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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

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• can be represented in a small number of

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• digits.

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• All digits of large values,

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• 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. Consequently, floating-point variables should 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 is a repeating fraction in binary and cannot be exactly precisely represented as a float or even as a binary floating-point number double.

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

This loop executes only nine times. (If the value of x is printed inside the loop, the following values appear: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.70000005, 0.8000001, 0.9000001.)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) {
  /* ... */
}

This produces an infinite loopThe 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 is used instead of a float to gain enough precision. 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

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TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Other Languages

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Automated detection of floating-point loop counters is straightforward.

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Tool	Version	Checker	Description
Parasoft Jtest	Include Page Parasoft_V Parasoft_V	CERT.NUM09.FPLI

Do not use floating point variables as loop

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indices
PVS-Studio	Include Page PVS-Studio_V PVS-Studio_V	V6108

Related Guidelines

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Bibliography

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Bibliography

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and Values"
[Seacord 2015]	Image Added NUM09-J. Do not use floating-point variables as loop counters LiveLesson

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Image Added Image Added Image Added Values|http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.2.3] \[[Bloch 2005|AA. Bibliography#Bloch 05]\] Puzzle 34: Down for the CountFLP06-J. Check floating point inputs for exceptional values      07. Floating Point (FLP)      FLP08-J. Avoid using floating point literals with the BigDecimal constructor