Introduction
Having a clear understanding of the task and the requirements of the program is crucial for developing a successful script. It’s important to identify the inputs, the desired outputs, and any intermediate steps required to achieve the desired results. Once the steps are identified, they can be translated into code in a selected programming language using the appropriate syntax, modules, and functions.
It’s also important to write the code in a structured and organized manner. This includes using comments to document the purpose of each section of code and breaking the code up into smaller, logical functions when appropriate.
In this tutorial, we’ll cover the basics of algorithm structure, including the key terminology, the structure of an algorithm, and the control structures used to control the flow of an algorithm. By the end of this tutorial, you’ll have a solid understanding of how algorithms work and be able to write simple algorithms yourself.
Algorithm
Creating algorithms is important regardless of programming language because algorithms are the foundation of any software development project. Even simple Bash scripts, which are used to automate tasks in a Unix-based operating system, rely on algorithms to perform their intended tasks.
Algorithms are essentially a set of logical step-by-step instructions that a computer program follows to solve a particular problem or perform a specific task. They’re used to process data, perform calculations, and automate repetitive tasks.
Algorithms are important because they allow us to automate tasks, perform calculations quickly and accurately, and process large amounts of data efficiently. They’re used in a wide range of applications, including tasks in various research projects.
Basic terminology
Before we dive into the structure of an algorithm, let’s define some key terms that we’ll be using throughout this tutorial:
term | description | notes |
---|---|---|
inputs | These are the values or data that are provided to an algorithm when it’s executed. | For example, if we’re writing an algorithm to calculate the area of a rectangle, the length and width of the rectangle would be the inputs. |
processing steps | These are the individual actions or instructions that make up an algorithm. | Each step should be clear and unambiguous, and they should be executed in a specific order. |
outputs | These are the results produced by an algorithm when it’s executed. | In the case of our rectangle example, the output would be the area of the rectangle. |
Algorithm structure
Regardless of the programming language being used, understanding the basics of algorithm structure is crucial for developing efficient and effective software. By writing well-structured algorithms, programmers can improve the performance of their programs and reduce the likelihood of errors or bugs. By developing strong algorithmic thinking skills, programmers can improve their ability to develop robust code.
At its core, an algorithm is made up of three basic parts: input
, processing steps, and output
.
Let’s take a look at how these three parts fit together in a simple algorithm.
Here's an example algorithm that calculates the area of a rectangle: |
Algorithm’s flowchart
We’ve defined basic terms, now let’s talk about how we can represent an algorithm in a visual way.
One common tool used to represent algorithms is a flowchart.
A flowchart is a diagram that shows the flow of an algorithm, including its inputs, outputs, and steps.
Flowcharts use specific symbols to represent different elements of the algorithm:
- a
rectangle
is used to represent a process or step, - a
diamond
is used to represent a decision point, and - an
oval
is used to represent the start or end of the algorithm.
Flowcharts can be a useful tool for visualizing and communicating the structure of an algorithm, especially for more complex algorithms. By using a flowchart, we can easily see the order in which steps are executed, where decisions are made, and what the inputs and outputs are.
Algorithm’s pseudocode
Now, let’s talk about how we can represent an algorithm in a more structured way.
One common way to represent algorithm’s structure is by using pseudocode.
Pseudocode is a way of writing out an algorithm in plain language, using specific keywords and syntax to indicate different parts of the algorithm. The syntax of the pseudocode is designed to be easy to read and understand, without being tied to any specific programming language.
Here’s an example of pseudocode for the rectangle example algorithm:
Start
Input length, width
Set area to 0
Process
Set area to length times width
Output area
End
In this pseudocode, we first define the inputs (length and width) and initialize the area variable to 0. Then, we move on to the processing step, where we calculate the area by multiplying the length and width. Finally, we output the result (the area) and end the algorithm.
Note that the specific syntax of pseudocode can vary depending on the context and individual preference. The goal is to provide a clear and structured outline of the algorithm’s logic, rather than to conform to a specific programming language’s syntax.
Pseudocode is often used in the early stages of algorithm development, as a way of outlining the basic structure of the algorithm before translating it into a specific programming language. By using pseudocode
, we can focus on the logic of the algorithm without worrying about a specific programming language’s syntax.
Control structure
In addition to the basic input
-processing
-output
structure, algorithms often include control structures. Control structures are statements that control the flow of the algorithm, allowing it to make decisions and perform different actions based on different conditions.
The figure shows a few flowcharts of algorithms that use various control structures, such as conditional statements (if
) or loops (until
, while
).
Conditional statements and loops are two examples of control structures that are commonly used in algorithms.
When you’re creating algorithms, it’s important to remember that different programming languages have different syntax
and available elements of the algorithm’s structure. For example, the syntax for loops and conditional statements in Bash
may be different from that in Python
or R
.
…about the syntax of various programming languages and the different elements of algorithm structure, consider visiting tutorials:
These resources can help you gain a deeper understanding of topics like conditional statements, loops, and other elements of algorithm structure, and how to use them effectively in your scripts or programs.
