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#010 DS&A - Time and Space Analysis

elkhatibomar profile image Omar ・12 min read

Introduction

Nothing to say more than hello 👋,
In Case your wondering who is this man in the banner it's the Khwarizmi the inventor of algorithms concept.
Make sure to follow because I will start more advanced series in future.

Algorithms

Time and Space Analysis

Introduction to asymptotic

In order to solve any problem in computer science we usually write a program , before writing program it's better to write it in formal description which is called Algorithm .

let's say we have a problem P we need to write a program , let's say we write it in c , to write program we need first to write an Algorithm , let's say P have many solutions s1,s2,s3,s4.... the thing that will differ solution from another is time and memory , the science of it is called Design and analysis of algorithm.

some of the notations used in Algorithms are these:

Asymptotic notation

Big(oh) represented by O

image-20200914233224043

if time Increasing as input increasing so the the worst case or upper bound is C*g(n) and satisfy those conditions

f(n) <= C*g(n) ; n >= n0 ; c>0 , n0 >= 1
f(n) = O(g(n))

let's take this example

f(n) = 3n+2 and g(n) = n
f(n) = O(g(n))
f(n) <= C * g(n) , c > 0 , n0 >= 1
3n+2 <= c*n
take c = 4
3n+2 <= 4n => n >= 2

we can pick g(n) = n^2 or n^3 ... n^nbecause f(n) can be written with respect to g(n) but it is preferred to take the smallest one which is n

Big omega represented by Ω

image-20200915171717083

f(n) >= C*g(n) ; n >= n0 ; c>0 , n0 >= 1
f(n) = O(g(n))

example

f(n) = 3n+2 and g(n) = n
f(n) = O(g(n))
f(n) >= C * g(n)
3n+2 >= c*n
take c = 1 and n0 >= 1
3n+2 = Ω(n)

example 2

g(n) = 3n+2 g(n)=n^2
3n+2 ?= Ω(n^2) 
3n+2 >= c*n^2 , n0

We can not find a C that satisfy this solution , so we need to pick things lower than n like log(n) or log(log(n))

Big theta represented by Θ

image-20200915172753442

f(n) = Θ(g(n))
c1*g(n) <= f(n) <= c2*gn(n) ; c1,c2 > 0 , n >= n0

example

f(n) = 3n+2 , g(n) = n
f(n) <= C2*g(n)
take c2 = 4
3n+2 <= 4n

f(n) >= c1*g(n)
take c1 = 1
3n+2 >= n , n0 >= 1

O this mean their is no bigger time than this , and it's called worst case.

Ω this mean their is no better time than this , and it's called best case.

Θ this mean it's the average case , and it's called the average case.

we usually don't care about best case , we care about worst case. If the worst and best cases are the same we generally go for average case.

example: take this array with n elements

0 1 2 3 4 5 6 7
5 7 1 2 4 10 20 11

let's say we need to find element x , best case is Ω(1) , worst case is O(n), average case is Θ(n/2)= Θ(n)

Time Complexity Analysis

the f(n) is not the real time is just approximation time that program take to finish.

there is 2 types of Algorithms :

iterative:
A()
{
    for i=1 to n
        max(a,b)
}

recursive:

A(n)
{
    if() A(n/2)
}

any iterative can be written as recursive and vise versa.

If the Algorithm doesn't have loops or recursion time complexity is constant O(1) , if the time complexity is O(n^2+n) we take the biggest degree so we take it O(n^2)

examples

A()
{
    int i;
    for(i = 1 to n) pf("text");
}

time complexity : O(n)
A()
{
    int i;
    for(i = 1 to n) 
        for(j = 1 to n)
            pf("omar");
}

time complexity : O(n^2)
A()
{
    int i;
    while (s <= n)
    {
        i++;
        s = s+i;
        pf("omar");
    }
}

time complexity : O(sqrt(n))
/*
Analysing :
s : 1 3 6 10 15 21 ...
i : 1 2 3  4  5  6 ...
s will stop on n
let's say k is the final i
and s is nothing but new i + all old i (6 = 3+2+1) so it will stop when it reach
k(k+1)/2 > n
(k^2+k)/2 > n
k = O(sqrt(n))
*/
A()
{
    int i;
    for(i=1;i^2<=n;i++) pf("omar");
}

time complexity : O(sqrt(n))
// Here all the cases are the same
A()
{
    int i,j,k,n;
    for(i=1 ; i<n ; i++)
    {
        for(j=1;j<=i;j++)
        {
            for(k=1 ; k <= 100 ; k++)
            {
                pf("omar");
            }
        }
    }
}

time complexity : O(n^2)

