Input Validation and Formatted Output in C | Programming in C

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Input Validation and Formatted Output in C

Programming in C · Chapter 7: Arrays, Strings, Input Validation, and Formatted OutputSection 7.7 · lesson 46 of 786 min · free with a free account

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In this lesson

Checking scanf input, reading lines with fgets, and aligning printf output.

  • Use the return value of scanf to detect bad input and end of input
  • Write a validation loop that clears the input buffer and checks a range
  • Read a whole line with fgets and remove its newline with strcspn
  • Format aligned output with printf width, precision, minus, and zero flags

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Transcript

Everything said in the video, section by section (807 words). Read along, or search it.

Opening Question

Players Who Type Banana or Minus Five

Ask a player how many tokens they want, and someone will type banana. Or minus five. Or five thousand. A program that trusts every answer breaks the first time a real person uses it. The cure has two halves. Check the data coming in, and shape the data going out, so it's easy to read.

Scanf's Return Value

One, Zero, or EOF

Every scanf returns a number: how many values it managed to fill in. Here, we print what it returns. Type forty, and scanf returns one: one number read. Type letters, and it can't make a number, so it returns zero. And tokens keeps whatever it held before, here zero. If the input has ended, scanf returns the special value EOF, which is minus one here. An empty file as input shows it. So the first guard of every careful program is checking that number.

The Input Buffer

Rejected Letters Stay Waiting

There's a catch. When scanf fails, the letters it didn't accept stay waiting in the input buffer, a small waiting line for typed characters. The next scanf sees the same letters, and fails again, forever. The fix: read and throw away characters, one at a time, until the end of the line.

Validation Loop

Check for a Number, Then the Range

Put the guards in a loop, which runs while no good answer has been given. Inside, two checks happen in order. First, did we get a number at all? Got not equal to one catches the word. The inner while reads one character at a time with getchar, until the newline or the end of input. That empties the buffer. Second, is the number in range? Tokens less than one, or more than a hundred, is refused. Only a good number sets valid to one, and gets out. Sam tests it: abc, then minus five, then two fifty, then forty. Each wrong answer gets its message, and only forty gets through.

Quick Check

Testing the Edges Zero and One Hundred

Quick check. Here's the same kiosk, written more compactly. Sam goes for the edges: he types zero, then a hundred. What's the last line the kiosk prints? Pause and predict. Buying 100 tokens. Zero is less than one, so it's refused, and the loop asks again. A hundred isn't more than a hundred, so it's in range: both ends, one and a hundred, are allowed.

Reading Whole Lines

Fgets and Removing Its Newline

Percent s stops at the first space, so it can't read Priya Sharma. The tool for whole lines is fgets: the array, its size, and stdin, the keyboard. It reads up to a newline, never more than size minus one characters, and always ends with the marker, so it can't overflow your array. With nothing to read, it returns NULL. One surprise: fgets keeps the newline from the Enter key. Priya Sharma has twelve characters, but the raw text has thirteen. Strcspn finds the box of the first newline, or the marker if there is none, and the program puts a marker there. Clean text: twelve.

Common Trap

Scanf Leaves a Newline Behind

Mixing scanf and fgets has a trap. After scanf reads twelve, the Enter key's newline is still waiting. The next fgets sees it first, and happily returns an empty line. The name has one character, that newline. Sam never got read. The fix is the getchar loop from before: clear the rest of the line after scanf. Now the name is Sam plus its newline: four characters.

Formatted Output

Width, Precision, Minus, and Zero

Clean output matters too. Between the percent sign and the letter, a number is a width: use at least that many columns, padded with spaces on the left. A minus sign flips it to left-aligned. A dot and a number is the precision: digits after the point. And a zero before the width pads with zeros. Run it. The prices line up, because every price takes exactly eight columns, so the decimal points fall in the same place. Percent zero five d turns forty-two into zero zero zero four two. And the brackets show percent five d padding on the left, and minus five on the right.

Quick Check

One Snack Row in Brackets

Quick check. One snack row in brackets: Soda, left-aligned in six, then two point five with width five and one decimal. What exactly prints between the brackets? Pause and predict. Soda, two spaces, the bar, then two spaces and two point five. The minus puts the name's padding after it. Numbers are right-aligned unless you ask otherwise, so the padding of two point five goes in front.

Story Resolved

Numbers Only, Please

Back at the kiosk. Sam once typed twelve, as a word, and the first version printed a price for zero tokens. Priya added the scanf check and the range loop. Now the kiosk says: numbers only, please. Try breaking it again, she said.

Key Takeaways

Check Input, Shape Output

To recap. Check what scanf returns: one, zero, or EOF. After a failure, throw the bad line away with getchar. Loop until the answer is a number, and in range. Read whole lines with fgets, and remove its newline with strcspn. And shape output with width, precision, minus, and zero.

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FAQ

Questions students ask us

About Input Validation and Formatted Output in C

What does scanf return in C?
scanf returns how many values it successfully stored. With scanf("%d", &tokens), typing 40 returns 1, typing letters returns 0 and leaves tokens unchanged, and reaching the end of input returns EOF, a negative constant that is usually -1. Checking this number is the first step of input validation, for example treating any result other than 1 as input that is not a number.
How do you clear the input buffer after scanf fails in C?
Read and discard characters with getchar until you reach the newline or the end of input: int c; while ((c = getchar()) != '\n' && c != EOF) { }. When scanf fails, the characters it rejected stay in the input buffer, so the next scanf sees them and fails again, forever. The same loop also removes the newline that scanf leaves behind before a later fgets call.
How do you read a full line with spaces in C?
Use fgets(name, sizeof name, stdin). It reads up to and including the newline, stores at most size minus one characters, and adds the null terminator, so it cannot overflow the array. Unlike %s, it does not stop at spaces, so Priya Sharma is read whole. Because fgets keeps the newline, remove it with name[strcspn(name, "\n")] = '\0';. It returns NULL when there is nothing to read.
How do you line up columns with printf in C?
Put a width between the % sign and the conversion letter. %8.2f uses at least 8 columns with 2 digits after the decimal point, so prices line up on their decimal points. Output is right-aligned by default; a minus sign, as in %-6s, left-aligns and pads on the right. A zero before the width, as in %05d, pads with zeros, so 42 prints as 00042.

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