List Comprehension — Expression, split(), and if Filters

What you will learn

• The list syntax: [expression for name in iterable if cond]
• Expression as type casting or math
• The split() pattern for space-separated values
• Filtering with if (positives only)
• How this replaces a for loop plus append

This continues from the Comprehensions overview.

The big idea

A list comprehension builds a new list from an iterable in one expression.

listobj = [expression for varname in iterable_object if test_cond]

From the teaching notes: expression is either type casting or a mathematical expression.

Traditional loop vs comprehension

lst = [4, 3, 7, -2, 6, 3]
newlst = []
for val in lst:
    newlst.append(val * 2)
print(newlst)

Same result in one line:

lst = [4, 3, 7, -2, 6, 3]
newlst = [val * 2 for val in lst]
print(newlst)

Output:

[8, 6, 14, -4, 12, 6]

Reading tokens — split()

Classroom programs often print a prompt and call input().split().

print("Enter List of values separated by space:")
lst = [float(val) for val in input().split()]

In the MahiDev playground, input() would block. Use a string with the same comprehension:

raw = "10 2 22 50 10 4 55 -3 0 22"
lst = [float(val) for val in raw.split()]
print("content of lst", lst)

split() with no argument splits on whitespace. Each token is cast with float(val).

You can also keep strings:

raw = "10 2 22"
lst = [str(val) for val in raw.split()]
print(lst)
print(type(lst))

Filter with if — positives only

Keep values greater than 0. Zeros and negatives are dropped.

tokens = "10 -3 0 22 5 -8 7"
pslist = [int(value) for value in tokens.split() if int(value) > 0]
print("List of +ve Values=", pslist)

Token	int(value) > 0	Result
10	True	Keep
-3	False	Drop
0	False	Drop
22	True	Keep

This is the same idea as filter(), written as a list comprehension.

Optional if

Without if, every item is kept (and transformed):

[val * 2 for val in [4, 3, 7]]

With if, only matching items enter the new list:

[val * 2 for val in [4, 3, 7] if val > 0]

Common mistakes

Calling input() in the playground — it cannot wait for the keyboard. Use a string + split().

Filter in the expression — if goes after the for clause, not inside the math.

Forgetting list brackets — {x for x in nums} is a set, not a list.

Interview cheat sheet

Question	Answer
List brackets?	[ ]
Cast tokens from text?	[float(v) for v in text.split()]
Double each value?	[v * 2 for v in lst]
Positives only?	if int(v) > 0
Loop equivalent?	empty list + append

Practice in the playground

1. Change * 2 to * 3 and confirm 4 becomes 12.
2. Change the positive filter to >= 0 — 0 should appear.
3. Use int(val) instead of float(val) on raw and compare types.

Next: Set comprehension.