DSC 40B Practice Problems

Problems tagged with "time complexity"

Problem #001

Tags: time complexity

What is the time complexity of the following function in terms of \(n\)? State your answer using asymptotic notation (e.g., \(\Theta(n)\)).


def foo(n):
    i = 0
    while i < n**2:
        i = i + 2
        j = 0
        while j < n:
            for k in range(n):
                print(i + j + k)
            j = j + 10

Solution

\(\Theta(n^4)\)

Problem #002

Tags: time complexity

What is the time complexity of the following function in terms of \(n\)?


def foo(n):
    for i in range(n**2):

        for j in range(i):
            print(i+j)

        for k in range(n):
            print(i+k)

        for x in range(n - i):
            print(x)

Solution

\(\Theta(n^4)\)

Problem #003

Tags: time complexity

What is the time complexity of the following function in terms of \(n\)?


from math import sqrt, log, ceil

def foo(n):
    for i in range(ceil(n**3 - 10*n + sqrt(n))):
        for j in range(ceil(log(n**2))):
            print(i, j)

Solution

\(\Theta(n^3 \log n)\)

Problem #004

Tags: time complexity

What is the time complexity of the following function in terms of \(n\)?


def foo(arr):
    """`arr` is a list containing n numbers."""
    for x in arr:
        if x > max(arr) / 2:
            print('large!')
        elif x < min(arr) * 2:
            print('small!')
        else:
            print('neither!')

Solution

\(\Theta(n^2)\)

Problem #021

Tags: time complexity

What is the time complexity of the following function?


import math

def foo(n):
    for i in range(math.floor(math.sqrt(n))):
        for j in range(math.floor(5*n**2 - math.sqrt(n)/1_000_000 + 100)):
            print(n * n)

Solution

\(\Theta(n^2 \sqrt n)\)

Problem #022

Tags: time complexity

What is the time complexity of the following function?


def foo(arr):
    """`arr` is an array with n elements."""
    x = max(arr) * min(arr)
    if sum(arr) > 10:
        return x
    else:
        return 0

Solution

\(\Theta(n)\)

Problem #024

Tags: time complexity

What is the time complexity of the following function?


def foo(n):
    i = 0
    while i < n:
        j = 0
        while j < i:
            print(i + j)
            j += 1
        i += 5

Solution

\(\Theta(n^2)\)

Problem #030

Tags: time complexity

What is the time complexity of the following function? State your answer using asymptotic notation (e.g., \(\Theta(n)\)).



def foo(n):
    for i in range(n):
        for j in range(n):
            for k in range(n**2):
                print(i + j + k)

Solution

\(\Theta(n^4)\)

Problem #039

Tags: time complexity

What is the time complexity of the following function? State your answer using asymptotic notation (e.g., \(\Theta(n)\)).



def foo(n):
    for i in range(n):
        for j in range(n**2):
            for k in range(n):
                print(i + j + k)

Solution

\(\Theta(n^4)\)

Problem #049

Tags: time complexity

Express the time complexity of the following code using asymptotic notation in as simplest terms possible.


def foo(n):
    for i in range(200, n):
        for j in range(i, 2*i + n**2):
            print(i + j)

Solution

\(\Theta(n^3)\)

Problem #050

Tags: time complexity

Express the time complexity of the following code using asymptotic notation in as simplest terms possible.


import math

def foo(arr):
    """`arr` is an array with n elements."""
    n = len(arr)
    ix = 1
    s = 0

    while ix < n:
        s = s + arr[ix]
        ix = ix * 5 + 2

    return s

Solution

\(\Theta(\log n)\)

Problem #052

Tags: time complexity

Express the time complexity of the following code using asymptotic notation in as simplest terms possible.


def foo(n):
    for i in range(n):
        for j in range(i):
            for k in range(n):
                print(i + j + k)

Solution

\(\Theta(n^3)\)

Problem #066

Tags: time complexity

Express the time complexity of the following code using asymptotic notation in as simplest terms possible.



def foo(n):
    for i in range(n**3):
        for j in range(n):
            print(i + j)
        for j in range(n**2):
            print(i + j)

Solution

\(\Theta(n^5)\)

Problem #072

Tags: time complexity

Suppose bar and baz are two functions. Suppose bar's time complexity is \(\Theta(n^3)\), while baz's time complexity is \(\Theta(n^2)\).

