How should I practice String Interpolation with a Missing-Value Mode?

Start by reading the full statement, solve the visible examples, then compare the pattern against related MongoDB phone screen problems in FastPrep.

Is String Interpolation with a Missing-Value Mode associated with MongoDB?

FastPrep tags this as a MongoDB phone screen coding practice problem when the problem catalog has that company and stage context.

String Interpolation with a Missing-Value Mode

MongoDB● MediumFULLTIMEPHONE SCREEN

Problem statement

Source note: A MongoDB phone screen for the Atlas Growth team asked for a library function that behaves like JavaScript string interpolation. The discussion centered on edge-case behavior, and the error-handling policy was left for the candidate to choose.

The report does not give the placeholder syntax, how values are passed, which edge cases came up, or the policy the candidate chose. Practice choices: ${name} placeholders with identifier names; values in parallel name and value lists; backslash escapes for $ and backslash; values are never scanned again; a mode argument that decides what a missing value does (error, keep, or empty); malformed placeholders are always an error; and the exact error strings, with only the first problem from the left reported.

The judged task matches the report at about 70%.

Write a small string-interpolation function, similar to JavaScript template strings. A placeholder is ${name}, where name starts with a letter or _ and continues with letters, digits, or _. Names are case-sensitive. The value of names[i] is values[i].

Scan text from left to right and build the result:

A backslash followed by $ or by another backslash outputs that second character, and both characters are used up. Any other backslash is copied as is.
${ starts a placeholder that ends at the next }. If there is no later }, or the characters between the braces are not a valid name, the placeholder is malformed.
A placeholder whose name has a value is replaced by that value. The value is inserted as is and is never scanned again.
A placeholder whose name has no value depends on mode: "error" stops with an error, "keep" keeps the placeholder text unchanged, and "empty" replaces it with nothing.
Every other character, including a $ that is not followed by {, is copied as is.

Return the finished string. If the scan stops with an error, return only the error for the first problem found: "ERROR: malformed placeholder at i", where i is the 0-based index of the placeholder's $ in text, or "ERROR: missing value for name" with the placeholder's name. A malformed placeholder is an error in every mode.

Function

interpolate(text: String, names: String[], values: String[], mode: String) → String

Examples

Example 1

text = "Hello, ${name}! You have ${count} new messages."names = ["name", "count"]values = ["Ana", "3"]mode = "error"return = "Hello, Ana! You have 3 new messages."

Both placeholders have values, so ${name} becomes Ana and ${count} becomes 3. The rest of the text is copied as is.

Example 2

text = "Price \\${cost} is ${cost} for ${user}"names = ["cost"]values = ["$5"]mode = "keep"return = "Price ${cost} is $5 for ${user}"

The backslash makes \$ a plain $, so the first ${cost} is ordinary text. The second ${cost} becomes $5; that value is not scanned again. user has no value, and in keep mode ${user} stays as it was.

Example 3

text = "${greeting}, ${who}"names = ["greeting"]values = ["Hi"]mode = "error"return = "ERROR: missing value for who"

${greeting} has a value, but ${who} does not. In error mode the result is the error string for the first placeholder without a value.

Example 4

text = "a ${x} b ${y} c ${"names = ["x"]values = ["1"]mode = "empty"return = "ERROR: malformed placeholder at 16"

In empty mode ${y} would be removed, but the ${ that starts at index 16 is never closed. A malformed placeholder is an error in every mode.

Constraints

0 <= text.length <= 2000
0 <= names.length <= 100
values.length == names.length
1 <= names[i].length <= 30
0 <= values[i].length <= 20
text and every values[i] contain only printable ASCII characters (codes 32 to 126).
Every names[i] is a valid name, and the names are distinct.
mode is "error", "keep", or "empty".