Unit 1 · Lesson 728 minAcademic review pending

Talk to the program—and to individual bits

Use formatted standard I/O, understand the three standard streams, read command-line arguments, and apply basic bitwise operations.

Choose explanation

After this lesson

You should be able to

Match printf and scanf conversion specifiers to argument types.
Use masks to test, set, and clear selected bits.

Three standard streams

stdin supplies ordinary input, stdout carries ordinary results, and stderr carries diagnostics. Keeping errors on stderr lets a user redirect normal output to a file without mixing it with error messages.

Format strings are contracts. %d expects an int, %lf in scanf expects a pointer to double, and scanf usually needs the address operator &: scanf("%d", &age). Always check scanf's return value before using the result.

Checked input

int age;

if (scanf("%d", &age) != 1) {
    fprintf(stderr, "Expected an integer\n");
    return 1;
}

printf("Age: %d\n", age);

Arguments and bit masks

With int main(int argc, char *argv[]), argc is the number of command-line strings and argv stores them. argv[0] is normally the program name. Text must be validated and converted before numeric use.

Bitwise & tests shared 1 bits, | sets bits, ^ toggles differing bits, ~ inverts bits, and shifts move bit patterns. A mask such as 1u << 3 selects bit 3. Test with value & mask, set with value | mask, and clear with value & ~mask.

Test bit 3

unsigned int mask = 1u << 3;

if ((value & mask) != 0u) {
    printf("bit 3 is set\n");
}

Try it yourself

Write expressions that set bit 2 and then clear bit 2 in an unsigned value flags.

Need a hint?

Create the mask with 1u << 2. Clearing requires the inverted mask.

Check the worked solution

Set with flags |= 1u << 2; and clear with flags &= ~(1u << 2);.

flags |= 1u << 2;
flags &= ~(1u << 2);

Quick check

Where should a validation error normally be written?

Select an answer to check your thinking.

Why this lesson exists

Syllabus mapping

Formatted input/output · stdin stdout stderr · Command-line arguments · Bitwise operations

Maps to course outcomes CO2, CO3.