Conditional Statements
Conditional statements allow an algorithm to perform different actions based on whether a certain condition is true or false. In programming, conditional statements are often written using the keywords if
, else if
, and else
.
The if
statement is followed by a block of code to be executed if the condition is true.
The else if
statement allows for additional conditions to be checked, followed by a block of code to be executed if that condition is true.
Finally, the else
statement provides a default option to be executed if none of the previous conditions are true.
Here’s an example of pseudocode
for a conditional statement:
if condition1 is true then
perform action1
else if condition2 is true then
perform action2
else
perform action3
end if
In this example, we have an if statement that checks if condition1 is true. If it is, the algorithm will perform action1. If condition1 is false, the algorithm will move on to the next condition, which is specified using the else if statement. Here, the algorithm checks if condition2 is true. If it is, the algorithm will perform action2. If both condition1 and condition2 are false, the algorithm will perform action3 using the else statement.
Here’s an example of a conditional statement that determines whether a number is even or odd:
1) code in Bash
num = 5
if ((num % 2 == 0)); then
echo "$num is even"
else
echo "$num is odd"
fi
What code in Bash does?
In this Bash script, we first initialize the variable num
to be equal to 5.
The conditional statement if
checks if the remainder of num divided by 2 is equal to 0 (i.e. whether num
is even). If it is, the program outputs "5 is even"
.
Otherwise, the else
condition is executed and it outputs "5 is odd"
.
The double parentheses around the condition are used to perform arithmetic operations in Bash.
2) code in Python
num = 5
if num % 2 == 0:
print(num, "is even")
else:
print(num, "is odd")
What code in Python does?
In this Python code, we first initialize the variable num
to be equal to 5.
The conditional statement if
checks if the remainder of num divided by 2 is equal to 0 (i.e. whether num
is even). If it is, the program outputs "5 is even"
.
Otherwise, the else
condition is executed and it outputs "5 is odd"
.
In Python, the syntax for conditional statements is if
followed by the condition, a colon, and a block of indented code to be executed if the condition is true. The else
statement provides a block of code to be executed if the condition is false.
Loops
Loops allow an algorithm to repeat a certain action multiple times, either for a fixed number of iterations or until a certain condition is met. In programming, there are two main types of loops: for
loops and while
loops.
example FOR loop
Here’s an example of a for
loop that prints the numbers 1 through 10:
0) pseudocode
For i = 1 to 10 with a step of 1, do the following:
Print i
End For
In this pseudocode, we use the For
keyword to start a loop that will iterate from 1 to 10, with a step of 1. The i
variable is initialized to 1 at the beginning of the loop. The loop will continue to execute as long as i
is less than or equal to 10. Inside the loop, we print the value of i
.
Below you can explore the executable code written in 1) Bash
and 2) Python
, based on the pseudocode of the algorithm.
1) code in Bash
#!/bin/bash
for (( i=1; i<=10; i++ ))
do
echo $i
done
What code in Bash does?
In this Bash script, we use the for
keyword to start a loop that will iterate from 1 to 10, with a step of 1. We initialize the loop variable i
to 1 using the i=1
expression. The loop will continue to execute as long as i
is less than or equal to 10. Inside the loop, we print the value of i
using the echo command. Then, at the end of each iteration, we increment the value of i
by 1 using the i++
expression.
2) code in Python
for i in range(1, 11):
print(i)
What code in Python does?
In this Python script, we use the for
keyword to start a loop that will iterate over a range of numbers from 1 to 10. We use the range()
function to generate a sequence of numbers, starting from 1 and ending at 11 (which is not included in the sequence). Inside the loop, we print the value of i
using the print()
function.
Example WHILE loop
Suppose we want to write an algorithm that counts down from 10 to 1 and prints each number on a new line. We could use a while loop to do this as follows:
0) pseudocode
Start
Set i to 10
While i is greater than 0
Print i
Subtract 1 from i
End While
End
In this example, we first initialize the variable i
to 10. Then, we enter a while loop, which continues as long as i
is greater than 0. Inside the loop, we first print the value of i
, and then subtract 1 from i
. This process continues until i
is no longer greater than 0, at which point the loop ends and the algorithm finishes.
The while
loop in this example is useful because it allows us to repeat a certain action (printing the value of i
) as many times as necessary (until i
is no longer greater than 0). By controlling the flow of the algorithm in this way, we can create more powerful and versatile programs that can handle a wider range of situations.
Below you can explore the executable code written in 1) Bash
and 2) Python
, based on the pseudocode of the algorithm.
1) code in Bash
#!/bin/bash
i=10
while [ $i -gt 0 ]
do
echo $i
i=$((i-1))
done
What code in Bash does?
In this Bash script, we first set the variable i
to 10. Then, we enter a while
loop using the while keyword, with the condition [ $i -gt 0 ]
, which means “while i is greater than 0”. Inside the loop, we print the value of i
variable using the echo
command, and then decrement i
by 1 using the $((i-1))
expression. This process continues until i
is no longer greater than 0, at which point the loop ends and the script finishes.