/*
Analysing :
i = 1 , j = 1 time  , k = 100   times
i = 2 , j = 2 times , k = 200   times
i = 3 , j = 3 times , k = 300   times
...
i = n , j = n times , k = j*100 times

1*100+2*100+3*100+...+n*100 
 = 100 (1+2+3+...+n)
 = 100 (n(n+1)/2) = O(n^2)
*/
int i,j,k,n;
for(i = 1 ; i <= n ;i++)
{
    for(j=1 ; j <= i^2 ; j++)
    {
        for(k=1 ; k <= n/2 ; k++)
        {
            pf("omar");
        }
    }
}

time complexity : O(n^4)

/*
Analysing :
i = 1 , j = 1 time  , k = n/2 * 1 
i = 2 , j = 4 times , k = n/2 * 4   
i = 3 , j = 9 times , k = n/2 * 9   
...
i = n , j = n^2 times , k = n/2 * n^2 times

n/2 * 1 + n/2 *4 + n/2 *  9 + ... + n/2 * n^2
 = n/2 * (n(n+1)(2n+1))/6
 = O(n^4)
*/
A()
{
    for(i = 1 ; i < n ; i = i*2)
        pf("omar");
}

time complexity : O(log(n))

/*
Analysing :
i :  1  , 2   , 4   ... n
    2^0 , 2^1 , 2^2 ... 2^k
2^k = n => k = log(n) = O(log(n))
since i is multiplied by 2 every step so log here is base 2
if i is multiplied by k we say log of base k
*/
A()
{
    int i,j,k;
    for(i=n/2 ; i <= n ; i++)
        for(j=1 ; j <= n2 ; j++)
            for(k=1 ; k <= n ; k=k*2)
                pf("omar");
}

time complexity : O(n^2 * log(n))

/*
Analysing :
n/2 * n/2 * log(n) = O(n^2 * log(n))
*/
A()
{
    int i,j,k;
    for(i=n/2 ; i <= n ; i++) // n/2
        for(j=1 ; j <= n ; i = 2*k) // log n
            for(k=1 ; k <= n ; k = k*2) // log n
                pf("omar");
}

time complexity : O(n*(log(n))^2)
A()
{
    // assume n >= 2
    while(n>1)
    {
        n = n/2;
    }
}

time complexity : O(log(n))
A()
{
    for(i = 1 ; i <= n ; i++) // n
        for(j = 1 ; j <= n ; i = j+i) // 
            pf("omar")
}

time complexity : O(n*log(n))

/*
Analysing :

i = 1 , j = 1 to n ; n   times
i = 2 , j = 1 to n ; n/2 times
i = 3 , j = 1 to n ; n/3 times
...
i = n , j = 1 to n ; n/n times

n(1+ 1/2 + 1/3 + ... + 1/n )
 = n (log n) = O(n * log(n))
*/
A()
{
    int n = (2^2)^k;
    for(i=1;i<=n;i++) // n
    {
        j = 2
        while(j <= n)
        {
            j = j^2;
            pf("omar");
        }
    }
}

time complexity : O(log(log(n)))

/*
Analysing :

k = 1 ; n = 4       ; j = 2,4               ; n * 2 times
k = 2 ; n = 16      ; j = 2,4,16            ; n * 3 times
k = 3 ; n = (2^2)^3 ; j = 2^1,2^2,2^4,2^8   ; n * 4 times

n = (2^2)^k => log n = 2^k => log(log(n))=k
n*(k+1) = n(log(log(n)) + 1) = O(log(log(n)))
*/

Time analysis of recursive

A(n)
{
    if(...)
    return A(n/2)+A(n/2);
}

T(n) = c+2*T(n/2)
A(n)
{
    if(n>1) return A(n-1);
}

T(n) = 1 + T(n-1) 
= 1 + 1 + T(n-2) 
= 2+1+T(n-3) 
= k+T(n-k) // k = n-1
= (n-1)+T(1) = n-1+1 = n

back substitution

T(n-1) = 1+T(n-2) -> 2
T(n-2) = 1+T(n-3) -> 3
T(n) = n + T(n)
T(n-1) = (n-1)+T(n-2)
T(n-2) = (n-2)+T(n-3)
-----------------------
T(n) = n + T(n-1)
     = n + (n-1) + T(n-2)
     = n + (n-1) + (n-2) + T(n-3)
     = n + (n-1) + (n-2)+...+(n-k)+T(n-(k+1))
with n-(k+1)=1 => n-k-1=1 => k=n-2
= n+(n-1)+(n-2)+...+(n-(n-2))+T(n-(n-2+1))
= n+(n-1)+(n-2)+...+1
= n(n-1)/2
= O(n^2)

