid
\\Valid, variable differs from firstname
\\Not Valid, begins with number
\\Not Valid, contains special character
\\Valid, but shouldn’t be used, starts with _ 9 9

10. Arithmetic operations
Informatik I / D-ITET
Informatik I / D-ITET
Arithmetic operations
There are 5 types of different Arithmetic Operators:
plus (+), minus (-), times(*), divided(/) and Modulo(%)
The first 4 are intuitive
Modulo returns the remainder of a division
For example:
5 % 3; //Result = 2 because 3 goes into the 5 one time with a rest of 2
21 % 5; //Result = 1 10 10

11. Arithmetic operations
Informatik I / D-ITET
Informatik I / D-ITET
Arithmetic operations
Arithmetic Operations are evaluated as follows:
Brackets before operators
Division, Multiplication and Modulo before Minus and Plus
If two arguments have the same precedence level, expression is evaluated left to right 11 11

12. Arithmetic operations
Informatik I / D-ITET
Arithmetic operations
Examples
5 * 4 + 1; //21
5 * (4 + 1); //25
2 * * 4 / 2; //12
9 % 4 + 1; //2
9 % (4 + 1); //4
5 * 3 % 4 * 2; //6
5 * (3 % 4) * 2; //30 12

13. Operators Informatik I / D-ITET But other operators also exist:
Assignment:
int x,y;
x = y;
x *= y; // x = x * y;
x /= y; x += y; x -= y; x %= y;
y = x++; // y = x; x = x + 1;
y = ++x; // x = x + 1; y = x;
y = x--; // y = x; x = x – 1;
y = --x; // x = x -1; y = x; 13

14. Operators Informatik I / D-ITET
But other operators also exist (will be discussed next week):
Comparisons
int x,y;
bool a,b;
a == b, x == y, a != b, x != y
x > y, x < y, x >= y, x <= y
Logical operators
!a, a && b, a || b
a | b, a & b, a ^ b 14

15. Type Conversion Informatik I / D-ITET Consider the following code:
float ft_var = 5.453;
int it_var = ft_var;
The Variable „it_var“ cannot save a real number. So what happens when this code is executed?
The Answer is, the value which will be assigned to „it_var“ will be converted. This process is called Conversion. 15

16. Type Conversion Informatik I / D-ITET
The assigned value will be converted to type of the variable to which it is assigned to
At our Example:
float ft_var = 5.453;
int it_var = ft_var;
This means that the value will be converted to an integer.
C++ does that by truncating the fractional part of the value
Thus it_var has the value 5 after the assignment 16

17. Type Conversion Informatik I / D-ITET Assignments:
Whenever a numerical value is assigned to a variable of another type, the value will be converted to the type of the receiving variable. 17 17

18. Type Conversion Informatik I / D-ITET Conversion in Expression:
A second time of the conversion of variables, is the conversion in Arithmetic expressions in which several types of variables are included like the following:
5.453f * 3 * 9.86
double
float
int 18

19. Type Conversion Informatik I / D-ITET
A rule of thumb in these cases is that C++ converts the expression into the more general type of the two variables.
Thus at our example
5.453f(float) * 3(int) * 9.86(double) =
16.359(float) * 9.86(double) =
(double) 19

20. Type Conversion Informatik I / D-ITET
A special note: divison of int variables
The resulting type of such a division is still an integer variable
Rational parts of the division will be truncated
Thus:
5 / 2 == 2 //The .5 is truncated
1 / 2 == 0 //Same reason here 20

21. Type Conversion Informatik I / D-ITET Even more examples
13.0f * 4 = 52.0 (float)
7.0 * (3 / 7) = 0.0 (double)
20 / (10 / 6) = 20 (int)
20 / (10.0 / 6) = 12 (double) 21 21

22. cin & cout Informatik I / D-ITET So lets get back to some programming
Cin and cout are your way to talk to the user
With these „objects“ you can print messages on the console
For example the program „Hello World“
But you can also print variables on it
You can even read in variables 22

