Motivation Homework
3. Cycles
Jan Dugacek
October 2, 2019
Jan Dugacek        3. Cycles
Motivation Homework			
Table of Contents			
wM Motivation w • While loop • For loop Exercise			
Homework			
Jan Dugacek		3. Cycles	
Motivation Homework
Motivation
While loop For loop
Exercise
• Write a program that reads 10 numbers and outputs their average
Jan Dugacek        3. Cycles
Motivation #2
Motivation Homework
While loop For loop
Exercise
• If you copy and paste something 10 times, editing it afterwards is annoying
• There might be a number that tells how many numbers will there be, allowing large numbers
• We need a better solution
Jan Dugacek        3. Cycles
Motivation #3
Motivation Homework
While loop For loop
Exercise
i n t   i   =  0 ;
float sum = 0; while  ( i < 10) { i n t   got;
std : : c i n » got ; sum += got ;
i++;
}
std : : cout « (sum /  10) « std : : end I ;
• We repeat the same part of the code over and over while the condition is met
Jan Dugacek        3. Cycles
Motivation Homework
While loop
While loop For loop
Exercise
unsigned   long   int  y = 1; while   (x > 1) {
y *= x;
x--;
}
while loop is much like if, but at the end of the block, it checks for the condition again and if it's still true, the block is executed again, after it is checked again and so it goes forever while the condition is true
This particular loop computes the factorial of x (the result is in variable y)
Jan Dugacek        3. Cycles
While loop #2
Motivation Homework
While loop For loop
Exercise
unsigned long int y = 1; while   (x > 1) {
y *= x;
if  (y > 100000000000000)
x —;
break ;
}
This is like the previous one, but we check if y is greater than a billiard, if it is, the loop is broken, immediately interrupted and the execution jumps out of it
A lesser variant of break is continue, which skips the rest of the block but goes to check for the condition again
If there is nothing to stop the cycle from looping, the program will freeze
while (1) {}
Jan Dugacek        3. Cycles
While loop #3
Motivation Homework
While loop For loop
Exercise
unsigned   long   int  y = 1; do {
y *= x ;
x--;
}  while   (x >  1);
• This is a variation of the while loop that starts executing the block and checks the condition if it's going to continue after
• The only difference is that the block is executed at least once regardless of the condition
• The code above does not correctly compute the factorial of x, if x is 0, the result is 0, which is incorrect because 0! = 1
Jan Dugacek        3. Cycles
Motivation Homework
For loop
While loop For loop
Exercise
i n t   X =  0 ; i n t   i = 0 ; while   ( i < 10) { x += i ;
i++;
}
• Defining a variable, changing it and checking it in a loop is so common that a new construct was created to shorten it:
i n t  x = 0 ; for   ( i n t   i =
x +=
}
0;   i < 10;   i++) {
i ;
This does the same as the code above
Jan Dugäcek        3. Cycles
Motivation Homework
While loop For loop
Exercise
For loop #2
i n t  x = 0 ;
for   (int   i = 0;   i < 10;   i++) {
if (i % 2 = 1) continue; x += i ;
}
9 The first part defines a variable (but may also assign a value to an existing variable or may be left blank), the second part is a condition that must be met if it's to be repeated and the last part is a change that happens after every loop (the condition is checked right afterwards, so in the case above, it never enters the block with i equal to 10)
• The % sign is modulo, the result of i % 2 is the remainder of division of i by 2
Q continue makes the loop restart, but i is increased before the check (unlike in while)
• The code above sums all even numbers between 0 and 10 (including 0, excluding 10)
• The variable change in each iteration of the loop is called iterator and usually named i (if there are more of them, they are named i, j, k, ...)
Jan Dugacek        3. Cycles
Motivation Homework
While loop For loop
Exercise
Nested loop
i n t  x = 0 ; for   (i n t i f o r
1; i < ( 'nt j
X +:
10;   i++) {
1;  j <= 10;  j++) {
}
}
■= i * j ;
Many applications require using loops within loops
In this case, we calculate the sum of all multiplications between numbers in range 1 to 10
Jan Dugacek        3. Cycles
Motivation Homework
While loop For loop
Exercise
Exercise
O Calculate the harmonic average of 10 numbers supplied by the user
O Same as previous, but keep asking for numbers until the user inputs zero
O Calculate the product of odd numbers between 23 and a
number supplied by the user O Calculate J^2sin(x2)dx
O Calculate tv by computing the ratio between the integral of a constant and the integral of the circle function y = ±Vl x2
9 Hint:
^include <cmath> int  main(int  argc,   char** argv)
float  x = 1;
float  y = sin(x);
float   z = sqrt(x);
{
Jan Dugacek        3. Cycles
Motivation Homework
Advanced exercise
While loop For loop
Exercise
• Find primes lesser than 10000 using Eratostenes' sieve
• Is it faster than trying to divide each number by all primes lesser than its square root?
Jan Dugacek        3. Cycles
Homework
Motivation Homework
• Find the values of a, b and c where
f(x) — a • ln(x • b) — b • V200 — x • c + c has the smallest sum of second powers of values of f(l), f(2), ■■■ f(100)
o You have two weeks to do it
Jan Dugacek        3. Cycles