C Academy · Lesson

Adding Variables

Store and recall values.

Lesson 4 of 413 steps

Adding Variables is a free C Academy lesson on CoddyKit. This is lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C Academy learning path, and your progress syncs across the web and the CoddyKit app. The C Academy course includes 4 lessons in total.

Beyond Arithmetic

To support x = 5 and later x + 1, the interpreter needs identifiers and a place to store their values. That store is the environment.

We will extend the lexer, parser, and evaluator to handle names and assignment.

Lexing Identifiers

An identifier starts with a letter or underscore and continues with letters, digits, or underscores. We copy the name into the token.

A fixed-size buffer keeps the example simple; real lexers intern names.

#include <ctype.h>
#include <string.h>

typedef struct { TokKind kind; int value; char name[32]; } Token;

static Token lex_ident(void) {
  Token t; t.kind = TOK_IDENT; int i = 0;
  while (isalnum((unsigned char)peek()) || peek() == '_')
    if (i < 31) t.name[i++] = advance(); else advance();
  t.name[i] = '\0';
  return t;
}

A Variable Node

The AST gains two node kinds: a variable reference and an assignment. Each stores the variable name.

Assignment also keeps the expression whose value it stores.

typedef struct Node {
  enum { N_NUM, N_BINOP, N_VAR, N_ASSIGN } kind;
  int value; char op; char name[32];
  struct Node *left, *right;  /* assign uses right as value expr */
} Node;

The Environment

The environment maps names to values. A simple linear array of name/value pairs suffices for a small interpreter.

Larger languages use hash tables for O(1) lookup and nested scopes.

#define MAX_VARS 64

typedef struct {
  char names[MAX_VARS][32];
  int  values[MAX_VARS];
  int  count;
} Env;

static Env env;  /* global for this tiny example */

Setting a Variable

Assigning either updates an existing slot or appends a new one. Linear search keeps the code short.

Returning the stored value lets assignments be used as expressions, like y = (x = 3).

#include <string.h>

static int env_set(const char *name, int v) {
  for (int i = 0; i < env.count; i++)
    if (strcmp(env.names[i], name) == 0) {
      env.values[i] = v; return v;
    }
  strcpy(env.names[env.count], name);
  env.values[env.count] = v;
  env.count++;
  return v;
}

Reading a Variable

Lookup scans for the name and returns its value. Referencing an undefined variable is a runtime error.

Catching this here gives a clear message instead of returning a garbage value.

#include <stdio.h>
#include <stdlib.h>

static int env_get(const char *name) {
  for (int i = 0; i < env.count; i++)
    if (strcmp(env.names[i], name) == 0)
      return env.values[i];
  fprintf(stderr, "undefined variable: %s\n", name);
  exit(1);
}

Parsing Assignment

Assignment has the lowest precedence and is right-associative. We parse a factor; if it is a bare identifier followed by =, we build an assign node.

Otherwise the identifier is just a variable read.

static Node *parse_assign(void) {
  if (cur().kind == TOK_IDENT) {
    char saved[32]; strcpy(saved, cur().name);
    bump();
    if (cur().kind == TOK_ASSIGN) {
      bump();
      Node *val = parse_assign();
      return assign_node(saved, val);
    }
    return var_node(saved);  /* not an assignment */
  }
  return parse_expr();
}

Evaluating Variables

The evaluator gains two cases. A variable node looks its name up in the environment; an assign node evaluates its value expression and stores it.

Both reuse the env_get and env_set helpers.

int eval(Node *n) {
  switch (n->kind) {
    case N_NUM:    return n->value;
    case N_VAR:    return env_get(n->name);
    case N_ASSIGN: return env_set(n->name, eval(n->right));
    case N_BINOP: {
      int l = eval(n->left), r = eval(n->right);
      switch (n->op) {
        case '+': return l + r; case '-': return l - r;
        case '*': return l * r; case '/': return l / r;
      }
    }
  }
  return 0;
}

A Working REPL Step

This program stores a variable, then reads it back in an expression — all through the environment. It models one line of a real interpreter.

Run it to see assignment and lookup cooperate.

#include <stdio.h>
#include <string.h>

#define MAX_VARS 64
static char names[MAX_VARS][32];
static int  values[MAX_VARS];
static int  count;

static int env_set(const char *n,int v){
  for(int i=0;i<count;i++) if(!strcmp(names[i],n)){values[i]=v;return v;}
  strcpy(names[count],n); values[count]=v; count++; return v;
}
static int env_get(const char *n){
  for(int i=0;i<count;i++) if(!strcmp(names[i],n)) return values[i];
  return 0;
}

int main(void){
  env_set("x", 5);            /* x = 5 */
  int r = env_get("x") + 1;   /* x + 1 */
  printf("x = %d\n", env_get("x"));
  printf("x + 1 = %d\n", r);
  return 0;
}

Scopes and Shadowing

A single flat environment is global. Real languages add nested scopes so a function's locals do not clobber outer names.

You implement this by chaining environments: lookup walks from the innermost scope outward.

typedef struct Env {
  char names[MAX_VARS][32];
  int  values[MAX_VARS];
  int  count;
  struct Env *parent;  /* enclosing scope */
} Env;

Where to Go Next

With variables in place you can add statements, conditionals, and functions. Each is a new node kind plus an eval case.

From here, the same lexer-parser-evaluator pipeline scales into a real little language.

Quick Check

Think about what the environment is responsible for.

Answer it in the interactive lesson

Recap

You added identifiers to the lexer, var/assign nodes to the AST, an environment with set and get, and eval cases for reads and assignments.

The interpreter now remembers state — the foundation for statements and functions.

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Frequently Asked Questions

Is the “Adding Variables” lesson free?

Yes — the full text of “Adding Variables” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C Academy course, upgrade to CoddyKit PRO. The C Academy course includes 4 lessons in total.

What will I learn in “Adding Variables”?

Store and recall values. You practise C Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C Academy?

No prior experience is required. C Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.

How long does the “Adding Variables” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

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