MISRA C++:2008 Rule 6-5-1

A for loop shall contain a single loop-counter which shall not have floating type

Description

Rule Definition

A for loop shall contain a single loop-counter which shall not have floating type.¹

Rationale

A for loop without a loop-counter can be replaced with a while loop. Use a while loop instead in these situations.

Floating-point types are rounded to fit into a finite representation. This causes rounding errors to be introduced with floating-point calculations which can cause unexpected results when performing comparisons.

Polyspace Implementation

This rule checker flags these situations:

• The for loop index has a floating point type.

• More than one loop counter is incremented in the for loop increment statement.

  For instance:

  for(i=0, j=0; i<10 && j < 10;i++, j++) {}

• A loop counter is not incremented in the for loop increment statement.

  For instance:

  for(i=0; i<10;) {}

  Even if you increment the loop counter in the loop body, Polyspace^® still raises a violation. According to the MISRA™ C++ specifications, a loop counter is one that is initialized in or prior to the loop expression, acts as an operand to a relational operator in the loop expression and is modified in the loop expression. If the increment statement in the loop expression is missing, the checker cannot find the loop counter modification and considers as if a loop counter is not present.

Troubleshooting

If you expect a rule violation but Polyspace does not report it, see Diagnose Why Coding Standard Violations Do Not Appear as Expected.

Examples

Only Use a Single Loop Counter

Because both loop counters x and i are incremented, the following example is noncompliant.

#include <iostream>

int main()
{
	int x = 0;
	for (int i = 0; i >= x; i = x++)   // Non-compliant
	{
		//...
	}
}

Avoid Using Floating Point Numbers as Loop Counters

In this example, because floating-point calculations can contain rounding errors, the for loop will only execute nine times. This is due to the value of x being slightly larger than 1.0f on the tenth loop.

#include <iostream>

int main()
{
	for (float x = 0.1f; x <= 1.0f; x += 0.1f) {   //Noncompliant
		std::cout << x << std::endl;
	}
}

Check Information

Group: Statements

Category: Required

Version History

Introduced in R2013b