Variable Assignment and Typecasting in C | Programming in C

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Variable Assignment and Typecasting in C

Programming in C · Chapter 3: Data Types, Variables, Math Library, and Console I/OSection 3.3 · lesson 15 of 786 min · free with a free account

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In this lesson

Assign values, use increment operators and cast types on purpose.

  • Explain how assignment and compound assignment operators work
  • Compare prefix and postfix increment and decrement
  • Place a cast so that a division is done in floating point
  • Predict truncation and narrowing when converting to smaller types

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Transcript

Everything said in the video, section by section (813 words). Read along, or search it.

Opening Puzzle

Where Did the Half Go

Priya shares seven prize tickets between two players, and stores the answer in a double. The screen says 3.0. I made it a double, she says. So where did the half go? To find out, we need to know exactly what an assignment does, and how to ask C for a conversion.

Assignment

Right Side First, Then Store

The assignment operator, a single equals sign, doesn't mean equals as in math class. It means: work out the right side, then store the answer in the box on the left. Think of it as an arrow pointing left. That's why tokens equals tokens plus five is legal: read twenty, add five, store twenty-five back. A compound assignment, such as plus equals five, means the same thing, shorter to write and harder to mistype. There's one for each operator: minus, times, divide, and remainder. Run it. Twenty, then 25, then 30, times three is 90, and the remainder after dividing by four is 2. The last line chains assignments. They run right to left: c gets seven, then b, then a, because an assignment produces the value it stored.

Common Slip

Double Equals Stores Nothing

A classic slip: typing double equals when you meant a single one. Double equals only asks whether two values are equal, and throws the answer away. Nothing is stored. Clang notices: equality comparison result unused. And a note suggests the fix: use a single equals, to turn it into an assignment. It's only a warning, so the program still runs, and tokens is still ten. One warning, and nothing stored.

Increment and Decrement

Prefix Versus Postfix

Adding or subtracting one happens so often that C gives it its own operators: plus plus, the increment, and minus minus, the decrement. They come in two flavors. Postfix, after the name, hands over the old value, then adds one. Prefix, before the name, adds one first, then hands over the new value. Run it. Games plus plus gives 5, the old value, and games is now 6. Plus plus games adds first, so it gives 7. On a line of its own, like games minus minus, the two flavors do the same thing. The difference only matters inside a bigger statement.

Undefined Behavior

Changing x Twice in One Statement

One rule: never use plus plus on a variable that appears twice in the same statement. Here, x is changed twice, and C doesn't say which change happens first. That's undefined behavior, so there's no answer worth printing. Clang warns: multiple unsequenced modifications to x. Give each plus plus its own line.

Quick Check

Lives Plus Plus Both Ways

Quick check. Lives starts at three. A gets lives plus plus, then b gets plus plus lives. What are a, b, and lives at the end? Pause and predict. It prints three, five, five. Postfix hands a the old value, three, and lives becomes four. Then prefix adds first: lives becomes five, and b gets that five. Four is never stored in a or b; it only lives in lives for a moment.

Typecasting

Where the Cast Sits Matters

In lesson 3.1, C converted types on its own. A cast is when you ask for a conversion, by writing the new type in parentheses in front of a value. Open paren double, close paren, tokens means: treat this int as a double, right here. It changes the value used in that one spot, not the variable. Where the cast sits decides everything. Run it. Wrong is 3.0: the division was done with ints. Right is 3.5, because the cast touched the seven before dividing. Late is 3.0 too. The cast came after the division, when the half was already gone.

Pitfalls

Truncation and Narrowing

Two more pitfalls in the same run. Truncation: int of 9.99 is nine, and int of minus 2.7 is minus two. Decimals are cut off toward zero. Nothing is rounded. And narrowing. A char is one byte, and 321 doesn't fit. On this machine it wraps around, and 65 comes out. Other setups may differ, so never rely on it.

Quick Check

Eleven Points Over Four Rounds

Quick check. Eleven points over four rounds. This time the cast sits on the right side of the division, and the average is then cast to an int. What does the program print? Pause and predict. It prints 2.75 and 2. A cast on either side, before the division, makes it a double division, so eleven divided by four is 2.75. Then int of 2.75 cuts the decimals off. It doesn't round to three.

Story Ending

A Cast on the Seven

Back to Priya's seven tickets. Sam traced the line: the division happened first, with ints, so her double only received the leftover 3. She added a double cast on the seven, and the screen said 3.5.

Key Takeaways

Assign, Increment and Cast

To recap. Assignment works out the right side, then stores it on the left, and compound operators like plus equals are shortcuts. Postfix plus plus hands over the old value, prefix the new one. Never change a variable twice in one statement. A cast must touch a value before the operation. Casting to int cuts toward zero, and squeezing into a char loses data.

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FAQ

Questions students ask us

About Variable Assignment and Typecasting in C

What is the difference between ++x and x++ in C?
The prefix form ++x adds one first and gives the new value, while the postfix form x++ gives the old value and then adds one. If lives is 3, a = lives++ stores 3 in a and makes lives 4, then b = ++lives makes lives 5 and stores 5 in b. On a line of its own, either form simply adds one.
How do you cast an int to a double in C?
Write the target type in parentheses before the value, as in (double)tokens. The cast changes the value used at that spot, not the variable itself. Its position matters: (double)7 / 2 gives 3.5 because one operand is a double before the division, but (double)(7 / 2) gives 3.0 because the integer division has already dropped the half.
What does a compound assignment operator like += do in C?
A compound assignment operator combines an operation with storing the result, so tokens += 5 means tokens = tokens + 5. C has one for each arithmetic operator, including -=, *=, /= and %=. Starting from 25, tokens += 5 gives 30, then tokens *= 3 gives 90, and tokens %= 4 gives 2. The short form is quicker to write and harder to mistype.
Does casting a double to an int round the number in C?
No, casting a double to an int truncates toward zero instead of rounding. (int)9.99 gives 9, and (int)-2.7 gives -2. To round to the nearest whole number, use a function such as round from math.h. A related risk is narrowing: 321 does not fit in a one-byte char, and on many systems it wraps to 65, a result that should never be relied on.

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Programming in C (Computer Science 240) carries a NCCRS credit recommendation for 3 semester credits. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities.
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