Conditions and Loops

if, elif and else choose which lines run; for and while loops repeat them. Indentation shows which lines belong to each block.

Key ideas

if checks a condition; elif is checked only if every condition above it was False; else runs when none was True.
In an if–elif chain, only the FIRST true branch runs, even if later ones are also true.
The lines inside an if or a loop are indented (usually 4 spaces); wrong indentation changes the program.
range(a, b, c) starts at a, goes up in steps of c, and stops BEFORE b; range(n) gives 0 to n − 1.
A while loop repeats as long as its condition is True; something inside must change so that it can stop.
In nested loops the inner loop runs completely on every turn of the outer loop.

Remember

range(5)	0, 1, 2, 3, 4 (five values)
range(1, 10, 3)	1, 4, 7
Sum 1 to n	range(1, n + 1), with s starting at 0
Nested loops	range(3) inside range(4): 12 runs
A grade chain	if marks >= 90: print('A') elif marks >= 75: print('B') else: print('C')
A while loop	n = 20 while n > 5: n = n - 6 print(n) # 2

Watch out

Thinking range(1, 10) includes 10: it stops at 9.
Expecting more than one branch of an if–elif chain to run.
Putting print inside the loop (indented) when it should run once after it.

Worked example

This program should print the first 7 even numbers starting from 2.
for i in range(2, 14, 2):
print(i)
(a) Write the numbers it actually prints. (b) How many numbers is that? (c) What is the smallest stop value for range that prints all 7 numbers?

Working: range stops before its stop value, so it must be more than 14

Answer: (a) 2, 4, 6, 8, 10, 12; (b) 6; (c) 15