Python Nested Loop explanation-CodePudding

I’m new to python coding and i dont understand why the nested for loop is only returning 0 1 2 for the firs Iteration.

Input:

x = 3
for i in range (x):
  for j in range (x):
    x = 2
    print (i, '',j)

Output:

CodePudding user response：

x is changed after it's passed to range to make range(3). You only see the effects of the change (i.e. range(2)) on the next loop.

CodePudding user response：

Long version

range is not a 'reserved' word; it is the name of a built-in type(class). Note : The syntax highlighter cheats when it highlights range as a reserved word. It does this, because this is generally useful as range is mainly used in association with for. But this does not change what range is. However, this can mislead you

As a result of the above, the correct typing is range(x) not range (x)

range(x) build an objet of type range and initialize it with x.

Short answer

x is interpreted when it is passed to range().

Code to print the range objects:

x = 3
range_i = range(x) 
print(f"i loop x: {x}, range(x):{range_i}")
for i in range_i:
    range_j = range(x) 
    print(f"j loop x: {x}, range(x):{range_j}")
    for j in range_j:
        x = 2
        print (i, ' ',j)

Output

i loop x: 3, range(x):range(0, 3)
j loop x: 3, range(x):range(0, 3)
0   0
0   1
0   2
j loop x: 2, range(x):range(0, 2)
1   0
1   1
j loop x: 2, range(x):range(0, 2)
2   0
2   1

Musing around

Note : what follows is NOT recommended

You can redefine range. (The example below is a very simplified redefinition: it does not take in account the second version of range : range(start, stop[, step]) neither it cares for other range specifications)

def range(n):
    i = 0
    while i <= n: # note the use of '<=' instead of '<'
        yield i
        i  = 1

And now 'range' does not behave as it should. Example:

for i in range(3):
    print(i)

gives:

Yes 0..3 not 0..2 as the 'true' range.