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Conditional Statements and Loops in Python

 2 years ago
source link: https://dev.to/sahilfruitwala/conditional-statements-and-loops-in-python-298p
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What if you want to do some specific tasks when the string is "Python" and some other tasks when the string is "Java". To achieve this conditioning we can use conditional statements in python or any other language. When we want to do some task for n number of times or we want to loop through some specific list, in this kind of situation, we can use different types of loops.

Conditional Statements in Python

In python, we have two types of conditional statements.

Both conditional statements seem almost similar, and actually, they are. There is just a minor difference between these two statements. We will understand this difference using code examples. If you know any other language(s), you might be wondering if python has a switch case or not. NO, python does not have a switch case.

if-else

Syntax:

if(conditional expresson):
    statement
else:
    statement
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If I have explained the if-else statement in plain English then it would be the "either this or that" statement.

Let's assume if the string is java, I want to do the sum of two numbers else I want to multiply two numbers.

num1 = 5
num2 = 2

lang = 'java'

if(lang=="java"): # check if value of lang variable is 'java'
    c = num1 + num2
    print(c)
else:
    c = num1 * num2
    print(c)

# OUTPUT: 7
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num1 = 5
num2 = 2

lang = 'java'

if(lang!="java"): # check if value of lang variable is NOT 'java'
    c = num1 + num2
    print(c)
else:
    c = num1 * num2
    print(c)

# OUTPUT: 10
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It is not compulsory to use else part. You can use only if part as well. For example, add string "Programming" to the existing string if it is "Python" or does nothing.

lang = 'Python'

if(lang=="Python"):
    lang = lang +" Programming"

print(lang)
# Output: Python Programming


lang = 'Java'

if(lang=="Python"):
    lang = lang +" Programming"

print(lang)
# Output: Java
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Sometimes, you can go into if statement using only variable as well. For example,

lang = 'Python'
if(0):
    lang = lang +" Programming"
print(lang)
# OUTPUT: Python

lang = 'Python'
if(1):
    lang = lang +" Programming"
print(lang)
# OUTPUT: Python Programming

lang = 'Python'
if(True):
    lang = lang +" Programming"
print(lang)
# OUTPUT: Python Programming

lang = 'Python'
if(False):
    lang = lang +" Programming"
print(lang)
# OUTPUT: Python

lang = 'Python'
if(""):
    lang = lang +" Programming"
print(lang)
# OUTPUT: Python

lang = 'Python'
if("random string"):
    lang = lang +" Programming"
print(lang)
# OUTPUT: Python Programming

lang = 'Python'
if(None):
    lang = lang +" Programming"
print(lang)
# OUTPUT: Python

lang = 'Python'
if([1]):
    lang = lang +" Programming"
print(lang)
# OUTPUT: Python Programming

lang = 'Python'
if([]):
    lang = lang +" Programming"
print(lang)
# OUTPUT: Python

lang = 'Python'
if({'a':1}):
    lang = lang +" Programming"
print(lang)
# OUTPUT: Python Programming

lang = 'Python'
if({}):
    lang = lang +" Programming"
print(lang)
# OUTPUT: Python
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If we want to combine multiple conditions we can use and and or within the if statement.

lang = 'PYTHON'

if(lang.isupper() and len(lang) > 2):
    print("Worked!")
# OUTPUT: Worked!

if(lang.islower() or len(lang) == 6):
    print("Worked again!")
# OUPUT: Worked again!
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if-elif-else

Syntax:

if(conditional expresson):
    statement
elif(conditional expresson):
    statement
else:
    statement
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We use this conditional statement when we want to test more than one scenario. Let's understand this by an example.

marks = 75

if(marks >= 34 and marks <= 50):
    print("Grade C!")
elif(marks > 50 and marks <= 75):
    print("Grade B!")
elif(marks > 75 and marks <= 100):
    print("Grade A!")
elif(marks >= 0 and marks < 34):
    print("FAIL!")    
else:
    print("Something is wrong!")

# OUTPUT: Grade A!
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Be aware when you use multiple if and if-elif. A single error in your logic and all your code-base can throw an error. To understand this, see the below example.

marks = 75

if(marks > 75 and marks <= 100):
    print("Grade A!")
elif(marks > 50):
    print("Grade B!")
elif(marks >=34):
    print("Grade C!")
else:
    print("Something is wrong!")

print("-----------------------------------")

marks = 75

if(marks > 75 and marks <= 100):
    print("Grade A!")
if(marks > 50):
    print("Grade B!")
if(marks >=34):
    print("Grade C!")