recursive tree method

T(n) = 2*T(n/2) + C ; n>1
     = C ; n = 1

image-20200917021118588

T(1) = T(n/n)
c+2c+4c+...+nc
c(1+2+4+...+n)
c(1+2+4+...+2^k)
c(1 (2^(k+1) - 1) / (2-1) )
c(2^k+1 - 1)
c(2n-1)
O(n)

T(n) = 2*T(n/2)+n ; n > 1
     = 1 ; n = 1

image-20200917025850624

comparing various functions

let's say we have two functions n^2 and n^3 they have n^2 as common so I rewrite it as 1 and n .

If f(n) is given and g(n) is given we take biggest degree and compare them. If they are constants like 2 and 4 we consider them the same.

examples

2^n                         n^2
n log(2)                    2 log(n)
n                           2*log(n)
consider n = 2^100
2^100                       2*log(2^100)
2^100                       200
2^100 >>>>>>>>>>>>>>>>>>>   200
so 2^n growing very large
3^n                         2^n
n*log(3)                    n*log(2)
cancel n and compare it
log(3)                      log(2)
n^2                         n*log(n)
concel common terms
n*n                         n*log(n)
n           >               log(n)
n                           log(n)^100
log(n)                      100*log(log(n))
take n = 2^128
128                         100*log(128)
128                         700
let's take n = 1024
1024                        100*log(1024)
1024                        1000
so n growing larger
n^log(n)                    n*log(n)
log(n)*log(n)               log(n)+log(log(n))
for n = 10^1024
1024*1024                   1034
for n = (2^2)^20        
2^20*2^20                   2^20+20
so n^log(n) is larger
sqrt(log(n))                log(log(n))
1/2 * log(log(n))           log(log(log(n)))
take n = (2^2)^10
5                           3.5
n^(sqrt(n))                 n^log(n)
sqrt(n)*log(n)              log(n)*log(n)
sqrt(n)                     log(n)
1/2 * log(n)                log(log(n))
n = 2^128
64                          7
f(n) =  {
            n^3         0<n<10000
            n^2         n>=10000
        }
g(n) =  {
            n           0 < n < 100
            n^3         n > 100
        }
0-99 100-9999 10,000 ....
f(n) n^3 n^3 n^2
g(n) n n^3 n^3

we take care about the function in infinity so g(n) is bigger in infinity

Masters theorem

first there is a different between log^2(n) and (log(n))^2 , because (log(n))^2 = log(n) * log(n) and log^2(n) = log(log(n))

masters theorem used to solve reclusive problems

Lightbox

examples

T(n) = 3T(n/2) + n^2
a = 3 , b = 2 , k = 2 p=0
a < b^k
3 < 4
so it's the case 3)a so T(n) = O(n^2 * log^0(n))
T(n) = 4*T(n/2) + n^2
a=4 , b=2 , k=2 , p=0
4=2^2
so it's case 2)a T(n) = O(n^log(4) * log(n)) = O(n^2*log(n))
T(n) = T(n/2)+n^2
a=1 , b=2 , k=2 , p=0
1 < 2^2
it's case 3)a T(n) = O(n^2 * log^0(n)) = O(n^2)
T(n) = 2^n*T(n/2)+n^n master theoreme is not applied
T(n) = 16*T(n/4)+n
a = 16 b=4 k=1 p=0
16>4
so it's 1)
T(n) = 2*T(n/2)+n*log(n)
a=2 b=2 k=1 p=1
2=2 , p>-1 so it's 2)a

