NUM09-J. Do not use floating-point variables as loop counters

Floating-point variables must not be used as loop counters. Limited-precision IEEE 754 floating-point types cannot represent

• All simple fractions exactly.
• All decimals precisely, even when the decimals can be represented in a small number of digits.
• All digits of large values, meaning that incrementing a large floating-point value might not change that value within the available precision.

For the purpose of this rule, a loop counter is an induction variable that is used as an operand of a comparison expression that is used as the controlling expression of a do, while or for loop. An induction variable is a variable that gets increased or decreased by a fixed amount on every iteration of a loop [Aho 1986]. Furthermore, the change to the variable must occur directly in the loop body (rather than inside a function executed within the loop.)

This rule is a subset of NUM04-J. Do not use floating-point numbers if precise computation is required.

Noncompliant Code Example

This noncompliant code example uses a floating-point variable as a loop counter. The decimal number 0.1 cannot be precisely represented as a float or even as a double.

for (float x = 0.1f; x <= 1.0f; x += 0.1f) {
  System.out.println(x);
}

Because 0.1f is rounded to the nearest value that can be represented in the value set of the float type, the actual quantity added to x on each iteration is somewhat larger than 0.1. Consequently, the loop executes only nine times and typically fails to produce the expected output.

Compliant Solution

This compliant solution uses an integer loop counter from which the desired floating-point value is derived:

for (int count = 1; count <= 10; count += 1) {
  float x = count/10.0f;
  System.out.println(x);
}

Noncompliant Code Example

This noncompliant code example uses a floating-point loop counter that is incremented by an amount that is typically too small to change its value given the precision:

for (float x = 100000001.0f; x <= 100000010.0f; x += 1.0f) {
  /* ... */
}

The code loops forever on execution.

Compliant Solution

This compliant solution uses an integer loop counter from which the floating-point value is derived. Additionally, it uses a double to ensure that the available precision suffices to represent the desired values. The solution also runs in strict floating-point (FP-strict) mode to guarantee portability of its results (see NUM53-J. Use the strictfp modifier for floating-point calculation consistency across platforms for more information).

for (int count = 1; count <= 10; count += 1) {
  double x = 100000000.0 + count;
  /* ... */
}

Risk Assessment

Using floating-point loop counters can lead to unexpected behavior.

Automated Detection

Automated detection of floating-point loop counters is straightforward.

Related Guidelines

Bibliography

[Aho 1986]
[Bloch 2005]	Puzzle 34, "Down for the Count"
[JLS 2015]	§4.2.3, "Floating-Point Types, Formats, and Values"
[Seacord 2015]	NUM09-J. Do not use floating-point variables as loop counters LiveLesson