I have doubt in these python nested for loop, that how the inner loop is executing can anyone resolve my problem here
for i in range(1, 4):
print(i)
for j in range(1, i):
print(j)
Basically, your for loops are counting numbers from 1 to the upper limit minus 1.
Your code will run as follows:
Starting with i=1, it will print out a 1 and then j will go from 1 to 0, that is it will take up no values and j won't be printed.
Next, i will become 2, and then j will go from 1 to 2, resulting in the output 1 from j.
This will continue, and the overall output will be:
(i)1
(i)2
(j)1
(i)3
(j)1
(j)2
The stuff in the brackets show which variable is being printed.
For one your code example is poorly formatted.
I am assuming this is how the code is.
for i in range(1, 4):
print(i)
for j in range(1, i):
print(j)
This is a very simple for loop example, I'll go over two iterations that should make it clear.
Let's look at the outer for loop, it goes from 1 to 3 (4 exclusive)
i=1 now, j will go from 1 to i i.e. 1 to 1.
The inner loop will now look as follows:
..
for j in range(1, 1):
print(j)
As range(1, 1), 1 will be exclusive, hence this loop will not run.
So out of Iteration 1:
1 # print(i)
=====
i=2, now, j will go from 1 to i i.e. 1 to 2.
outer loop will print 2, and now inner loop will look like:
..
for j in range(1, 2):
print(j)
This will print 1, as 2 is excluded limit.
Now output will be
2 # outer loop
1 # Inner loop.
On similar retrospection, the last iteration at i=3
Will give output:
3 # outer loop print(i)
1 # inner loop print(j)
2 # inner loop print(j)
Combining all three the final output would be
1 # outer
2 # outer
1 # inner
3 # outer
1 # inner
2 # inner
Related
I have a very simple piece of code but it's driving me nuts to understand the logic.
for a in range(6):
print("\t\t")
for j in range(a):
print(a, end=" ")
The output is:
1
2 2
3 3 3
4 4 4 4
5 5 5 5 5
the first value to be printed is supposed to be 0 since the range(6) starts from 0 and a and j wrapped in for loops both are supposedly getting 0 as first element but it starts from 1 instead.
That comes, because the for loop in range 0 will not be executed. You can not execute something if that has to go 0 times. To make it run as you would like to be, you would have to write following code:
for a in range(6):
print("\t\t")
for j in range(a + 1):
print(a, end=" ")
The value of a is the number of iterations of the inner loop. (Note that the question should have the j loop indented, so that it is inside the a loop, in order to get the output shown.)
On the first iteration, a is 0, and then range(0) will give an empty sequence, which give zero iterations when iterating over it. (Specifically a StopIteration will occur the first time a value is fetched.)
On the next iteration of the outer loop, there will be one j value when iterating, and so forth.
So 0 is printed 0 times (i.e. not at all), then 1 is printed once, 2 is printed twice, ... , 5 is printed 5 times.
You are not using the values of j at all, but they will be:
when a is 0: no iterations
when a is 1: j is 0 only
when a is 2: j is 0, 1
... etc, up to ...
when a is 6: j is 0, 1, 2, 3, 4, 5
Note that in fact it is conventional to use a variable name _ for a loop variable whose value you do not use (at least assuming that it not inside a loop already using _). If you adhere to this convention (i.e. use _ instead of j) and also fix the indentation, then your code might look like:
for a in range(6):
print("\t\t")
for _ in range(a):
print(a, end=" ")
I am new to python. I have executed for loop with runtime input.
Loop 1:
a = int(input("enter\n"))
for i in range(1, a+1):
print(i)
The output prints all values from 1 to 5.
Loop2:
for i in range(1, 5):
print(i)
The output print the values from 1 to 4.
Please someone explain the different between the above two for loop.
In the range function in python, the last argument is exclusive. This means that it represents the stopping point of the range but is not included in the output. Since the first loop has a+1 as the last argument, a will be included in the output.
Did you have a look at https://docs.python.org/fr/3/library/stdtypes.html#range ?
To keep things easy, range(a, b) will return an iterator that goes from a to b - 1.
Thus, your first loop will print 1 to (a + 1) - 1 = a, and your second loop will print 1 to 5 - 1 = 4.
for i in range(0,5):
print(i)
Prints values from 0 to 4 because when the iteration comes to the value 5 it becomes excluded and it won't print the value.