2) code in Python
i = 10
while i > 0:
print(i)
i -= 1
What code in Python does?
In this Python script, we first set the variable i
to 10. Then, we enter a while
loop using the while keyword, with the condition i > 0
, which means “while i is greater than 0”. Inside the loop, we print the value of i
using the print function, and then decrement i
by 1 using the i -= 1
shorthand for i = i - 1
. This process continues until i
is no longer greater than 0, at which point the loop ends and the script finishes.
In this section, we will provide several examples of simple algorithms and walk through how they work. We will create pseudocode to represent these algorithms and explain how control structures are used in these examples. These examples will demonstrate how the basic concepts of algorithms and control structures can be applied in various situations.
Sum of two numbers
input: two numbers, num1 and num2
output: sum of the two numbers, sum
set sum = num1 + num2
output sum
EXPLANATION
This algorithm takes in two numbers as input and outputs their sum. There are no control structures used in this algorithm.
Factorial of a number
input: a number, num
output: the factorial of num, fact
set fact = 1
for i from 1 to num:
set fact = fact * i
output fact
EXPLANATION
This algorithm takes in a number as input and outputs its factorial. A for
loop is used to iterate from 1 to the input number and multiply each number in the range together to get the factorial.
Greatest common divisor
input: two numbers, num1 and num2
output: the greatest common divisor of num1 and num2, gcd
while num2 is not 0:
set temp = num2
set num2 = num1 % num2
set num1 = temp
output num1
EXPLANATION
This algorithm takes in two numbers as input and outputs their greatest common divisor. A while
loop is used to repeatedly calculate the remainder of dividing the larger number by the smaller number until the remainder is 0. The final value of the smaller number is the greatest common divisor.
Linear search
input: an array of numbers, arr, and a value to search for, val
output: the index of val in arr, or -1 if val is not in arr
for i from 0 to length of arr - 1:
if arr[i] == val:
output i
output -1
EXPLANATION
This algorithm searches for a value in an array using the linear search algorithm. A for
loop is used to iterate through each element of the array and compare it to the search value within a simple if
condition statement.
Binary search
input: a sorted array, arr, and a value to search for, val
output: the index of val in arr, or -1 if val is not in arr
set left = 0
set right = length of arr - 1
while left <= right:
set mid = (left + right) / 2
if arr[mid] == val:
output mid
else if arr[mid] < val:
set left = mid + 1
else:
set right = mid - 1
output -1
EXPLANATION
This algorithm searches for a value in a sorted array using the binary search algorithm. A while
loop is used to repeatedly cut the search range in half by comparing the value to the middle element of the current range and adjusting the endpoints accordingly within the if
-else if
-else
condition statement.
Bubble sort
input: an array of numbers, arr
output: the sorted array
set n = length of arr
for i from 0 to n - 1:
for j from 0 to n - i - 1:
if arr[j] > arr[j+1]:
swap arr[j] and arr[j+1]
output arr
EXPLANATION
This algorithm sorts an array of numbers using the bubble sort algorithm. Two nested for
loops are used to compare adjacent elements of the array and swap them if
they are in the wrong order, repeatedly iterating through the array until no more swaps are needed.
Reverse a string
input: string s
output: reversed string r
Algorithm:
1. Initialize an empty string called r.
2. Get the length of s and store it in a variable called len.
3. Loop from len-1 to 0 with a step size of -1.
4. For each iteration, add the character at index i of s to r.
5. Return r as the output.
EXPLANATION
The above pseudocode assumes a 0-indexed
string, meaning the first character of the string is at index 0. If your string is 1-indexed
(i.e., the first character is at index 1), then you’ll need to adjust the loop accordingly by starting at len
and ending at 1
with a step size of -1
.
Pseudocode is a high-level description of a computer program that uses a mix of natural language and structured programming language-like syntax. Pseudocode
can be more descriptive using text, where it provides detailed explanations of each step, or more like a template of the code, where it resembles actual programming code without adhering to a specific programming language’s syntax.
Transforming the descriptive pseudocode from above example to code-like pseudocode can be a great exercise to help you understand how to translate an algorithm’s logic into actual code.
see example code-like solution
Here’s how you would implement the algorithm in code-like pseudocode:
input: string s
output: string r, as reversed string s
set r = "" # Initialize an empty string called r.
set len = length of s # Get the length of s and store it in a variable called len.
FOR i FROM len-1 TO 0 STEP -1 DO: # Loop from len-1 to 0 with a step size of -1.
r = r + s[i] # For each iteration, add the character at index i of s to r.
END FOR
RETURN r # Return r as the output.
see realistic Python implementation
Here’s how you would implement the algorithm in Python:
def reverse_string(s):
r = ""
length = len(s)
for i in range(length - 1, -1, -1):
r += s[i]
return r
# Usage example:
result = reverse_string("hello")
print("Reversed string:", result)
Enclosing the algorithm in a Python function enhances code modularity, reusability, and readability by encapsulating the logic within a named unit that can be easily called and utilized throughout the codebase.