Python Control Flow: if‑elif‑else, while, and for Loops

This article introduces Python's fundamental control‑flow statements: if‑elif‑else , while , and for loops, each accompanied by concise explanations and executable code snippets.

1. if‑elif‑else statement The first condition that evaluates to True executes its block; if none are true, the else block runs. Python uses indentation to define blocks.

>> x = 5
>>> if x < 5:
...     y = -1
...     z = 5
... elif x > 5:
...     y = 1
...     z = 11
... else:
...     y = 0
...     z = 10
...
>>> print(x, y, z)
5 0 10

2. while loop The loop continues as long as its condition remains True . Inside the loop you can nest other control structures.

>> u, v, x, y = 0, 0, 100, 30
>>> while x > y:
...     u = u + y
...     x = x - y
...     if x < y + 20:
...         v = v + x
...         x = 0
...     else:
...         v = v + y + 2
...         x = x - y - 2
...
>>> print(u, v)
60 40

3. for loop A for loop iterates over any iterable (e.g., lists, tuples). The example demonstrates type checking with isinstance , using continue , and finding the first integer divisible by seven.

>> item_list = [3, "string1", 23, 14.0, "string2", 49, 64, 70]
>>> for x in item_list:
...     if not isinstance(x, int):
...         continue
...     if not x % 7:
...         print("found an integer divisible by seven: %d" % x)
...         break
...
found an integer divisible by seven: 49

Additional notes: the not operator negates a boolean value; not False is True . The article also lists common string‑formatting specifiers such as %s , %d , %f , and their meanings.

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