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Equality Operators Should Not be Used in "for" Loop Termination Conditions

High
correctnessreliabilitysafety

What is it?​

This practice addresses the use of equality operators (== and !=) in the termination conditions of for loops, which can lead to infinite loops if not used carefully.

Why apply it?​

An incorrect termination condition using an equality operator can lead to infinite loops, making your program less reliable and potentially causing it to hang indefinitely. Using broader relational operators like <, <=, >, or >= generally offers a safer approach.

How to fix it?​

Replace equality operators in loop termination conditions with appropriate relational operators that form a more robust termination criterion, thus preventing unintentional infinite loops.

Examples​

Example 1:​

Negative​

The negative example incorrectly uses the inequality operator, which causes the loop to potentially never terminate because the increment step skips the condition.

for (int i = 1; i != 10; i += 2) {  // Noncompliant - infinite loop
  System.out.println("i = " + i);
}
System.out.println("This line will never be reached due to infinite loop.");

Example 2:​

Negative​

In this negative example, the use of != can be problematic if the loop's incremental logic mistakenly skips the condition, especially with floating-point numbers.

for (int i = 0; i != 8; i += 2) {  // Noncompliant - might skip
  System.out.println("I reached: " + i);
}
System.out.println("This line might be part of an infinite loop.");

Example 3:​

Positive​

The positive example uses a relational operator to ensure the loop terminates correctly when the index surpasses a defined limit.

for (int i = 1; i <= 10; i += 2) {  // Compliant
  System.out.println("i = " + i);
}
System.out.println("Loop terminated correctly.");

Negative​

This negative example uses an equality operator with decrement logic, which risks non-termination due to overshooting.

for (int j = 10; j != 0; j--) {  // Noncompliant - risks overshooting
  System.out.println("j = " + j);
}
System.out.println("May never reach this statement due to infinite loop.");

Example 4:​

Positive​

Here, the positive example employs a broader relational operator and functional logic that safely terminates the loop.

int array[] = {1, 2, 3, 4, 5, 6, 7, 8};
for (int i = 0; i < array.length; i++) {  // Compliant
  if (array[i] % 2 == 0) {
    System.out.println("Even number: " + array[i]);
  }
}
System.out.println("Processed all elements of the array.");

Example 5:​

Positive​

A positive example where the loop correctly uses a decrementing logic with a relational operator, ensuring safe termination.

for (int j = 10; j >= 1; j--) {  // Compliant
  System.out.println("j = " + j);
}
System.out.println("Countdown complete.");