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?

Answer 1

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.