1.10 for loops
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1.10 for loops#
This lesson introduces for loops in python.
Important
This lesson is still under development.
Just like while loops, for loops allow an instruction to be executed
a certain number of times. How many times? It depends upon iterator.
As per wikipedia 11 people have received Nishan-e-Haider 1
award in Pakistan. Let’s say we want to iterate over this list.
NH_receivers = ['Saif Ali Janjua', 'Muhammad Sarwar', 'Tufail Muhammad',
'Aziz Bhatti', 'Rashid Minhas', 'Muhammad Akran',
'Shabbir Sharif', 'Muhammad Husain Janjua', 'Muhammad Mahfuz',
'Sher Khan', 'Lalak Jan']
for shaheed in NH_receivers:
print(shaheed)
Saif Ali Janjua
Muhammad Sarwar
Tufail Muhammad
Aziz Bhatti
Rashid Minhas
Muhammad Akran
Shabbir Sharif
Muhammad Husain Janjua
Muhammad Mahfuz
Sher Khan
Lalak Jan
In above example, a list is acting as an iterator. In fact it can be a tuple, string or any other sequence. The basic syntax of for loop in python is:
for variable in sequence:
do something
The variable in above syntax is assigned a new new value from sequence upon every iteration.
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We can also run a for loop on items of a dictionary. If we want to iterate over both keys and values of a dictions, we would do as below
scholars = {
"Baqir al sadr": 1980,
"Murtaza Mutahri": 1979,
"Allama Iqbal": 1938,
"Jamal ul din Afghani": 1897,
"Ali Shariati": 1977,
"Moh Husain Tabatabai": 1981
}
for scholar, date_of_death in scholars.items():
print(scholar, "died in ", date_of_death)
Baqir al sadr died in 1980
Murtaza Mutahri died in 1979
Allama Iqbal died in 1938
Jamal ul din Afghani died in 1897
Ali Shariati died in 1977
Moh Husain Tabatabai died in 1981
for with else#
Just like while loops, the code under else gets executed if everything goes well within for loop. If let’s say, we are running for loop to search an item in the iterator, and when we find the item, there is not point in continuing the for loop further. Hence we break out of for loop. In such a case code under else will not be executed.
for variable in sequence:
do something
else:
do at last
for shaheed in NH_receivers:
if shaheed == 'Rashid Minhas':
print("Person from air force found")
break
else:
print('Search completed')
Person from air force found
We can create a simple sequence using range function
range(5)
range(0, 5)
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We can alo iterate over a list using range.
for i in range(5):
print(NH_receivers[i])
Saif Ali Janjua
Muhammad Sarwar
Tufail Muhammad
Aziz Bhatti
Rashid Minhas
However, the above example is not a very clever approach as we could
have simple done for i in NH_receivers. A more useful example would be:
for i in range(2, 5):
print(NH_receivers[i])
Tufail Muhammad
Aziz Bhatti
Rashid Minhas
We can also include step argument in range, which decides how big the jump/step we want to have in our iterator. In this way we can skip every nth value in a squence/iterator.
for i in range(2, 8, 2):
print(NH_receivers[i])
Tufail Muhammad
Rashid Minhas
Shabbir Sharif
We can also go backwards in the iterator
for i in range(8, 2, -1):
print(NH_receivers[i])
Muhammad Mahfuz
Muhammad Husain Janjua
Shabbir Sharif
Muhammad Akran
Rashid Minhas
Aziz Bhatti
for i in range(8, 0, -2):
print(NH_receivers[i])
Muhammad Mahfuz
Shabbir Sharif
Rashid Minhas
Tufail Muhammad
nested for loops#
for name in NH_receivers:
for char in name:
print(char)
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accessing index#
If we want to access index itself, we can do this by using enumerate.
for index, item in enumerate(NH_receivers, start=0): # default value of start is zero.
