LangChain Expression Language (LCEL) basics - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is LangChain Expression Language (LCEL)?

LCEL is a simple language designed to write expressions that LangChain can evaluate to manipulate data or control flow in chains.

beginner

How do you write a variable reference in LCEL?

You write a variable name directly, like userName, to access its value in the expression.

beginner

Which operators are commonly used in LCEL for arithmetic?

LCEL supports + (add), - (subtract), * (multiply), and / (divide) for arithmetic operations.

intermediate

How do you write a conditional expression in LCEL?

Use the ternary style: condition ? valueIfTrue : valueIfFalse to choose between two values.

intermediate

What is the purpose of functions in LCEL?

Functions let you perform tasks like string manipulation or data transformation inside expressions, e.g., <code>toUpperCase(name)</code>.

In LCEL, how do you access the value of a variable named age?

A Write <code>{age}</code>

B Write <code>$age</code>

C Just write <code>age</code>

D Write <code>var age</code>

In LCEL, variables are accessed by their name directly without extra symbols.

Which operator would you use in LCEL to multiply two numbers?

A /

B -

C +

D *

The asterisk (*) is used for multiplication in LCEL.

What does this LCEL expression do? score > 50 ? 'Pass' : 'Fail'

A Checks if score is greater than 50 and returns 'Pass' or 'Fail'

B Multiplies score by 50

C Always returns 'Fail'

D Adds 50 to score

This is a conditional expression returning 'Pass' if score is over 50, else 'Fail'.

How do you convert a string variable name to uppercase in LCEL?

A toUpperCase(name)

B uppercase(name)

C name.toUpperCase()

D upper(name)

LCEL uses toUpperCase() function to convert strings to uppercase.

Which of these is NOT a valid LCEL arithmetic operator?

A +

B %

C -

D /

LCEL does not support the modulus (%) operator in its basic arithmetic.

Explain how to write and use a conditional expression in LangChain Expression Language (LCEL).

Think about how you choose between two values based on a condition.

Describe how variables and functions work in LCEL and why they are useful.

Consider how you might change or check data inside an expression.

Practice

1. What is the main purpose of LangChain Expression Language (LCEL)?

easy

A. To create full applications without coding

B. To write small expressions that control LangChain chains

C. To replace Python in LangChain completely

D. To design user interfaces for LangChain

Solution

1. Step 1: Understand LCEL's role

   LCEL is designed to write expressions that help control how LangChain chains behave.

2. Step 2: Compare options

   Only To write small expressions that control LangChain chains correctly describes LCEL's purpose; others describe unrelated tasks.

3. Final Answer:

   To write small expressions that control LangChain chains -> Option B

4. Quick Check:

   LCEL purpose = control chains with expressions [OK]

Hint: LCEL is for small expressions controlling chains [OK]

Common Mistakes:

• Thinking LCEL replaces full programming
• Confusing LCEL with UI design tools
• Assuming LCEL builds entire apps

2. Which of the following is the correct syntax to use a variable named input in an LCEL expression?

easy

A. {{input}}

B. {input}

C. $input

D. input

Solution

1. Step 1: Recall LCEL variable usage

   In LCEL, variables are used directly by their name without extra symbols.

2. Step 2: Evaluate options

   Only input uses the variable name plainly, which is correct syntax in LCEL.

3. Final Answer:

   input -> Option D

4. Quick Check:

   Variable usage = plain name [OK]

Hint: Use variable names directly without braces or symbols [OK]

Common Mistakes:

• Adding braces or dollar signs around variables
• Using double curly braces like in templates
• Confusing LCEL with other templating syntaxes

3. Given the LCEL expression input + ' world' where input is 'Hello', what is the output?

medium

A. 'input world'

B. 'Hello+ world'

C. 'Hello world'

D. Error: invalid operation

Solution

1. Step 1: Understand the expression

   The expression adds the string ' world' to the variable input which holds 'Hello'.

2. Step 2: Perform string concatenation

   Concatenating 'Hello' + ' world' results in 'Hello world'.

3. Final Answer:

   'Hello world' -> Option C

4. Quick Check:

   String concat = 'Hello world' [OK]

Hint: Adding strings concatenates them in LCEL [OK]

Common Mistakes:

• Treating + as a literal character
• Expecting variable name output instead of value
• Assuming syntax error on string addition

4. What is wrong with this LCEL expression: if input == 'yes' then 'ok' else 'no'?

medium

A. LCEL does not support if-then-else syntax like this

B. The variable name input is invalid

C. The strings should use double quotes instead of single quotes

D. The expression is correct and will work

Solution

1. Step 1: Check LCEL conditional syntax

   LCEL uses ternary-like syntax: condition ? true_value : false_value, not if-then-else.

2. Step 2: Analyze the given expression

   The expression uses if-then-else which is not valid in LCEL, causing a syntax error.

3. Final Answer:

   LCEL does not support if-then-else syntax like this -> Option A

4. Quick Check:

   Conditional syntax = ternary only [OK]

Hint: Use ternary ?: for conditions, not if-then-else [OK]

Common Mistakes:

• Using if-then-else like in other languages
• Assuming single quotes cause errors
• Thinking variable names are restricted

5. How would you write an LCEL expression to return the length of a variable text only if it is not empty, otherwise return 0?

hard

A. text != '' ? length(text) : 0

B. length(text) if text else 0

C. text ? length(text) : 0

D. if text != '' then length(text) else 0

Solution

1. Step 1: Understand LCEL conditional syntax

   LCEL uses ternary syntax: condition ? true_value : false_value.

2. Step 2: Check condition for non-empty string

   To check if text is not empty, use text != ''.

3. Step 3: Combine condition and function

   Use text != '' ? length(text) : 0 to return length if not empty, else 0.

4. Final Answer:

   text != '' ? length(text) : 0 -> Option A

5. Quick Check:

   Conditional length check = text != '' ? length(text) : 0 [OK]

Hint: Use ternary with condition text != '' for non-empty check [OK]

Common Mistakes:

• Using if-then-else syntax
• Assuming text alone is false if empty
• Using Python-style if expressions