How do for loops work in python?

[shivajikobardan]

TL;DR Summary

how for loops work in python

str1="hello"

for stringElement in str1:
    print("str1","=",stringElement)

Strings are arrays in python and they are represented like this.

Now I want to understand how the for loop is working here.

My dry run-:

 for h in str1
     print h

 for e in str1
     print e

and so on till o...

My confusions-:

This is what I have read about python for loop from a book called python power.

https://i.stack.imgur.com/qApVt.png

My confusion is that-:

1) How can I see array index 0 when h is being printed
2) How does python knows to increment array index by 1 after the end of each for loop? I heard strings are iterables in python. But I am not very clear what do they really mean here in this case. So some guidance will be really valuable.

 

[PhDeezNutz]

I think what you want to do is create an empty string at the beginning of the for loop and add to the empty string with each iteration

str1 = "hello"
str2 = ""

for str_element in str1:
    str2 = str2 + str_element
    print(str2)

Or if you want to just create the entirety of the new string (a copy) and then print it all at the end (instead of printing at the end of each iteration) you just change the indent

str1 = "hello"
str2 = ""

for str_element in str1:
    str2 = str2 + str_element

print(str2)

Did I understand your question correctly?

 

[Gaussian97]

I think you have big misconceptions.

My dry run-:

 for h in str1
     print h

 for e in str1
     print e

and so on till o...

Have you tried to run these codes? both are exactly equal and will produce the exact same result and the same all others 'till o'. You are just changing the variable, but there's no difference in the result.

My confusion is that-:

1) How can I see array index 0 when h is being printed
2) How does python knows to increment array index by 1 after the end of each for loop? I heard strings are iterables in python. But I am not very clear what do they really mean here in this case. So some guidance will be really valuable.

For 1) I think the question arises from your misconceptions, you can obtain the index of the iteration using the enumerate function.

For 2), python doesn't know to increment any index by 1, this is not what is doing. It is iterating over all the elements of the string (which are length 1 strings), so the only thing Python needs to know is what is the current element and what will be the next.

 

[jack action]

Is this what you are looking for:

str1="hello"

for index in range(len(str1)):
    print("str1[",index,"] = ",str1[index])

Which should give:

str1[ 0 ] =  h
str1[ 1 ] =  e
str1[ 2 ] =  l
str1[ 3 ] =  l
str1[ 4 ] =  o

 

[anorlunda]

You could increase your understanding by studying Python iterators and generators.

https://docs.python.org/3.6/tutorial/classes.html#iterators
https://docs.python.org/3.6/tutorial/classes.html#generators
https://docs.python.org/3.6/tutorial/classes.html#generator-expressions

 

[shivajikobardan]

Is this what you are looking for:

str1="hello"

for index in range(len(str1)):
    print("str1[",index,"] = ",str1[index])

Which should give:

str1[ 0 ] =  h
str1[ 1 ] =  e
str1[ 2 ] =  l
str1[ 3 ] =  l
str1[ 4 ] =  o

exactly. thanks...LOTS OF THANKS. This was in my book but index=0 was done there so I got confused.