Flowchart guide

Every flowchart is a real program. AlgoLab validates its structure, then executes it one node at a time.

Nodes

Node	Outputs	Behaviour
Start	next	Where execution begins. Exactly one per flowchart.
Input	next	Binds the problem's input values to variables. Created for you from the problem contract.
Assign	next	Stores the value of an expression in a variable (count) or an element (result[i]).
Decision	true / false	Evaluates a condition and follows exactly one branch.
While	body / exit	Repeats the body while the condition is true. The body must lead back to the While node.
Return	—	Ends execution and returns the expression as the answer.

Expressions use a small Python-like language. They are parsed by AlgoLab — never executed as JavaScript or Python.

+ - *

Arithmetic. + also joins strings and arrays.

/

True division (7 / 2 = 3.5)

// %

Floor division and modulo, Python semantics (-7 // 2 = -4, -7 % 3 = 2)

== != < <= > >=

Comparison. == compares arrays element by element.

and or not

Boolean logic, short-circuiting (&&, ||, ! also accepted)

x in s, x not in s

Membership in a string or array

xs[i], grid[r][c]

Indexing; negative indexes count from the end

[1, 2, 3], []

Array literals

true false null

Literals (True / False / None also accepted)

len(x)

Length of an array or string (length(x) also works)

append(xs, v)

New array with v added at the end

slice(x, start, end?)

Part of an array or string, like x[start:end]

array(n, fill?)

New array of n copies of fill (default 0)

min(...), max(...)

Smallest / largest of the arguments, or of one array

abs(n)

Absolute value

str(x), int(x)

Convert to text / integer

lower(s), upper(s)

Change letter case

is_digit(s), is_letter(s)

Character class checks

• Every output port must be connected, and every node must be reachable from Start.
• A variable must be assigned on every path before it is used — the validator tells you which one is missing.
• Arrays are copied on assignment: after b = a, changing b[0] does not change a.
• Conditions must be true or false; numbers are not treated as booleans.
• Runs stop after 200,000 steps or 2 seconds, so an infinite loop shows an error instead of freezing the page.