How does the Python s range function work

Question

If I write  for i in range 5         print i   Then it gives 0  1  2  3  4 Does that mean Python assigned 0  1  2  3  4 to i at the same time  However if I wrote   for i in range 5         a i 1   Then I call a  it only gives 5 But if I add   print a   it gives 1  2  3  4  5 So my question is what is the difference here  Is i a string or a list or something else     Or maybe can anyone help me to sort out   for l in range 5        vs fs rs are all m n matrixs got initial values in i e vs 0  fs 0  rs 0  are known      want use this foor loop to update them     vs l 1  vs l  fs l      fs l 1    rs l -re l       rs l 1  rs l  vs l   then this code gives vs fs rs   If I run this kind of code  then I will get the answer only when l 5 How can I make them start looping     i e l 0 got values for vs 1  fs 1  rs 1   then l 1 got values for vs 2  rs 2  fs 2       and so on  But python gives different arrays of fs vs rs  correspond to different value of l    How can I make them one piece

User · Answer

range x  returns a list of numbers from 0 to x - 1    gt  gt  gt  range 1   0   gt  gt  gt  range 2   0  1   gt  gt  gt  range 3   0  1  2   gt  gt  gt  range 4   0  1  2  3    for i in range x   executes the body  which is print i in your first example  once for each element in the list returned by range    i is used inside the body to refer to the    current    item of the list  In that case  i refers to an integer  but it could be of any type  depending on the objet on which you loop

User · Answer

for i in range 5     is the same as  for i in  0 1 2 3 4

User · Answer

It is looping  probably the problem is in the part of the print     If you can t find the logic where the system prints  just add the folling where you want the content out   for i in range len vs        print vs i      print fs i      print rs i

User · Answer

When I m teaching someone programming  just about any language  I introduce for loops with terminology similar to this code example   for eachItem in someList      doSomething eachItem        which  conveniently enough  is syntactically valid Python code   The Python range   function simply returns or generates a list of integers from some lower bound  zero  by default  up to  but not including  some upper bound  possibly in increments  steps  of some other number  one  by default    So range 5  returns  or possibly generates  a sequence  0  1  2  3  4  up to but not including the upper bound    A call to range 2 10  would return  2  3  4  5  6  7  8  9  A call to range 2 12 3  would return  2  5  8  11  Notice that I said  a couple times  that Python s range   function returns or generates a sequence   This is a relatively advanced distinction which usually won t be an issue for a novice   In older versions of Python range   built a list  allocated memory for it and populated with with values  and returned a reference to that list   This could be inefficient for large ranges which might consume quite a bit of memory and for some situations where you might want to iterate over some potentially large range of numbers but were likely to  break  out of the loop early  after finding some particular item in which you were interested  for example    Python supports more efficient ways of implementing the same semantics  of doing the same thing  through a programming construct called a generator   Instead of allocating and populating the entire list and return it as a static data structure  Python can instantiate an object with the requisite information  upper and lower bounds and step increment value      and return a reference to that   The  code  object then keeps track of which number it returned most recently and computes the new values until it hits the upper bound  and which point it signals the end of the sequence to the caller using an exception called  StopIteration     This technique  computing values dynamically rather than all at once  up-front  is referred to as  lazy evaluation    Other constructs in the language  such as those underlying the for loop  can then work with that object  iterate through it  as though it were a list   For most cases you don t have to know whether your version of Python is using the old implementation of range   or the newer one based on generators   You can just use it and be happy   If you re working with ranges of millions of items  or creating thousands of different ranges of thousands each  then you might notice a performance penalty for using range   on an old version of Python   In such cases you could re-think your design and use while loops  or create objects which implement the  lazy evaluation  semantics of a generator  or use the xrange   version of range   if your version of Python includes it  or the range   function from a version of Python that uses the generators implicitly   Concepts such as generators  and more general forms of lazy evaluation  permeate Python programming as you go beyond the basics   They are usually things you don t have to know for simple programming tasks but which become significant as you try to work with larger data sets or within tighter constraints  time performance or memory bounds  for example     Update  for Python3  the currently maintained versions of Python  the range   function always returns the dynamic   lazy evaluation  iterator   the older versions of Python  2 x  which returned a statically allocated list of integers are now officially obsolete  after years of having been deprecated

User · Answer

A  for loop  in most  if not all  programming languages is a mechanism to run a piece of code more than once   This code   for i in range 5       print i   can be thought of working like this   i   0 print i i   1 print i i   2 print i i   3 print i i   4 print i   So you see  what happens is not that i gets the value 0  1  2  3  4 at the same time  but rather sequentially   I assume that when you say  call a  it gives only 5   you mean like this   for i in range 5       a i 1 print a   this will print the last value that a was given  Every time the loop iterates  the statement a i 1 will overwrite the last value a had with the new value   Code basically runs sequentially  from top to bottom  and a for loop is a way to make the code go back and something again  with a different value for one of the variables   I hope this answered your question

User · Answer

The range function wil give you a list of numbers  while the for loop will iterate through the list and execute the given code for each of its items   for i in range 5         print i   This simply executes print i five times  for i ranging from 0 to 4   for i in range 5         a i 1   This will execute a i 1 five times  Since you are overwriting the value of a on each iteration  at the end you will only get the value for the last iteration  that is 4 1   Useful links  http   www network-theory co uk docs pytut rangeFunction html http   www ibiblio org swaroopch byteofpython read for-loop html

[python] How does the Python's range function work?

Examples related to python

Examples related to for-loop

Examples related to range