Variables
Conditions
Homework
2. Variables and Conditions
Ján Dugáček
September 11, 2018
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Table of Contents
1 Variables
Why we need them
Available types
Usage
Exercises
Shortcuts
2 Conditions
Condition
Exercise
3 Homework
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Why we need them
Available types
Usage
Exercises
Shortcuts
Advanced exercise
Do this only if you already know how to use variables!
Calculate π using the Monte Carlo method (scatter many
points randomly in a square, calculate the fraction of them
that is closer to its centre than a half of the square’s side)
Hint: you may use rand() to generate random numbers
Why is the result so imprecise?
Challenge: Do it without computing any square root (neither
manually nor in the program)
Second powers of the same numbers are computed over and
over. Would it be useful to store the computed second powers
of numbers for later use?
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Why we need them
Available types
Usage
Exercises
Shortcuts
Variables
Everything in digital format is a number or a group of
numbers (addresses, texts, pictures, programs, ...)
There are several formats for numbers, depending on the
required size and need to support negative numbers and
decimals
Numbers are always binary code, groups of ones and zeroes, a
bit is a single value that can be zero or one, a byte is a group
of eight bits (82 = 256 possible values)
On computers, numbers usually can be saved on 1 byte (256
values), 2 bytes (216 = 65536 values), 4 bytes
(232 = 4294967296 values) or 8 bytes
(264 = 18446744073709551616 = 1.8 · 1019 values)
A number stored someplace with a name is called variable
A single number is called primitive data type
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Why we need them
Available types
Usage
Exercises
Shortcuts
Usage
#i n c l u d e <iostream >
i n t main ( i n t argc , char ∗∗ argv ) {
i n t x ;
x = 2;
std : : cout << x << std : : endl ;
r e t u r n 0;
}
We first create variable x
The compiler will recognise x as an integer variable
Then we set value 2 to x
We can write its value to the program’s output
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Why we need them
Available types
Usage
Exercises
Shortcuts
Usage #2
#i n c l u d e <iostream >
i n t main ( i n t argc , char ∗∗ argv ) {
i n t x = 2;
std : : cout << x << std : : endl ;
r e t u r n 0;
}
We can set its value at the same line as when creating it
This is the recommended way to do it, because if you forget to
set it, it will have an unpredictable value
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Why we need them
Available types
Usage
Exercises
Shortcuts
Available types
int - standard sized integer (usually int32_t, range
-2147483648 to 2147483647)
short int - short sized integer (usually int16_t, range
-32768 to 32767)
char - very short sized integer, often used to store letters
(usually int8_t, range -128 to 127)
long int - short sized integer (usually int64_t, range
-9223372036854775808 to 9223372036854775807)
unsigned int - integer for non-negative values (usually
uint32_t, range 0 to 4294967295)
There are unsigned versions of all other sized integer types
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Why we need them
Available types
Usage
Exercises
Shortcuts
Available types
float - stores numbers with decimal point (usually 32-bit, 6
decimals, greatest numbers are around 1038)
double - stores numbers with decimal point (usually 64-bit, 15
decimals, greatest numbers are around 10308)
bool - can have only two values, false which is 0 or true
which is 1
std::string - stores text, works quite differently
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Why we need them
Available types
Usage
Exercises
Shortcuts
Usage #3
#i n c l u d e <iostream >
i n t main ( i n t argc , char ∗∗ argv ) {
i n t x = −1024 − 2;
s h o r t i n t y = x ∗ x ;
i n t z = x / 4;
std : : cout << y << std : : endl ;
std : : cout << z << std : : endl ;
r e t u r n 0;
}
We first create variable x and save -1026 into it
Then we create variable y and save the square of x into it,
which does not fit there
After, we create variable z and set its value to x divided by 4,
because both x and 4 are integers, the result is an integer,
rounding the value down
The resulting values of y and z are written into the terminal
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Why we need them
Available types
Usage
Exercises
Shortcuts
Usage #3
#i n c l u d e <iostream >
i n t main ( i n t argc , char ∗∗ argv ) {
f l o a t x = 15 / 2;
f l o a t y = 15.0 / 2;
f l o a t z = ( f l o a t )15 / 2;
f l o a t w = x / 2;
std : : cout << "Computed x=" << x << " y= " << y
<< " z= " << z " w= " << w << std : : endl ;
r e t u r n 0;
}
We first divide 15 by 2, rounding down because both numbers are integers and
result is integer, recalculate it to float and save it into x
Then we divide 15.0 by 2, because 15.0 is a decimal, it is a float, arithmetic
between a float and an int yields a float, the resulting float is saved into y
After, we convert the integer 15 to float, divide it by 2, the resulting float is
saved into z
Next, we divide the float x by 2 and save it into variable w
The resulting values of variables are written into the terminal
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Why we need them
Available types
Usage
Exercises
Shortcuts
Exercises
1 Set 17 to x, divide it by 4 (rounded down), set x2 − 12 to y,
add 18 to the result and write out the result
2 Calculate
(3 + 2 − 12) · ((9 − 2) · 5) + (3 + 2 − 12) · (8 + ((9 − 2) · 5))
without writing 3 + 2 - 12 or (9 - 2) · 5 more than once
or calculating anything yourself
3 Calculate 3+2−12
(9−2)· 5 + (3 + 2 − 12) · (8 + (9−2)· 5
3+2−12 ) without
writing 3 + 2 - 12 or (9 - 2) · 5 more than once or
calculating anything yourself
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Why we need them
Available types
Usage
Exercises
Shortcuts
Shortcuts
Lines like x = x + 4 are used a lot, so they can be shortened
to x += 4
Analogically, you can use x -= y * 2 (subtract 2 multiplied
by y from x and save it into x), x /= 1.5 or x *= 1.01
x += 1 can be further shortened to x++ or ++x
Analogically, there is also x–– or ––x for x -= 1
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Condition
Exercise
Advanced exercise
Do this only if you already know how to use if, while and
for!
