Loops

Imagine printing a greeting for 100 students by writing 100 print lines. Loops let you write the work once and repeat it as many times as needed.

for: once per item

A for loop takes each item of a sequence in turn and runs the indented block with it:

range(): counting

range() produces a sequence of numbers on demand. Like slicing, the stop value is not included.

The accumulator pattern

Start with an empty total, then add to it on every pass. This is one of the most common patterns in programming:

enumerate(): the index and the item

while: repeat until a condition is false

Use while when you don't know how many repetitions you need, only when to stop:

break and continue

• break leaves the loop immediately.
• continue skips the rest of this pass and moves on to the next item.

Nested loops

A loop inside a loop runs the inner loop completely for every pass of the outer one:

Recap

• for item in sequence: runs once per item; range() gives you numbers to loop over.
• while condition: repeats until the condition is False. Make sure something changes!
• break exits a loop; continue skips to the next pass.
• Accumulate totals with total += x; use enumerate() when you need positions.