String Concat In Loop

ID

swift.string_concat_in_loop

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Efficiency

Language

Swift

Tags

efficiency, performance

Description

Reports a += whose right-hand side contains a string literal (or interpolation literal) inside a loop body. Each iteration allocates a new String; on long inputs the cumulative cost is quadratic.

for x in xs {
    s += "\(x), "                              // FLAW
}

let s = xs.map { "\($0)" }.joined(separator: ", ")   // OK

Rationale

Swift String is a value type with copy-on-write storage. Each s += literal reallocates s’s storage to fit the appended bytes; on `n iterations the total work is O(n²) in characters. Built-in combinators (joined(separator:), reduce(into:)) write to a single buffer and run in O(n).

Remediation

Build a collection first and join once, or write into a single var s allocated to the expected capacity:

let s = xs.map { "\($0)" }.joined(separator: ", ")

// or, when joined doesn't fit the shape:
var s = ""
s.reserveCapacity(xs.count * 4)
for x in xs {
    s.append("\(x), ")
}

The rule is conservative: it requires the RHS to contain a string literal, so it doesn’t flag numeric += or array appends.

References