Course: CSE 1209 - Computer Fundamentals and Programming.

tags: cse-1209 c fundumental error

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NOTE:

In scanf, whitespace in the format string means:
“Skip any amount of whitespace in input (spaces, tabs, newlines)”

Whitespace = ' ', \n, \t

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1. Space BEFORE a format specifier

scanf(" %d", &x);

What the space does

• Skips all leading whitespace
• Then reads an integer

When you NEED it

• After previous input
• After reading a character
• After pressing Enter earlier

Example (important)

int x;
scanf(" %d", &x);

Input:

    42

✔ Works

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2. Space BEFORE %c (VERY IMPORTANT)

scanf(" %c", &ch);

Why this matters

%c does NOT skip whitespace by default

Bad:

scanf("%c", &ch);

Input:

A↵

It may read:

'\n'

Good:

scanf(" %c", &ch);

✔ Skips newline, then reads the character

👉 Rule:

Always put a space before %c unless you explicitly want whitespace.

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3. Space BETWEEN format specifiers

scanf("%d %d", &a, &b);

What this means

• Read first integer
• Skip whitespace
• Read second integer

Input:

10 20

✔

Input:

10
20

✔

Input:

10     20

✔

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4. What if I don’t put space?

scanf("%d%d", &a, &b);

Surprise:

Still works.

Why?

• %d already skips leading whitespace
• So space between %d is optional

But:
👉 Readable code always uses space

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5. Comma in format string (MOST COMMON MISTAKE)

scanf("%d, %d", &a, &b);

This means:

Input MUST contain a comma

Input:

10, 20

✔

Input:

10 20

❌ FAILS

Key rule

Commas, letters, symbols in format string must appear EXACTLY in input

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6. Literal characters in scanf

scanf("(%d)", &x);

Input must be:

(25)

Otherwise → fail

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7. Newline \n in scanf ❌ (TRAP)

scanf("%d\n", &x);   // BAD

Why it’s bad

• \n means “skip whitespace”

• But scanf will WAIT for non-whitespace

• Program appears “stuck”

NEVER do this

scanf("%d\n", &x);
scanf("%d ", &x);   // same problem

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8. %d vs %i (quick note)

scanf("%d", &x); // decimal only
scanf("%i", &x); // auto base (10, 8, 16)

Input:

010   -> 8   (with %i)
0x10  -> 16

For beginners:
👉 Use %d

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9. Mixed inputs (int + char) – classic bug

❌ Buggy code:

int x;
char ch;
 
scanf("%d", &x);
scanf("%c", &ch);  // reads '\n'

✅ Correct:

scanf("%d", &x);
scanf(" %c", &ch);

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10. Multiple inputs with characters

char ch1, ch2;
scanf(" %c %c", &ch1, &ch2);

Input:

A B

✔

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11. Always check return value (real fix)

if (scanf("%d %d", &a, &b) != 2) {
    // input error
}

This alone prevents half your bugs.

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12. Golden rules (print & remember)

1. Whitespace in format → skip whitespace
2. %d, %f, %s skip whitespace automatically
3. %c does NOT → add space before it
4. Commas & symbols must exist in input
5. Never use \n at end of scanf
6. Always check return value
7. scanf doesn’t clear input buffer

Mental model (simple)

Think of scanf like this:

“I will match input EXACTLY as my format string says,
except that whitespace means ‘skip all whitespace’.”

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