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Iterate through lists using inner for-loops

Iterate through lists using inner for-loops#

We’ve already seen nested for-loops in which we iterate through a list using the inner for-loop:

for i in range(n):
    for j in range(len(cases)):
        print(cases[j])

This is the code the researcher’s friend provided to print multiple copies of elements in cases.

Notice that the outer for-loop is just to repeat our inner for-loop for n times, while the inner for-loop is the one that iterates through cases and prints out the elements.

Iterate through lists using outer for-loops#

We can also use the outer for-loop to iterate through a list, and do something with each element in the list using the inner for-loop!

Consider the following example code snippet:

The code above uses a nested for-loop, and it will calculate the factorial of each element in lst and print it out.

The code above uses a nested for-loop, and it will calculate the factorial of each element in lst and print it out. The output is:

Factorial of the element at index 1 is: 1
Factorial of the element at index 2 is: 2
Factorial of the element at index 3 is: 6
Factorial of the element at index 4 is: 24

The following snippet shows how the loop gets unrolled to reach the above output. Notice how the number of iterations of the inner loop is dependent on the value of n, the elements of the list being traversed using the outer for-loop.

#In the 1st iteration:

n = 1
factorial = 1 #factorial is set to 1
#The inner for loop equivalently becomes:
for j in range(1, 2):
    factorial *= j
#The inner loop gets executed once and we get factorial = 1


#In the 2nd iteration:

n = 2
factorial = 1 #factorial is reset to 1
#The inner for loop equivalently becomes:
for j in range(1, 3):
    factorial *= j
#The inner loop gets executed twice and we get factorial = 1*2 = 2



#In the 3rd iteration:

n = 3
factorial = 1 #factorial is reset to 1
#The inner for loop equivalently becomes:
for j in range(1, 4):
    factorial *= j
#The inner loop gets executed thrice and we get factorial = 1*2*3 = 6


#In the 4th iteration:

n = 4
factorial = 1 #factorial is reset to 1
#The inner for loop equivalently becomes:
for j in range(1, 5):
    factorial *= j
#The inner loop gets executed four times and we get factorial = 1*2*3*4 = 24

Iterate through lists using both outer for-loops and inner for-loops?#

In fact, as you may probably guess, we can iterate through a list using the outer for-loop while at the same time iterate through a list using the inner for-loop! Since this is just a combination of the two previously discussed ways of using nested for-loops, we won’t get into it here. How about you try some exercises on it yourself?

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