So we add 1 to the final value(maximum) to print till the value we want.
for i in range(0,6):
print(i)
GIVES:
0
1
2
3
4
5
I don't understand how the i in the 2nd for loop of the code below works.
di = [96, 15, 33, 87]
for i in range(len(di)):
total = di[i]
for j in range(i+1,len(di)):
print(i)
0
0
0
1
1
2
Why is the output 0,0,0,1,1,2. How does i in the 2nd for loop get affected from the first loop? Is there some inheritance? Pardon the newbie here.
len(di) is 4. So the loop
for i in range(len(di)):
will repeat 4 times. Because of the way range works (from lower bound, which is 0 by default if not specified, to 1 below the upper bound), i will be 0 in the first repetition, 1 in the second repetition, and so on. To calculate out how many objects range(x, y) generates, in this case, how often for i in range(x, y)will repeat, you can simply do number of repetitions = y - x. So in this case: len(di) - 0 (default lower bound) = 4.
The loop
for j in range(i+1, len(di)):
print(i)
will repeat the print(i) command len(di) - (i + 1) times. Keep in mind, i is defined by the outer loop. So, during the first loop of
for i in range(len(di)):
i equals 0, so the print(i) command will be executed 4 - (0+1) = 3 times - it will print i(=0) 3 times. In the second loop, iequals 1, so it will be printed 2 times, and so on. So here's whats happening, formatted as code for better readability:
First outer loop:
i = 0
total = di[i] = di[0] = 96
--> first inner loop of first outer loop:
j = i + 1 = 1
i is printed -> prints 0
second inner loop of first outer loop:
j = j+1 = 2
i is printed -> prints 0 again
third inner loop of first outer loop:
j = j+1 = 3 --> since len(di) = 4, the upper bound of range(i+1, len(di)) is reached, so this is the last Repetition
i is printed -> prints 0 again
Second outer loop:
i = 1
total = di[1] = 15
--> first inner loop of second outer loop:
j = i+1 = 2
i is printed -> prints 1
second inner loop of second outer loop:
j = i+1 = 3 -> upper bound of range reached, last repetition
i is printed -> prints 1 again
Third outer loop:
i = 2
total = di[2] = 33
--> first inner loop of third outer loop:
j = i+1 = 3 -> upper bound of range is reached, only Repetition
i is printed -> prints 2
Fourth (and final) outer loop:
i = 3 -> upper bound of range(len(di)) reached, last Repetition
total = di[3] = 87
since j = i+1 = 4, the inner loop does not get executed at all (both bounds of range are equal), so 3 doesn't get printed
end of code.
In programming language, a variable is available for usage within a scope. When you start for loop with a new variable then it would be available for usage until you end it.
As you are beginning the journey to learn python, one of the really good practice is to read official document. https://docs.python.org/3/tutorial/controlflow.html
To help your understanding, try this:
di = [96, 15, 33, 87]
for i in range(len(di)):
print("first loop, i =", i)
total = di[i]
for j in range(i+1,len(di)):
print("second loop, j =", j)
print("second loop, i =", i)
The i is the same in both loops. each time the outer loop runs, it then runs the inner loop until the "for" statement is complete.
(Newbie question, please bear with me...)
For the following code:
x = 4
for j in range(x):
for i in range(x):
print(i)
x = 2
Python prints the following:
0
1
2
3
0
1
0
1
0
1
... and I do not understand why it prints the first iteration of 0 1 2 3 when there is no print(j) command, only print(i). What purpose at all do the line "for j in range(x):" and j specifically serve?
This is an example from the book "Introduction to Computation and Programming Using Python" by John Guttag (used in the MIT courses).
The outer loop for j in range(x): is evaluated one time with x=4, it means that the inner loop will be evaluated 4 times with j=0,1,2,3.
You have then to ask yourself what is the value of x when the expression for i in range(x): is evaluated.
At the first evaluation x is 4, the inner loop is executed with 0,1,2,3 for i.
At the second evaluation x is 2, the inner loop is executed with 0,1 for i
For the third and fourth evaluation x is also 2, and the inner loop is executed with 0,1 for i.
situation:
2 loops, expect the outer loop run once, then inter loop runs completely, then outer loop runs its 2nd...
question: why the outer loop run twice before the inter loop get a chance to run?
Code:
def sort(a_list):
for i in range(1,len(a_list)):
print("i=",i)
for j in range(i-1,0,-1):
print("j=",j)
Test:
L=[9,6,1,3]
sort(L)
Result:
i= 1
i= 2 # here, the outer lopp ran twice then inter loop began.
j= 1
i= 3
j= 2
j= 1
It's because when i = 1 the inner loop becomes range(0,0,-1) which is empty. So you don't print anything and move onto i = 2.
Because here in inner loop for the first iteration it would be
For j in range(1-1,0,-1):
Which equates to
For j in range(0,0,-1):
That range is an empty range so at i=1 inner loop won't run since condition evaluate to false