# OUTPUT:
Grade B!
-----------------------------------
Grade B!
Grade C!
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Loops in Python

In python, we have two kinds of loops.

for Loop in Python

Syntax:

for iterator in sequence:
    statement(s)
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For loop is used when we want to traverse through the sequential datatype. For example traversing through lists, tuples, strings etc.

fruits = ['apple', 'banana', 'mango', 'grapes', 'peach']

for fruit in fruits:
    print(fruit)
"""
# OUTPUT:
apple
banana
mango
grapes
peach
"""

for i in range(len(fruits)):
    print(fruits[i])
"""
# OUTPUT:
apple
banana
mango
grapes
peach
"""
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In the first example above, in the fruit variable, we are directly getting each element of the fruits list. Whereas, in the second example, we are accessing each element by index reference. The range(n) function returns a sequence of numbers.
Syntax: range(start, stop, step).

for i in range(2, 13, 3):
    print(i)

""" OUTPUT: 
2
5
8
11
"""
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We can use for loop with dictionaries as well.

dict1 = {"lang": "python", "year": "2021", "month": "november"}

for item in dict1:
    print(item, ":" , dict1[item])

""" OUTPUT:
lang : python
year : 2021
month : november
"""
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while Loop in Python

Syntax:

while expression:
    statement(s)
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When you need to do some work repeatedly until some condition satisfies, in that situation you can use a while loop.

Let's assume we need to print "Hello World" until the count variable became 5.

count = 0
while (count < 5):   
    print("Hello World")
    count = count + 1
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Loop Control Statements

There are times when you need to break the loop or skip some iterations in the loop or do nothing. This kind of thing can be done using Loop Control Statements. In python, we have three loop control statements.

  1. Break statement
  2. Continue statement
  3. Pass statement

We use a break statement when we want to break the loop when certain conditions match. For example,

fruits = ['apple', 'banana', 'mango', 'grapes', 'peach']

for fruit in fruits:
    if(fruit == 'mango'):
        break
    print(fruit)

""" OUTPUT: 
apple
banana
"""
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While the break statement breaks the loop, the continue statement makes the loop skip the current iteration and forces it to go to the next iteration.

fruits = ['apple', 'banana', 'mango', 'grapes', 'peach']

for fruit in fruits:
    if(fruit == 'mango'):
        continue
    print(fruit)

""" OUTPUT: 
apple
banana
grapes
peach
"""
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Pass statement is a meaningless piece of code that suggests the interpreter to do nothing. It is similar to the comment but comments are ignored by interpreters while pass statement executed by the interpreter and as per the name does nothing. Pass statement used with function and class as well, which I will explain in the next section.

fruits = ['apple', 'banana', 'mango', 'grapes', 'peach']

for fruit in fruits:
    if(fruit == 'mango'):
        pass
    print(fruit)

""" OUTPUT:
apple
banana
mango
grapes
peach
"""
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for...else & while...else

During the development journey, there are times when you might need to use for..else and while...else. These are just normal for and while loop with special else cases. Let's see an example to understand it.

fruits = ['apple', 'banana', 'mango', 'grapes', 'peach']

for fruit in fruits:
    if(fruit == 'mango'):
        break
    print(fruit)
else:
    print("Loop Did Not Break!")

""" OUTPUT :
apple
banana
"""
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fruits = ['apple', 'banana', 'mango', 'grapes', 'peach']

for fruit in fruits:
    if(fruit == 'abc'):
        break
    print(fruit)
else:
    print("Loop Did Not Break!")

""" OUTPUT :
apple
banana
mango
grapes
peach
Loop Did Not Break!
"""
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So, you might have guessed. When loop does not break OR loop completes its all the iteration without interruption, else block is executed. The same happens with the while loop as well.

i = 1
while i < 6:
  if(i == 3):
    break
  print(i)
  i += 1
else:
  print("All iteration of While are executed successfully.")

""" OUTPUT :
1
2
"""
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Conclusion

Finally! We are at the end of this section 😁.

I know, it is a lot to take in at a time. But, you don't need to remember everything that I have mentioned here. I just showed you so that you can recall what is possible and what is not. There are some other methods as well that I have not mentioned here.

If you want to find out more about control statements and loops in python, check out on Programiz.

That was it. Thank you for reading.

I know it is a lot but I hope, you were able to absorb some knowledge. If you have any questions or doubt mention them in the comment section or reach out to me on Twitter, LinkedIn or Instagram.

If you want me to explain any specific topic, mention that in the comment section below 👇.


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