Relational Operators: Testing Relationships Between Values

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Relational Operators: Testing Relationships Between Values

Programming in C · Chapter 4: Conditional Logic, Decision Structures, and Practical ApplicationsSection 4.2 · lesson 20 of 787 min · free with a free account

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In this lesson

Compare values with C's six relational operators and avoid classic traps.

  • Use C's six relational operators and explain why each returns 1 or 0
  • Predict comparisons that mix arithmetic or character codes
  • Compare doubles with fabs and a tolerance instead of ==
  • Recognize the = versus == slip and chained comparison bugs

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Transcript

Everything said in the video, section by section (964 words). Read along, or search it.

Opening Question

Is 250 at Least 300

A prize machine at the Starlight Arcade has a sign: need three hundred points to win. Maya has two hundred fifty. Somewhere inside, a program asks one tiny question. Is two fifty at least three hundred? The answer isn't maybe, and it isn't a number of points. It's a flat yes or no. This lesson is about those questions, and about one typing slip that has fooled programmers for decades.

Six Operators

Every Comparison Gives 1 or 0

A relational operator compares two values and says how they relate. C has six of them. Less than, greater than, less than or equal, and greater than or equal. Then two equals signs, for is equal to, and an exclamation mark with an equals sign, for is not equal to. The two-character operators have no space inside, and the equals sign always comes second. Here they are in a program, with Maya's score of 250 and the target of 300. Each result is printed with %d, which prints a whole number. That's exactly what a comparison gives: 1 for true, 0 for false. Build and run it. Less than gives one, and greater than gives zero. Less or equal, one. Greater or equal, zero. Equal, zero, and not equal, one. Two patterns to notice. Equal and not equal are always opposites. And for any two numbers, exactly one of less, equal, and greater is true.

Comparisons as Numbers

Storing and Adding Results

Many languages have a special true-or-false value. Classic C doesn't. A comparison produces a plain int, so you can store it like any other number. Enough is tickets at least fifty, so it's one. Perfect is tickets equal to a hundred, so it's zero. And adding them gives one. In tickets plus ten greater than seventy, C does the arithmetic first, getting seventy-four, and compares second. Relational operators rank below plus and minus. And letters compare by their codes. Capital A is sixty-five, and capital B is sixty-six. But lowercase a is ninety-seven, so it is not less than capital B. A tip: name a stored comparison like a yes-or-no question, such as enough, is empty, or has ticket. Then the line reads almost like English.

Quick Check

Seven Against Seven and Lowercase b

Quick check, with three new comparisons in one line. Seven against seven, twice, and then a lowercase b against a capital B. What does it print? Pause and predict. Build and run it. It prints zero, one, one. Seven is not less than seven, but it is less than or equal to seven. And lowercase b has a bigger code than any capital letter, so b greater than capital B is one.

Comparing Decimals

Using fabs With a Tolerance

Whole numbers are exact, so double equals is safe for them. Decimals are not. A double stores numbers in binary. A value as simple as zero point one has no exact binary form, just as one third has no exact decimal form. So add zero point one and zero point two, and compare the sum with zero point three. Printing seventeen decimals shows the tiny errors. The sum lands a hair above, and the stored zero point three a hair below. So double equals says zero. The fix asks how far apart they are. The fabs function, from math dot h, gives the distance without a minus sign. If that distance is under a tiny tolerance, they're close enough, and the test says one. One millionth is plenty for money, but fit the tolerance to your numbers.

Classic Slip

One Equals Sign in an if

Now the famous slip. One equals sign changes a variable. Two equals signs ask about it. They're one keystroke apart. Sam, the arcade's tester, wanted to ask whether the machine is out of tokens. But he typed only one equals sign. The if statement comes fully in lesson 4.4. For now, read it as: run the next block only when the test is true, meaning not zero. Compile with warnings on, and clang speaks up: using the result of an assignment as a condition without parentheses. Its note says exactly what to do: use double equals, to turn this into an equality comparison. GCC gives a similar warning, in different words. Run it anyway, and it says tokens left, zero, though the machine held ten. The single equals stored zero, and that value, zero, counts as false. Add the second equals sign and rebuild. Now it asks the question, and tokens stays ten. Treat that warning as a bug until proven otherwise.

Math Habit Trap

Chained Comparisons Mislead

One more trap, this time from math class. To check a score is between zero and a hundred, you might chain two comparisons. C reads it from the left. Zero less than or equal to one fifty gives one. And one is always less than or equal to a hundred. So the test answers one, even for a score of one hundred fifty. Clang warns that comparisons like this don't have their mathematical meaning. Asking between takes two separate questions, joined with the AND operator. That's lesson four point three.

Quick Check

Zero Lives Becomes Five

Quick check. In this new program, a player starts with zero lives, and the test has a single equals sign with a five. Which message appears? Pause and predict. Run it. It prints play on, lives five. The single equals stores five in lives, and the value of that assignment is five. Five isn't zero, so it counts as true, and the first block runs. Clang warns here too, exactly as before. So a slipped equals sign can go either way. With zero, the block never runs. With anything else, it always does.

Key Takeaways

Six Operators, Ints and Tolerance

To sum up. C has six relational operators, and each one gives an int: one for true, zero for false. Arithmetic happens before comparison, and letters compare by their codes. Compare doubles with a tolerance, using fabs, never with double equals. And one equals sign changes, two equals signs ask. Compile with warnings on, and take that warning seriously.

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FAQ

Questions students ask us

About Relational Operators: Testing Relationships Between Values

What are the relational operators in C?
C has six relational operators: <, >, <=, >=, == and !=. Each compares two values and gives an int, 1 when true and 0 when false. The two-character operators have no space inside, with the equals sign second. For any two integers, exactly one of less than, equal to and greater than is true, and == and != give opposite results.
What is the difference between = and == in C?
A single = assigns a value, while == compares two values and gives 1 or 0. Writing if (tokens = 0) by mistake stores 0 in tokens, and because the value of that assignment is 0, the condition is false. With if (lives = 5), the value is 5, which counts as true, so the block runs every time. Clang and GCC warn about an assignment used as a condition.
How do you compare two doubles for equality in C?
Compare two doubles by checking whether they are close enough rather than using ==. Use fabs from math.h to get the distance without a sign, as in fabs(a - b) < 0.000001. Because values such as 0.1 are stored as binary approximations, 0.1 + 0.2 == 0.3 gives 0. Choose the tolerance to suit the size and precision of the numbers being compared.
Why does 0 <= score <= 100 not check a range in C?
C evaluates 0 <= score <= 100 from left to right as (0 <= score) <= 100. The first comparison gives 0 or 1, and both are less than or equal to 100, so the whole test is true for any score, including 150. Clang warns that such comparisons lack their mathematical meaning. A correct range test uses two comparisons joined with AND: score >= 0 && score <= 100.

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