Introduction to C: variables, types and operators

The last four weeks of the semester are C. Why C? Because C sits right above the level you have learned so far: a C variable is a location in memory, a C expression is a few LC-3 instructions, a function call is JSR and stack operations. That is why the textbook builds C on top of the LC-3. Most of the C questions on the exam are of the “What is printed?” type: you run the code in your head and write the output.

Golden rules:

1. int / int = int, the fraction is truncated.
2. True = nonzero, false = 0. Comparisons produce 1 or 0.
3. x++ gives the old value, ++x gives the new value.
4. && and || short-circuit.
5. A variable declared inside a block shadows the one outside.

The first program

#include <stdio.h>

int main(void) {
    int count = 3;
    printf("count = %d\n", count);
    return 0;
}

count = 3

• #include <stdio.h>: a preprocessor command for printf/scanf. Before compiling, the contents of that file are inserted here.
• main: the program starts here.
• Every statement ends with ;. Blocks are { }.

A C program is compiled: source code → preprocessor → compiler → assembly/machine code → linker → executable file. On the LC-3 this means producing assembly of the kind you wrote in week 10.

Variables and types

Every variable has a type, a name and a value. The type says how many bits will be reserved in memory and how the bits will be read (week 2).

In LC-3 C, int is 16 bits; on your computer it is usually 32 bits. The numbers used in the exam fit in both.

Constants: 17 is decimal, 0x1F is hex (31), 017 is octal (15). Putting a zero in front makes the number octal: watch out.

Scope

• A local variable is declared inside a function or block and is seen only there. In the LC-3 it lives on the stack (runtime stack).
• A global variable is declared outside all functions and is seen everywhere. In the LC-3 it lives in the global data region (R4 points to this region).

A variable with the same name in an inner scope shadows the outer one:

int g = 10;

void show(void) {
    int g = 3;            /* a new, local g */
    printf("show: %d\n", g);
}

int main(void) {
    show();
    printf("main: %d\n", g);
    return 0;
}

show: 3
main: 10

PITFALL · a local with no initial value. If you write int x; and use x, its value is undefined (whatever was left in that memory location). Global variables, on the other hand, start automatically at 0.

Arithmetic operators

printf("%d %d %d %d\n", 7 / 2, -7 / 2, 7 % 3, -7 % 3);

3 -3 1 -1

• Division truncates toward zero: -3.5 → -3.
• The sign of the result of % is that of the left number.
• If one is a double, the division is a real division: 7 / 2.0 = 3.5.

Assignment and compound assignment

int n = 10;
n -= 3;   /* 7  */
n *= 2;   /* 14 */
n /= 4;   /* 3  */
n %= 3;   /* 0  */
printf("%d\n", n);

0

Increment and decrement

int i = 3;
int k = i++ * 2;     /* 3 * 2, then i = 4 */
printf("%d %d\n", i, k);

4 6

If you had written ++i * 2, i would become 4 first, and k = 8.

PITFALL · changing the same variable twice in one expression. Expressions like x = x++ + ++x; are undefined in C (undefined behavior). They give different results depending on the compiler. If you meet them in exam questions, the answer may be “undefined”.

Relational and logical operators

< <= > >= == != give 1 (true) or 0 (false) as the result. && (and), || (or), ! (not) count every nonzero value as true.

int a = 3, b = 0;
printf("%d %d %d %d\n", a && b, a || b, !b, a == 3);

0 1 1 1

Short-circuit: in A && B, if A is false, B is not computed. In A || B, if A is true, B is not computed. This makes guarded expressions like (i != 0) && (10 / i > 1) possible.

The conditional operator: condition ? a : b. If the condition is true, a; otherwise b.

Bitwise operators

~x = -x - 1 (2’s complement). Do not mix up & and &&: 12 & 10 = 8, 12 && 10 = 1.

char arithmetic

Since a char is an integer, you can calculate with it:

char c = 'b';
c = c - 'a' + 'A';      /* lowercase → uppercase */
printf("%c\n", c);
printf("%d\n", '7' - '0');

B
7

Precedence and associativity

If you are not sure, use parentheses; in the exam, solve step by step according to the table:

2 + 3 * 4 % 5
2 + 12 % 5        * and % are at the same level, the left one first
2 + 2
4

From C to the LC-3

a = b + c; (all three local) is roughly translated by the compiler like this (R5 is the frame pointer, week 12):

        LDR  R0, R5, #-1    ; R0 = b
        LDR  R1, R5, #-2    ; R1 = c
        ADD  R0, R0, R1
        STR  R0, R5, #0     ; a = R0

So every C variable is a memory location, and every operation is one or a few instructions.

Common mistakes

printf("%d %d\n", 17 / 5, 17 % 5);

int x = 5;
int y = x++;
printf("%d %d\n", x, y);

int x = 5;
int y = ++x;
printf("%d %d\n", x, y);

double d = 7 / 2;
printf("%.1f\n", d);

printf("%d\n", 10 > 5 > 1);

int i = 0;
int r = (i != 0) && (10 / i > 1);
printf("%d\n", r);

printf("%d %d %d\n", 12 & 10, 12 | 10, 12 ^ 10);

printf("%d %d\n", 5 << 2, 40 >> 3);

char c = 'A' + 2;
printf("%c %d\n", c, c);

printf("%d\n", 2 + 3 * 4 % 5);

int x = 4;
printf("%d %d %d\n", !x, !!x, x > 3 ? 10 : 20);

int g = 10;

void show(void) {
    int g = 3;
    printf("show: %d\n", g);
}

int main(void) {
    show();
    printf("main: %d\n", g);
    return 0;
}