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

Because floating-point numbers can represent fractions, programmers often mistakenly assume that they can represent any simple fraction exactly. In fact, floating-point numbers are subject to range limitations just as integers are. Furthermore, limited-precision binary floating-point numbers cannot represent all decimals precisely, even when the decimals can be represented in a small number of digits.

In addition, because floating-point numbers can represent large values, programmers often mistakenly assume that they can represent all digits of those values. To gain a large dynamic range, floating-point numbers maintain a fixed number of bits of precision and an exponent. Incrementing a large floating-point value might not change that value within the available precision.

As a result, floating-point variables must not be used as loop counters.

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 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 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.

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

      03. Numeric Types and Operations (NUM)