Suppose foo is defined as below:



def foo(n):
    if n < 1_000:
        bar(n)
    else:
        baz(n)

What is the asymptotic time complexity of foo?

Solution

\(\Theta(n^2)\)

Problem #076

Tags: time complexity

The function below counts how many elements What is the time complexity of the following function in terms of \(n\)? Remember to state your answer in the simplest terms possible.


from math import sqrt, log, ceil

def foo(n):
    for i in range(ceil(n**3 - 10*n + sqrt(n))):
        for j in range(ceil(log(n**2))):
            print(i, j)

Solution

\(\Theta(n^3 \log n)\)

Problem #077

Tags: time complexity

What is the time complexity of the following function in terms of \(n\)?


def foo(arr):
    """`arr` is a list containing n numbers."""
    for x in arr:
        n = len(arr)
        if x > sum(arr) / n:
            print('large!')
        elif x < sum(arr) / n:
            print('small!')
        else:
            print('neither!')

Solution

\(\Theta(n^2)\)

Problem #079

Tags: time complexity

What is the time complexity of the following function in terms of \(n\)?


import math

def foo(n):
    for i in range(math.floor(math.sqrt(n))):
        for j in range(i):
            print(i + j)

Solution

\(\Theta(n)\)

Problem #090

Tags: time complexity

What is the time complexity of the following function in terms of \(n\)? State your answer using asymptotic notation (e.g., \(\Theta(n)\)).


def foo(n):
    for i in range(n**3):
        for j in range(n):
            print(i + j)
    for k in range(n):
        for l in range(n**2):
            print(k * l)

Solution

\(\Theta(n^4)\)

Problem #092

Tags: time complexity

Suppose bar and baz are two functions. Suppose bar's asymptotic time complexity is \(\Theta(n^4)\), while baz's is \(\Theta(n)\).

Suppose foo is defined as below:



def foo(n):
    if n < 1_000_000:
        bar(n)
    else:
        baz(n)

What is the asymptotic time complexity of foo?

Solution

\(\Theta(n)\) If you were to plot the function \(T(n)\) that gives the time taken by foo as a function of \(n\), you'd see something like the below:

This function starts off looking like \(n^4\), but at \(n = 1_000_000\), it "switches" to looking like \(n\).

Since asymptotic time complexity is concerned with the behavior of the function as \(n\) gets large, we can ignore the part where \(n\) is "small" (in this case, less than \(1{,}000{,}000\)). So, asymptotically, this function is \(\Theta(n)\).

Problem #094

Tags: time complexity

Suppose bar and baz are two functions. Suppose bar's time complexity is \(\Theta(n^2)\), while baz's time complexity is \(\Theta(n)\).

Suppose foo is defined as below:



def foo(n):
    # will be True if n is even, False otherwise
    is_even = (n % 2) == 0
    if is_even:
        bar(n)
    else:
        baz(n)

Let \(T(n)\) be the time taken by foo on an input of sized \(n\). True or False: \(T(n) = \Theta(n^2)\).

True False
Solution

False.

This function is not \(\Theta(n^2)\). For that matter, it is also not \(\Theta(n)\). It is\(O(n^2)\) and \(\Omega(n)\), though.

This function cannot be \(\Theta(n^2)\) because there are no positive constants \(c, n_0\) such that \(T(n) > c n^2\) for all \(n > n_0\). You can see this by imagining the plot of the time taken by foo as a function of \(n\). It "oscillates" between something that grows like \(n\) and something that grows like \(n^2\). If you tried to lower bound it with \(cn^2\), \(T(n)\) would eventually dip below \(cn^2\), since \(cn^2\) grows faster than \(n\).