if it doesn't directly look like theorem we need to refactoring it

T(n) = 2*T(n/2)+n/log(n)
     = 2T(n/2)+n*log^-1(n)
     a=2 , b =2 , k=1 p=-1
     s = 2^1 so it's case 2)b
T(n) = 2*T(n/4)+n^0.51
a=2 b=4 k=051 p=0
2 < 4^0.51
case 3)a
T(n) = 05*T(n/2)+1/n
a=0.5 not valid
T(n) = 6*T(n/3)+n^2*log(n)
a=6 b=3 k=2 p=1
6 < 3^2
so it's 3)a
T(n) = 64 T(n/8) - n^2 * log(n)
can not apply master theorem
T(n) = 7*T(n/3)+n^2
a=7 b=3 k=2 p=0
7 < 3^2
case 3)a
T(n) = 4*T(n/2)+log(n)
a=4 b=2 k=0 p=1
4 > 2^0
case 1
T(n) = sqrt(2)*T(n/2)+log(n)
a=sqrt(2) b=2 k=0 p=1
sqrt(2) > 2^0
case 1
T(n) = 2*T(n/2)+sqrt(n)
a=2 b=2 k=1/2 p=0
2>2^1/2
case 1
T(n) = 3*T(n/2)+n
a=3 b=2 k=1 p=0
3 > 2^1
case 1
T(n) = 3*T(n/3)+sqrt(n)
a=3 b=3 k=1/2 p=0
3>3^1/2
case 1
T(n) = 4*T(n/2)+C*n
a=4 b=2 k=1 p=0
4 > 2^1
case 3)b
T(n)=3*T(n/4)+(n*log(n))
a=3 b=4 k=1 p=1
3 < 4
case 3)a

Analysis Space Complexity

same as time complexity we have space complexity for Iterative programs and recursive programs. Some times we sacrifice time for space.

Algo(A,1,n)
{
    int i;
    for(i=1 to n) A[i] = 0;
}

this space complexity is constant O(1) because we don't take the initial input into count. So we calculate extra spaces such as i

Algo(A,1,n)
{
    int i;
    create B[n];
    for(i=1 to n) B[i] = A[i];
}

the amount of space required is O(n) because we declare B[n] that contain n element. Same as Time complexity we didn't take in count the constants in other word we take higher degree.

Algo(A,1,n)
{
    create B[n,n];
    int i,j;
    for(i=1 to n)
        for(j=1 to n) B[i,j]=A[i]
}

space complexity is O(n^2)

A(n)
{
    if(n>=1)
    {
        A(n-1);
        pf(n);
    }
}

because the program is small we are going to use the tree method , take n=3

image-20200919192742044

image-20200919192808057

the output is 1 2 3 because every time I end call I print it

the space complexity is the number of stacks which is O(kn) where k is constant so we write it as O(n)

time complexity is T(n) = T(n-1)+1 it's not form where we can apply master theorem so we gonna use back substitution

T(n)  =T(n-1)+1
T(n-1)=T(n-2)+1
T(n-2)=T(n-3)+1
T(n) = T(T(n-2)+1)+1
     = T(n-2) +2
     = (T(n-3)+1) +2
     = T(n-3)+3
     = T(n-k)+k
     = T(n-n)+n
     = T(0)+n
     = 1+n
     = O(n)

so time and space complexity is O(n)

A(n)
{
    if(n>=1)
    {
        A(n-1);
        pf(n);
        A(n-1);
    }
}

number of recursive calls are

A(3) = 15 = 2^(3+1) - 1
A(2) =  7 = 2^(2+1) - 1
A(1) =  3 = 2^(1+1) - 1
A(n) = (2^n) - 1

this is not the space complexity because the stack will only need 4 cells A(0),A(1),A(2),A(3) in the stack in order to compute it where the stack will start to empty it self every time it reach A(0) , so it's (n+1)*k where k is the size occupied by one cell in stack so space complexity is nothing more than O(nk) = O(n).

To optimize it We can use Dynamic programming which is to store the already computed values for not compute them again.

A(n) -> T(n)
{
    if(n>=1)
    {
        A(n-1); -> T(n-1)
        pf(n); -> 1
        A(n-1); -> T(n-1)
    }
}
T(n)    = 2*T(n-1)+1 ; n > 0
T(n-1)  = 2*T(n-2)+1
T(n-2)  = 2*T(n-3)+1

T(n)    = 2(2T(n-2)+1)
         = 4T(n-2)+2+1
         = 4(2T(n-3)+1)+2+1
         = 8T(n-3)+7
         = 2^k * T(n-k) + 2^(k-1)+...+2^2+2+1
         = 2^n * T(0)+2^n-1 + 2^n-2 + ... + 2^2 + 2 + 1
         = 2^n + 2^n-1 + 2^n-2 + ... + 2^2 + 2 + 1
         = O(2^n+1) = O(2^n)

the time complexity is very big O(2^n) we can lower it with Dynamic Programming as we said.

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Omar

@elkhatibomar

Computer Scientist , Full stuck software engineer , Linux user since childhood , Classical computer Science books lover , DevOps in progress...

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