For Loop Examples in CS Thinking

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of For Loop.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in CS Thinking.

Concept Recap

A control structure that repeats a block of code a specific number of times or once for each item in a collection. The loop variable is automatically set to each successive value in the range or collection, eliminating the need to manually update it.

A for loop is like 'do this for each...' — for each student in the class, print their name. For each number from 1 to 10, add it to the total.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: For loops are best when you know how many times to repeat or when iterating over a collection. The loop variable takes each value automatically.

Common stuck point: range(5) gives 0 to 4, not 1 to 5. Off-by-one errors are the most common for-loop bug.

Sense of Study hint: When using a for loop, first identify what you are iterating over—a range of numbers or a collection of items. The loop variable is automatically assigned each value in turn. Remember that range(n) produces 0 to n-1 (not 1 to n), and check your loop bounds carefully.

Worked Examples

Example 1

easy
Trace: FOR i = 0 TO 4: OUTPUT i * i.

Answer

0, 1, 4, 9, 16

First step

1
Step 1: i=0: output 0. i=1: output 1. i=2: output 4.

Full solution

  1. 2
    Step 2: i=3: output 9. i=4: output 16.
  2. 3
    Step 3: Outputs: 0, 1, 4, 9, 16 (perfect squares).
FOR loops are used when the number of iterations is known in advance. The loop variable automatically increments each iteration.

Example 2

medium
Convert this FOR loop to a WHILE loop: FOR i = 1 TO 5: OUTPUT i.

Example 3

medium
Trace: `total=0; for i in range(2,8): total+=i`. Final total?

Example 4

medium
Trace nested: `c=0; for i in range(4): for j in range(3): c+=1`. Final c?

Example 5

medium
Trace: `s=0; for i in range(1,6): for j in range(1,i+1): s+=j`. Final s?

Example 6

medium
Trace: `nums=[3,1,4,1,5]; mx=nums[0]; for x in nums[1:]: if x>mx: mx=x`. Final mx?

Example 7

medium
Trace: `p=1; for i in range(1,6): if i%2==0: p*=i`. Final p?

Example 8

medium
Trace: `out=[]; for x in [1,2,3]: for y in [10,20]: out.append(x+y)`. Final out?

Example 9

medium
Trace: `out=[]; for i in range(4): out.append(i*(i+1))`. Final out?

Example 10

hard
Trace: `out=[]; for i in range(5): for j in range(5): if i+j==4: out.append((i,j))`. Final out?

Example 11

hard
Trace: `s=0; for i in range(1,6): s += i*i`. Final s?

Example 12

hard
Trace: `n=5; out=[]; for i in range(n): row=[]; for j in range(n): row.append(1 if i==j else 0); out.append(row)`. How many 1s in out?

Example 13

challenge
Trace: `out=[]; for i in range(1,5): for j in range(1,5): if i!=j and (i+j)%2==0: out.append((i,j))`. How many tuples?

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium
Write a FOR loop that calculates 5!=5×4×3×2×15! = 5 \times 4 \times 3 \times 2 \times 1.

Example 2

medium
What does this code output? SET scores = [85, 92, 78, 95, 88] SET highest = scores[0] FOR i = 1 TO LENGTH(scores) - 1 IF scores[i] > highest THEN SET highest = scores[i] END IF END FOR OUTPUT highest

Example 3

easy
What values does `for i in range(5)` give i?

Example 4

easy
How many times does `for i in range(5)` iterate?

Example 5

easy
Trace: `total=0; for i in range(4): total+=i`. Final total?

Example 6

easy
What does `range(2, 6)` produce?

Example 7

easy
What does `range(0, 10, 2)` produce?

Example 8

easy
Trace: `out=[]; for c in 'cat': out.append(c)`. Final out?

Example 9

easy
Trace: `p=1; for i in range(1,4): p*=i`. Final p?

Example 10

easy
Trace: `count=0; for x in [4,7,2,9]: if x>5: count+=1`. Final count?

Example 11

medium
Trace: `total=0; for i in range(1,6): total+=i`. Final total?

Example 12

medium
Trace: nested `c=0; for i in range(3): for j in range(2): c+=1`. Final c?

Example 13

medium
Trace: `s=0; for i in range(1,5): for j in range(i): s+=1`. Final s?

Example 14

medium
Trace: `out=[]; for i in range(3): for j in range(3): if i==j: out.append((i,j))`. Final out?

Example 15

medium
Trace: `out=[]; for i in range(4): out.append(i*i)`. Final out?

Example 16

medium
Trace: `for i in range(5,0,-1): print(i)`. What prints?

Example 17

medium
Trace: `out=[]; nums=[1,2,3]; for i in range(len(nums)): out.append(nums[i]*2)`. Final out?

Example 18

medium
Trace: `seen=[]; for x in [1,2,2,3,3,3]: if x not in seen: seen.append(x)`. Final seen?

Example 19

medium
Trace: `total=0; for i in range(10): if i==5: continue; total+=i`. Final total?

Example 20

challenge
Trace: `out=[]; data=[1,2,3,4]; for x in data: if x%2==0: data.remove(x)`. What is data afterward, and why is this buggy?

Example 21

challenge
Trace: `m=[[1,2],[3,4],[5,6]]; total=0; for row in m: for v in row: total+=v`. Final total, and how many additions occur?

Example 22

challenge
Trace: `out=[]; for i in range(1,4): for j in range(i+1,4): out.append((i,j))`. Final out and its length?

Example 23

easy
How many iterations does `for i in range(7)` perform?

Example 24

easy
Trace: `total=0; for i in range(5): total+=2`. Final total?

Example 25

easy
Trace: `s=''; for c in 'abc': s = c + s`. Final s?

Example 26

easy
Trace: `out=[]; for x in [3,1,4]: out.append(x+1)`. Final out?

Example 27

easy
Trace: `count=0; for x in [2,4,6,8]: if x%2==0: count+=1`. Final count?

Example 28

easy
Why might `for i in range(1, 5)` instead of `for i in range(5)` be the better choice?

Example 29

medium
Trace: `out=[]; for i in range(5): if i%2: out.append(i)`. Final out?

Example 30

medium
Trace: `for i in range(3, 12, 3): print(i)`. What prints?

Example 31

medium
Trace: `total=0; for i in range(1,11): if i==7: break; total+=i`. Final total?

Example 32

medium
When is a `while` loop preferable to a `for` loop?

Example 33

hard
What's wrong with: `nums = [1,2,3,4]; for x in nums: nums.append(x)`?

Example 34

hard
What is the Big-O time of two nested for loops, each running n times, with O(1) work inside?

Example 35

challenge
A for loop over a list of n items contains another for loop over the same list. Each iteration does O(1) work. What is the total time complexity in terms of n?
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Related Concepts

IterationWhile LoopArray

Background Knowledge

These ideas may be useful before you work through the harder examples.

iterationwhile loop