print(item, ' is Nishan - Haider receiver number ', index)
Saif Ali Janjua is Nishan - Haider receiver number 0
Muhammad Sarwar is Nishan - Haider receiver number 1
Tufail Muhammad is Nishan - Haider receiver number 2
Aziz Bhatti is Nishan - Haider receiver number 3
Rashid Minhas is Nishan - Haider receiver number 4
Muhammad Akran is Nishan - Haider receiver number 5
Shabbir Sharif is Nishan - Haider receiver number 6
Muhammad Husain Janjua is Nishan - Haider receiver number 7
Muhammad Mahfuz is Nishan - Haider receiver number 8
Sher Khan is Nishan - Haider receiver number 9
Lalak Jan is Nishan - Haider receiver number 10
which is equivalent to
Saif Ali Janjua is Nishan - Haider receiver number 0
Muhammad Sarwar is Nishan - Haider receiver number 1
Tufail Muhammad is Nishan - Haider receiver number 2
Aziz Bhatti is Nishan - Haider receiver number 3
Rashid Minhas is Nishan - Haider receiver number 4
Muhammad Akran is Nishan - Haider receiver number 5
Shabbir Sharif is Nishan - Haider receiver number 6
Muhammad Husain Janjua is Nishan - Haider receiver number 7
Muhammad Mahfuz is Nishan - Haider receiver number 8
Sher Khan is Nishan - Haider receiver number 9
Lalak Jan is Nishan - Haider receiver number 10
This is another way to keep track that how many times the loop has been executed.
Iterating over more than one sequences#
If we want to iterate over more than one sequences, we can do this
using built-in function zip.
scholars = ['Baqir al sadr', 'Murtaza Mutahri', 'Allama Iqbal', 'Jamal ul din Afghani',
'Ali Shariati', 'Moh Husain Tabatabai']
date_of_death = [1980, 1979, 1938, 1897, 1977, 1981]
for scholar, dod in zip(scholars, date_of_death):
print(scholar, ' died in year ', dod)
Baqir al sadr died in year 1980
Murtaza Mutahri died in year 1979
Allama Iqbal died in year 1938
Jamal ul din Afghani died in year 1897
Ali Shariati died in year 1977
Moh Husain Tabatabai died in year 1981
date_of_birth = [1935, 1919, 1877, 1838, 1933, 1904]
for scholar, dod, dob in zip(scholars, date_of_death, date_of_birth):
print(scholar, ' was born in ', dob, ' and died in year ', dod)
Baqir al sadr was born in 1935 and died in year 1980
Murtaza Mutahri was born in 1919 and died in year 1979
Allama Iqbal was born in 1877 and died in year 1938
Jamal ul din Afghani was born in 1838 and died in year 1897
Ali Shariati was born in 1933 and died in year 1977
Moh Husain Tabatabai was born in 1904 and died in year 1981
But what if lengths of lists are not equal?
scholars = ['Baqir al sadr', 'Murtaza Mutahri', 'Allama Iqbal', 'Jamal ul din Afghani',
'Ali Shariati', 'Moh Husain Tabatabai']
date_of_death = [1980, 1979, 1938, 1897, 1977, 1981, 1989]
print(len(scholars), len(date_of_death))
for scholar, dod in zip(scholars, date_of_death):
print(scholar, ' died in year ', dod)
6 7
Baqir al sadr died in year 1980
Murtaza Mutahri died in year 1979
Allama Iqbal died in year 1938
Jamal ul din Afghani died in year 1897
Ali Shariati died in year 1977
Moh Husain Tabatabai died in year 1981
Simple zip will iterate over the point when all lists are equal and
ignore if any sequence is larger than the others. If we want to iterate
until the longest sequence, we have to use zip_longest from itertools
from itertools import zip_longest
scholars = ['Baqir al sadr', 'Murtaza Mutahri', 'Allama Iqbal', 'Jamal ul din Afghani',
'Ali Shariati', 'Moh Husain Tabatabai']
date_of_death = [1980, 1979, 1938, 1897, 1977, 1981, 1989]
for scholar, dod in zip_longest(scholars, date_of_death):
print('name: ', scholar, ' date of death: ', dod)
name: Baqir al sadr date of death: 1980
name: Murtaza Mutahri date of death: 1979
name: Allama Iqbal date of death: 1938
name: Jamal ul din Afghani date of death: 1897
name: Ali Shariati date of death: 1977