23. cin & cout Informatik I / D-ITET Enable use of cin and cout by:
#include <iostream>
using namespace std;
Use cin and cout with the shift operator ( << or >> ):
cout << “using cout“; // printing
int i;
cin >> i; // reading 23

24. cout Informatik I / D-ITET The order is important:
cout << “using cout“; // valid
cout >> “using cout“; // invalid
“using cout“ >> cout; // invalid
Concatenation is valid:
cout << “pri“ << “nting“; // valid
Use of variables and numbers is also valid:
cout << “var: “ << i << “ num: “ << 3;
Note the differenence:
cout << “i“;
cout << i; 24

25. cout Informatik I / D-ITET New line:
cout << “line1\nline2“; // 2 lines
cout << “line1“;
cout << “line2“; // just 1 line!
cout << “line1“ << “\n“;
cout << “line2“; // ok
cout << “line1“ << endl << “line2“; 25

26. cin Informatik I / D-ITET Usage: int i; cin >> i;
Can also concatenate:
int a,b;
cin >> a >> b;
Converts to relavent types:
int a;
cin >> a; // waits for user input
// if user inputs a string then a=0 26

27. cin Informatik I / D-ITET
Parses by spaces and new line, but waits for new line:
int a,b;
cin >> a; // waits for user input
// if user input is “65 73“ and Enter
// then a = 65;
cin >> b; // b = 73 without waiting
cin >> a; // waits for user input again 27 27

28. Structure of a function
Informatik I / D-ITET
Structure of a function
return type
function name
argument type
argument
int main ( int argc, char* argv[] ) {
cout << “Hello, World!” << endl;
return 0; }
argument list
function body
return value
function name: name of the function
function body: statements to be executed
argument: variable whose value is passed into function body from the outside
argument type: type of the argument
return value: value that is passed to the outside after function call
return type: type of the return value (void if there is no return value) 28

29. Functions Informatik I / D-ITET Advantages Readability int main() {
cout << “Please enter two integers: ";
int a,b;
cin >> a >> b;
int res = (a + b) * (a + b);
cout << “The result is: " << res << endl;
return 0; } 29

30. Functions Informatik I / D-ITET Advantages Readability
int square_of_sum(int i1, int i2) {
int r = i1 + i2;
return r*r; }
int main()
cout << “Please enter two integers: ";
int a,b;
cin >> a >> b;
int res = square_of_sum(a,b);
cout << “The result is: " << res << endl;
return 0; 30

31. Functions Informatik I / D-ITET Advantages Code re-use
int square_of_sum(int i1, int i2) {
int r = i1 + i2;
return r*r; }
int main()
cout << “Please enter four integers: ";
int a,b,c,d;
cin >> a >> b >> c >> d;
int res1 = square_of_sum(a,b);
int res2 = square_of_sum(c,d);
cout << “the results are: " << res1 << “,” << res2 << endl;
return 0; 31

32. Functions Informatik I / D-ITET Arguments and return value
Both are optional
Invocation of a function must have ()
Decleration must be before invocation
int square_of_sum(int i1, int i2)
res1 = square_of_sum( a, b);
void print_message() {
cout << “This is also a function“ << endl; }
Function name
Variable name
Should be declared first
print_message(); 32 32

33. Scope Informatik I / D-ITET Errors?
Scope = validity/visibility of a variable
int nonsense(int input) {
return input * x; }
int main()
int x = 3;
if (x == 3)
int y = 15;
int z = nonsense(y);
cout << z << endl;
return 0;
Errors? 33

34. Scope Informatik I / D-ITET int nonsense(int input) {
return input * x; }
int main()
int x = 3;
if (x == 3)
int y = 15;
int z = nonsense(y);
cout << z << endl;
return 0;
x is not visible here;
only visible inside «main» function
y is not valid outside { }
we cannot use y as an argument
for function «nonsense».
Scope of x 34