Input and Output with printf and scanf

Input is not a value until a program converts it, stores it in the right place, and checks that the conversion succeeded. Learn the complete, typed path from standard input through scanf to a clear printf result.

Follow data in both directions

Output makes the program state observable

printf starts with a format string. Ordinary characters appear as written; a placeholder reserves a place for the next value after the string. The placeholder must describe that value accurately. In this course, use %d for an int, %f for a double, and %s for a text reference. Each placeholder consumes one following argument, from left to right.

#include <stdio.h>

int main(void) {
  const char *activity = "C input practice";
  int available_seats = 8;

  printf("%s: %d seats available\n", activity, available_seats);
  return 0;
}

Edit the program, predict its output, then run it.

Input begins as characters, not as an int or double

When a person types 18, the environment first provides characters. scanf attempts to convert those characters according to its format string. On success, it stores the converted numeric value in a variable. That is why a variable must be declared before the scan, and why the format and target type must agree.

Read one whole number with scanf

The source below declares attendees without an initial value because input will provide its first value. &attendees means “the address of attendees”: it gives scanf a place to store the converted number. Edit the Standard input panel below, predict the final output, and run the program. The browser runner reads its input panel in token order; a local terminal reads what you type there.

#include <stdio.h>

int main(void) {
  int attendees;

  printf("Standard input supplies one whole number.\n");
  scanf("%d", &attendees);
  printf("Attendees: %d\n", attendees);
  return 0;
}

Edit the program, predict its output, then run it.

Match scanf formats to the target type

For scanf, use %d with an int target and %lf with a double target. The extra l matters here: it tells scanf to write a double. This differs from printf, where %f displays a double. Memorizing one format character is less reliable than reading the entire pair: function, placeholder, target type, and address.

#include <stdio.h>

int main(void) {
  int sessions;
  double hours_per_session;
  double total_hours;

  scanf("%d %lf", &sessions, &hours_per_session);
  total_hours = sessions * hours_per_session;
  printf("Sessions: %d\n", sessions);
  printf("Total study time: %f hours\n", total_hours);
  return 0;
}

Edit the program, predict its output, then run it.

Treat failed conversion as a real error state

If the program expects %d but the next input token is many, no valid integer is stored. In a complete C program, check the scanf return value before using the target variable. This course introduces that check concept now; the next lesson supplies the if statement needed to express the full recovery branch safely. The browser runner stops instead of pretending that invalid input produced a usable value.

Repair an address mistake before reading the value

This source has a single defect: the scanf call passes open_seats instead of its address. Leave the Standard input value in place, run the program, read the message, and add only the missing &. Then run it again and explain why the repaired program can safely store the number.

#include <stdio.h>

int main(void) {
  int open_seats;

  scanf("%d", open_seats);
  printf("Open seats: %d\n", open_seats);
  return 0;
}

Edit the program, predict its output, then run it.

Do not use scanf for beginner text input yet

scanf("%s", name) is not a safe first text-input pattern. It needs a writable character array with a known capacity, a field width that prevents overflow, and a plan for spaces and the remaining newline. Those are array and string concepts, so this lesson intentionally limits interactive input to numeric int and double values. Later, you will compare bounded scans with line-based input such as fgets.

Compile locally and read input from a terminal

Save an example as input.c and compile it with warnings enabled. In a terminal, the program waits at scanf until you type the requested values and press Enter. The local compiler is the authority for the full language and will help identify a format or argument mismatch that the browser subset deliberately refuses.

Independent lab: build an input-driven workshop report

Use the starter to generate a report from two user-supplied values. Change the workshop label, then provide a new whole-number registration count and decimal duration in Standard input. Add one meaningful calculation using the input—perhaps extra spaces, total minutes, or a different group size—and print it with a matching format. Keep every value's unit clear in its name and in the output label.

#include <stdio.h>

int main(void) {
  const char *workshop = "C practice session";
  int registered;
  double duration_hours;
  int group_size = 4;
  int full_groups;
  int learners_left;

  scanf("%d %lf", &registered, &duration_hours);
  full_groups = registered / group_size;
  learners_left = registered % group_size;

  printf("%s\n", workshop);
  printf("Registered: %d\n", registered);
  printf("Duration: %f hours\n", duration_hours);
  printf("Full groups: %d, learners left: %d\n", full_groups, learners_left);
  return 0;
}

Edit the program, predict its output, then run it.

Lesson review

You now know how typed data crosses a program boundary. printf turns known values into readable terminal text. scanf attempts to turn incoming characters into typed values at the addresses you provide. Reliable input code matches every format to its target, explains the expected shape to the user, verifies conversion before using data, and postpones text scanning until memory capacity is understood.

• I can match %d, %f, and %s with the values passed to printf.
• I can declare an int or double before scanning, then pass its address with &.
• I can explain why scanf uses %d for an int and %lf for a double, while printf uses %f to display a double.
• I can describe conversion failure, whitespace-separated input order, and why text input must wait for arrays, bounds, and string-safety techniques.