Variable Examples in CS Thinking

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

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 named container in a program that stores a value, which can be read, updated, or replaced. Variables have a name (identifier), a value (the data stored), and in many languages a type (what kind of data it holds).

A labeled box you can put things in, take things out, or change what's inside.

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: Variables give meaningful names to data values so programs can store, retrieve, and modify them.

Common stuck point: = means 'assign' not 'equals'. x = x + 1 is valid (not a paradox).

Sense of Study hint: When using variables, choose descriptive names that explain what the data represents (e.g., 'total_score' not 'x'). Initialize variables before using them, and keep track of how each variable's value changes as the program runs.

Worked Examples

Example 1

easy

Trace: SET x = 7. SET y = x + 3. SET x = y * 2. OUTPUT x, y.

Answer

x = 20, y = 10.

First step

Step 1: x = 7.

Full solution

2
Step 2: y = 7 + 3 = 10.
3
Step 3: x = 10 * 2 = 20. Output: x = 20, y = 10.

Variables are named containers for storing values. When we reassign a variable, the old value is replaced. Other variables that used the old value are not affected.

Example 2

medium

Show how to swap the values of two variables a and b using a temporary variable. Trace with a = 5, b = 9.

Practice Problems

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

Example 1

easy

What is the output? SET score = 0. SET score = score + 10. SET score = score + 5. OUTPUT score.

Example 2

medium

Trace this swap algorithm: SET A = 3. SET B = 8. SET temp = A. SET A = B. SET B = temp. What are A and B at the end, and why is temp needed?

Example 3

easy

Trace:

x = 5

;

x = x + 2

. What is

x

Example 4

easy

After

a = 3

;

b = a

;

a = 9

, what is

b

Example 5

easy

What three things does a variable typically have?

Example 6

easy

Trace:

count = 0

;

count = count + 1

;

count = count + 1

. Final count?

Example 7

easy

Which is a clearer variable name for a person's age: 'x' or 'age'?

Example 8

easy

Trace:

s =

'hi';

s = s + '!'

. What is

s

Example 9

easy

Using a variable before assigning it any value typically causes what kind of error?

Example 10

easy

In 'total = price', which side receives the value: total or price?

Example 11

medium

Trace:

a=2

;

b=3

;

a=b

;

b=a

. Final

a

and

b

— does this swap them?

Example 12

medium

Trace:

x=4

;

y=x

;

x=x+1

;

y=y+10

. Final

x

and

y

Example 13

medium

A counter starts at 0 and is incremented inside a loop running 6 times, but is reset to 0 each pass. Final value?

Example 14

medium

Trace:

x = 3

;

y = x * 2

;

x = 10

;

z = x + y

. Final

z

Example 15

medium

In a language where '=' assigns and '==' compares, what does 'x == 5' evaluate to when x is 5?

Example 16

medium

Trace:

list = [10, 20]

;

list[0] = 99

. What is the list now?

Example 17

medium

A program:

n =

'5' (a string);

n = n + 3

. Why might this error or misbehave?

Example 18

medium

Trace:

sum=0

; for

v

[3, -1, 4]

sum = sum + v

. Final sum?

Example 19

medium

Trace:

flag =

true;

flag =

NOT

flag

;

flag =

NOT

flag

. Final flag?

Example 20

challenge

Trace:

a=1

;

b=2

;

c=3

;

a=b

;

b=c

;

c=a

. Final

a,b,c

Example 21

challenge

A loop intends to track a maximum but initializes

best = 0

. For input

[-5,-2,-9]

, what wrong answer results and what is the fix?

Example 22

challenge

Trace:

x=2

; for

i

in range(3):

x = x * x

. Final

x

(range(3) is 3 passes)?

Example 23

easy

Trace:

a = 10

;

a = a - 3

. Final

a

Example 24

easy

Trace:

x = 3

;

y = x

;

x = 99

. Final

y

Example 25

easy

Trace:

n = 4

;

n = n * n

. Final

n

Example 26

easy

Which variable name is clearer for a student's grade: 'g' or 'grade'?

Example 27

easy

Trace:

s =

'cat';

s = s + 's'

. Final

s

Example 28

easy

What is wrong with:

\text{print}(x)

;

x = 5

Example 29

medium

Trace:

a = 5

;

b = a + 1

;

a = b + 1

;

b = a + 1

. Final

a, b

Example 30

medium

Trace swap without a temp using arithmetic:

a=7

;

b=3

;

a=a+b

;

b=a-b

;

a=a-b

. Final

a, b

Example 31

medium

Trace:

count=0

; for

v

[1,2,3,4,5]

: if

v\%2==1

count = count + 1

. Final

count

Example 32

medium

Trace:

total=0

;

items=[10, 20, 30]

; for

i

items

total = total + i

. Final

total

Example 33

medium

Trace:

x = 'A'

;

x = x * 3

(string repeat). Final

x

Example 34

medium

Trace:

a = [1, 2]

;

b = a

;

a.append(3)

. Final

b

Example 35

medium

Trace:

best = -\infty

; for

v

[3, -1, 4, 1]

: if

v > best

best = v

. Final

best

Example 36

medium

Trace:

x = 1

;

y = x

;

x = x + y

;

y = x + y

. Final

x, y

Example 37

medium

\text{if } x = 5

(single

=

) versus

\text{if } x == 5

(double

=

), which one performs comparison?

Example 38

medium

Trace:

sum=0

;

i=1

; while

i \le 5

sum = sum + i

;

i = i + 1

. Final

sum

Example 39

medium

Trace:

flag =

true;

count = 0

; while flag:

count = count + 1

; if

count == 3

flag =

false. Final

count

Example 40

medium

Trace:

a, b = 3, 5

;

a, b = b, a

. Final

a, b

Example 41

hard

Trace:

a = 1

;

b = 2

;

c = 3

;

a = a + b + c

;

b = a - b - c

;

c = a - b - c

. Final

a, b, c

Example 42

hard

In a scope-aware language,

x = 1

is defined globally and a function sets

x = 2

locally. After the function returns, what is the global

x

Example 43

hard

Trace:

n=5

;

f=1

; while

n>1

f = f * n

;

n = n - 1

. Final

f

Example 44

hard

Trace:

a = [1, 2, 3]

;

b = a[:]

(copy);

a.append(4)

. Final

b

Example 45

challenge

Trace:

a = 2

; for

i

in range(4):

a = a * a

. Final

a

? (range(4) = 4 passes)

Example 46

challenge

Trace:

a = 0

;

b = 1

; for

i

in range(6):

a, b = b, a + b

. Final

a

← Back to Variable Practice Mode

Related Concepts

Data Types Assignment