Variables, Types, and Operators

A program becomes useful when names stand for changing information. Learn the small, reliable path from a typed declaration to a calculation, an updated value, and terminal output—without losing track of what each value means.

Make every value carry a clear promise

A variable is a named place for a typed value

Read a declaration from left to right: the type says what kind of value the name may hold, the name gives that value a useful role, and the expression after = supplies the first value. In int remaining = capacity - enrolled;, the program evaluates the right side first, then stores the result in a new whole-number variable named remaining.

Keep three actions separate in your mental model. A declaration introduces a name and type. Initialization gives that newly declared name its first value. An assignment later replaces the value held by an already declared, mutable name. The equal sign is not a math claim; it means “compute the right side, then store it in the left-side variable.”

#include <stdio.h>

int main(void) {
  int capacity = 24;
  int enrolled = 18;
  int remaining = capacity - enrolled;

  printf("%d seats remain.\n", remaining);
  return 0;
}

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

Use a type as a useful boundary

C has more types than a first lesson needs. Start with the ones that make everyday data intentions clear. A type does not merely label a value for the compiler: it determines the operations and representation rules that apply to it. Use the smallest type concept that accurately describes the problem, then let later lessons add detail about sizes, arrays, pointers, and conversion rules.

Update a variable by reading, calculating, then storing

An assignment can reuse the current value of the same variable. C evaluates the expression on the right before it changes the left. In checked_in = checked_in + 1;, the old count is read, one is added, and the new count replaces the old value. Nothing “changes itself”; each step has a clear order.

#include <stdio.h>

int main(void) {
  int checked_in = 12;

  checked_in = checked_in + 1;
  printf("%d learners have checked in.\n", checked_in);
  return 0;
}

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

Calculate with operators—and read their order

Arithmetic operators transform numeric values. Multiplication, division, and remainder happen before addition and subtraction. Parentheses make a grouping explicit when the calculation matters to a human reader. Rather than memorizing a long precedence table, write a small named intermediate value or use parentheses whenever a reader could reasonably misread your intent.

#include <stdio.h>

int main(void) {
  int pages_per_chapter = 8;
  int chapters = 3;
  int total_pages = pages_per_chapter * chapters;
  int whole_groups = total_pages / 5;
  int spare_pages = total_pages % 5;

  printf("Total pages: %d\n", total_pages);
  printf("Groups of five: %d, spare: %d\n", whole_groups, spare_pages);
  return 0;
}

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

Match a value's type to its output format

printf needs a format string and values that match it. In this lesson, pair %d with an int, %f with a double, and %s with a text reference such as const char *. A mismatched format can make a real C program behave unpredictably, so treat each placeholder as part of the value's contract.

#include <stdio.h>

int main(void) {
  double session_hours = 1.5;
  double weekly_hours = session_hours * 3.0;

  printf("Weekly study time: %f hours\n", weekly_hours);
  return 0;
}

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

Protect a fixed rule with const

Use const when a name represents a rule or fixed configuration that this function must not overwrite. The broken source below tries to change max_attempts after promising it is constant. Read the runner message, then make one intentional repair: either preserve the fixed rule by removing the assignment, or make the value truly mutable by changing only const int to int.

#include <stdio.h>

int main(void) {
  const int max_attempts = 3;

  max_attempts = max_attempts + 1;
  printf("Maximum attempts: %d\n", max_attempts);
  return 0;
}

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

Compile with conversion warnings visible

Save a version of these examples as values.c. A local compiler understands the full C language and your system's implementation choices. Use warning flags while you experiment so a suspicious conversion or formatting mismatch is visible before it becomes a hard-to-find output bug.

Independent lab: report a study session

Use the starter to produce a short, readable terminal report. Change the session name, seats, booked count, and duration. Then add a new int calculation that has a clear unit—perhaps reserved_seats or remaining_after_waitlist—and print it using the correct placeholder. Your program should have a label, a whole-number calculation, a fractional value, and a visible success status.

#include <stdio.h>

int main(void) {
  const char *session_name = "C study lab";
  int seats = 32;
  int booked = 19;
  int open_seats = seats - booked;
  double session_hours = 1.5;

  printf("%s\n", session_name);
  printf("%d seats are open.\n", open_seats);
  printf("Length: %f hours\n", session_hours);
  return 0;
}

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

Lesson review

You now have the core data flow that every later C program will use: choose a type, give the value a meaningful name, initialize it, calculate with the correct operators, update it only when the program's rules permit it, and format the output according to the value's type. The next lesson will add input, which means users will start supplying some of those values themselves.

• I can distinguish a declaration, initialization, and later assignment in a C source file.
• I can choose int for a whole-number count, double for a fractional quantity, and const char * for a beginner text label.
• I can trace + - * / %, state the difference between integer division and remainder, and add parentheses or a named intermediate value when clarity needs it.
• I can match %d, %f, and %s to the values passed to printf, and use const to protect a fixed rule.