name: Moh Husain Tabatabai date of death: 1981
name: None date of death: 1989
Notice the last printed line. If we want to access the previous and next value during iteration, we must start from 1 and end before last item in order to print correct values
for i in range(1, len(NH_receivers) - 1):
print(NH_receivers[i], ' came before ', NH_receivers[i + 1], ' and after ', NH_receivers[i - 1])
Muhammad Sarwar came before Tufail Muhammad and after Saif Ali Janjua
Tufail Muhammad came before Aziz Bhatti and after Muhammad Sarwar
Aziz Bhatti came before Rashid Minhas and after Tufail Muhammad
Rashid Minhas came before Muhammad Akran and after Aziz Bhatti
Muhammad Akran came before Shabbir Sharif and after Rashid Minhas
Shabbir Sharif came before Muhammad Husain Janjua and after Muhammad Akran
Muhammad Husain Janjua came before Muhammad Mahfuz and after Shabbir Sharif
Muhammad Mahfuz came before Sher Khan and after Muhammad Husain Janjua
Sher Khan came before Lalak Jan and after Muhammad Mahfuz
We can change loop variable inside the loop. However, this is not a good practice and is prone to bugs. Let’s say we want to iterate over sequence from 0 to 10 i.e 0,1,2,3,4,5,6,7,8,9 but after 5 we want to jump to 8 and then carry on. It means we should change the loop variable inside the loop. A naive mind might thing following approach would work %%
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But we had wished the output as 0,1,2,3,4,5,8,9. The reasons is, for loop iterated over 0,1,2,3,4,5,6,7,8,9 and if we changed current value of i, it will not affect next value of i at next iteration. We have to use while loop in such a scenario.
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If we have a list of lists, and we want to flatten all the elements of that list into one list, we can use a nested for loop to achieve this.
prime_ministers = [['zafrullah jamali', 'chaudhry shujaat', 'shaukat aziz'],
['yousuf raza gilani', 'raja pervaiz ashraf'],
['nawaz sharif', 'shahid khaqan']]
print(len(prime_ministers))
all_pms = []
for era in prime_ministers:
for pm in era:
all_pms.append(pm)
print(all_pms)
3
['zafrullah jamali', 'chaudhry shujaat', 'shaukat aziz', 'yousuf raza gilani', 'raja pervaiz ashraf', 'nawaz sharif', 'shahid khaqan']
print(len(all_pms))
7
list comprehension#
One of the reasons, python is beautiful is because of its poetry like syntax. Consider following loop
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
Now, what if I told you that we can acheive this in one line?
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
The method of writing for loop inside the list as we have done above is called list comprehension.
We can also have if and else statement in list comprehension
['Muhammad Sarwar', 'Tufail Muhammad', 'Muhammad Akran', 'Muhammad Husain Janjua', 'Muhammad Mahfuz']
[0, 1, 8, 27, 64, 125, 36, 49, 64, 81]
list comprehension for nested for loops#
continue#
The continue keyword is used inside the for loop when we want to
skip some commands in a particular iteration.
clinton: Let's go to war
bush: Let's go to war
obama: Let's go to war
biden: Let's go to war
Above we did not want to print “Let’s go to war” when the value
of president was equal to trump so we used continue statement.
That was a too simple example. We would have better to avoid writing trump in the list instead of adding two lines inside the for loop. Usually, the conditioning variable (president in our case above) appears after doing some calculations inside for loop.
break#
The break keyword is used if we want to stop the iterations of for loop.
no more purchase please!
If you want to dig deep into how the for loops work in python,
you can jump to 3.16 magic methods.
Total running time of the script: ( 0 minutes 0.016 seconds)