Calculate x in x + 1 = 1
x
You may assume that x is positive
Challenge: Do not calculate anything more than 1000 times,
but limit your precision only by the maximum decimals that
can be stored in primitive types and use no prior knowledge
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Condition
Exercise
Condition
i n t x ;
std : : c i n << x ;
i f ( x < 0)
x ∗= −1;
std : : cout << x << std : : endl ;
First, we let the user insert a number
Then, we check if x is lesser than 0
Only if x is lesser than 0, multiply by -1
This will replace x by its absolute
x is printed at the end of the program
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Condition
Exercise
Condition #2
bool changed = f a l s e ;
i f ( x >= 0) {
x ∗= −1;
changed = t r u e ;
}
Here, we check if x is greater than or equal to 0
If the condition is met, multiply x by -1 and set variable
changed to 1
Variables defined in a block (the part in curly brackets) are not
available outside of it
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Condition
Exercise
Condition #3
i n t changed = 0;
bool equals = ( x == y ) ;
i f ( x > y | | x < 2 ∗ y ) {
changed = 1;
i f ( equals ) {
changed = 2;
}
}
Here, we check if x is greater than y or x is less than two
times y
We also check if x equals y and save the result of the
comparison into variable equals
If the first condition is met, 1 is assigned to changed and we
check if x was previously found to be equal to y
The result of comparison can be 1 (true) or 0 (false)
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Condition
Exercise
Condition #4
i n t z = 0;
i f ( x > y && x != 1) {
z = 1;
i f ( x = y − 1) {
z = 2;
}
}
Here, we check if x is greater than y and x is not equal to 1
If the condition is met, 1 is assigned to z and y - 1 is
assigned to x and if x is non-zero (true), 2 is assigned to z
Do not confuse = (variable assignment) with ==
(comparison)! It is a huge source of errors!
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Condition
Exercise
Condition #5
i n t z = 0;
i f ( x > y && ( x = y | | y == 1)) {
z = 1;
}
Here, we check if x is greater than y and if that is true, we
assign y into x and if the result is non-zero (true) or y is equal
to 1, the condition is met
If the condition is met, 1 is assigned to z
If x is not greater than y, the condition is never true and the
rest is ignored, thus y is never assigned to x
Do not confuse && and || with & and |, they mean something
else but usually lead to different outcomes, so a program using
& instead of && may seem okay but then behave weirdly
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Condition
Exercise
Condition #6
i n t z = 0;
i f ( ! ( x > y ) && ( x == 1 | | ( x = y ) ) ) {
z = 1;
}
Here, we check if it’s not true that x is greater than y and if
that condition is met, we check if x is equal to one, if that is
false, we assign y into x, check if it’s non-zero and go inside
the block if the one of these two conditions is met
If x is equal to 1, the condition is true regardless of the value
of y and the next condition is ignored, thus y is never assigned
to x
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Condition
Exercise
Inline condition
i n t z = ( ! ( x > y ) && ( x == 1 | | ( x = y ) ) ) ? 1 : 0;
This does the same as the previous, if the condition is met, z
is initialised with 1, otherwise it’s initialised with 0
It is useful only when assigning values into a variable
depending on a condition
i n t z = ( x > 1) ? (( y > 1) ? 2 : 1) : 0;
It can be nested too
If x is greater than 1, then if y is greater than 1, 2 is set into
z, otherwise 1, if x is not greater than 1, 0 is set into z
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Condition
Exercise
Exercise
1 Create a program that reads a number and tells if it’s even or
odd
2 Create a program that reads a number and reports if it’s the
square of an integer (the number will not be greater than 20)
3 Create a program that reads two numbers as coordinates of a
point and prints the point’s distance from point (2, 3)
4 Create a program that reads two numbers as coordinates of a
point and determines if the point lies within a circle with
centre at (2, 3) and radius 4
Ján Dugáček 2. Variables and Conditions
Variables
Conditions
Homework
Homework
No homework
Ján Dugáček 2. Variables and Conditions