Simple and Two-Way Decisions: if and if-else, Explained

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Simple and Two-Way Decisions: if and if-else

Programming in C · Chapter 4: Conditional Logic, Decision Structures, and Practical ApplicationsSection 4.4 · lesson 22 of 785 min · free with a free account

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In this lesson

Making a C program choose whether to run or skip code.

  • Write an if statement that runs a block only when its test is non-zero
  • Use if-else so exactly one of two blocks runs
  • Explain why braces should surround every if block
  • Identify the bug caused by a semicolon after an if test

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Transcript

Everything said in the video, section by section (670 words). Read along, or search it.

Opening Idea

Programs That Respond to What Happens

So far, every program you've written ran its lines from top to bottom, no matter what. Real machines don't. The arcade's token slot starts the game only if enough tokens went in. Otherwise, it flashes: insert more. That fork in the road is called a decision. With it, a program stops being a recording, and starts responding to what happens.

The if Statement

A Gate With a Test

An if statement is a gate with a test on it. The test goes in parentheses. If the test is true, meaning not zero, the program runs the block: the lines inside the curly braces. If it's false, the block is skipped. Either way, the program carries on after the closing brace. Here's a prize sign as a program. Win fifty tickets or more, and you earn the plush dragon. Sam, the arcade's tester, types in the tickets he won. First, sixty-four. The test is true, so the dragon line runs, and then thanks for playing. Run it again with twenty. The test is false, so the block is skipped. Thanks for playing sits outside the braces, so it appears both times.

Quick Check

Exactly Fifty Tickets

Quick check. Sam runs the same program once more, and this time types exactly fifty. Which lines does it print? Pause and predict. Run it. It prints both. Fifty is greater than or equal to fifty, so the or-equal part counts, and the dragon line runs. That's why Sam tests forty-nine and fifty, not just easy values. The boundary is where mistakes hide.

if-else

Exactly One of Two Blocks Runs

Often there's something to do in both cases. Add the word else, and a second block. The program always runs exactly one of the two blocks. Never both, and never neither. A game costs four tokens. With ten in hand, the game starts, and four are used, leaving six. With three, the else block runs instead: not enough tokens. Nothing is spent. Tokens now comes after both blocks, so it prints in both runs. And a block can hold as many lines as you like.

Common Mistake

Missing Braces Hide a Bug

C allows a shortcut. When the block holds only one statement, you may leave out the braces. Then the if controls only the very next statement. That's dangerous, because your eye reads indentation, and the compiler doesn't. Here's what Sam found. Both printf lines are indented under the if, so both look guarded. And with two tokens, the game shouldn't start. Build and run. No warnings, and start the game is skipped. But have fun still prints. Only the first line belonged to the if. GCC's usual warnings catch this, but clang's wall and W extra stay silent. Add the misleading indentation flag, and clang explains: statement is not part of the previous if. The cure costs two keystrokes. Always write the braces, even for one line. Then what the eye sees is what C does.

Common Mistake

A Semicolon That Empties the if

One more trap. Don't end the if line with a semicolon. In C, a lone semicolon is a complete statement that does nothing. So this line means: if the test is true, do nothing. The block underneath has no if in front of it any more, so it always runs. With two tokens, the game starts anyway. Clang does catch this one: if statement has empty body. Braces every time, and no semicolon after the parenthesis, prevent both bugs.

Quick Check

An Unguarded Subtraction

Quick check, with a new program. A player holds three tokens, and the if has no braces, but two indented lines. What does the last line print? Pause and predict. Build and run it. It prints tokens now, minus one. Three isn't at least four, so the start line is skipped. But the subtraction isn't guarded. It always runs, so the player loses four tokens for a game that never started.

Key Takeaways

Simple and Two-Way Decisions in Review

To sum up. An if runs its block only when the test is true, meaning not zero. An if-else always runs exactly one of its two blocks. Write braces every time, and never put a semicolon after the test. And test with values right at the edge of the rule, the way Sam does.

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FAQ

Questions students ask us

About Simple and Two-Way Decisions: if and if-else

How does an if statement work in C?
An if statement runs its block only when the test in parentheses is true, which in C means any non-zero value. If the test is zero, the block is skipped. Either way, the program continues after the closing brace. For example, if (tickets >= 50) prints a prize line only for 50 or more tickets, while a line after the brace prints every time.
What is the difference between if and if-else in C?
A plain if either runs its block or skips it, while if-else always runs exactly one of its two blocks, never both and never neither. For a game that costs 4 tokens, if (tokens >= 4) can start the game and subtract 4, and the else block can print that more tokens are needed. Code placed after both blocks runs in either case.
Why should you always use braces with an if statement in C?
Without braces, an if controls only the very next statement, no matter how the following lines are indented. Two indented printf lines under a brace-less if look guarded, but the second one runs every time, because the compiler reads statements, not indentation. Writing braces even for a single line makes the code do what it looks like, and clang's -Wmisleading-indentation flag warns about the brace-less version.
What happens if you put a semicolon after an if condition in C?
The semicolon becomes the entire body of the if, because a lone semicolon is an empty statement that does nothing. After if (tokens >= 4); the test controls nothing, and the block underneath is no longer attached to the if, so it always runs. Clang warns that the if statement has an empty body. Removing the semicolon fixes the bug.

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