Variables and types

Introduction

In C, a variable is a named piece of memory. Before you use one, you must say what type it is, so C knows how many bytes it needs and what the bytes mean.

Python worked out types for you, but C wants to know exactly what you are storing, because it needs to reserve the right number of bytes. That small effort is what makes C programs so fast and so predictable.

How it works

A new variable holds garbage

This is the biggest difference from Python. A local variable that you declare but do not assign does not start at 0. It holds whatever bytes were left in that memory before:

#include <stdio.h>

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

StepVioz shows such a variable as ?. Real C would quietly print a random number here. StepVioz stops and tells you instead, so you see the bug at once.

Characters are small numbers

A char stores one byte. 'A' is really the number 65 (its ASCII code), so you can do arithmetic with characters:

#include <stdio.h>

int main(void) {
    char c = 'A';
    printf("%c is %d\n", c, c);
    c = c + 2;
    printf("now %c\n", c);
    char lower = 'a' + (c - 'A');
    printf("lower case: %c\n", lower);
    return 0;
}

Syntax

int count = 0;
double price = 19.5;
char letter = 'A';
long long big = 9000000000LL;

Example

#include <stdio.h>

int main(void) {
    int age = 19;
    double cgpa = 3.75;
    char grade = 'A';
    age = age + 1;
    printf("Age %d, CGPA %.2f, grade %c\n", age, cgpa, grade);
    printf("An int uses %d bytes, a double %d\n", (int)sizeof(int), (int)sizeof(double));
    return 0;
}

Step through and look at the memory panel on the right. Each variable shows:

• its name and its address (where it lives in memory, in hexadecimal such as 0x7fffefe8);
• its value.

Line by line

1. Three declarations each reserve memory and give it a first value.
2. age = age + 1; reads the old value and stores the new one in the same box.
3. The first printf uses a different format code for each type.
4. (int)sizeof(int) asks how many bytes a type uses, and converts that to an int for %d.

Expected result

Age 20, CGPA 3.75, grade A
An int uses 4 bytes, a double 8

In the real world

Choosing a type is a real engineering decision. A sensor sending millions of readings might use one byte each to save memory, and banking software avoids double for money because decimals are approximate.

Common mistakes

• Using a variable before giving it a value (it holds garbage).
• Putting a double into an int: int x = 3.9; stores 3, because the fraction is cut off.
• Mixing up 'A' (one character, single quotes) and "A" (a string, double quotes).

Best practices

• Initialise each variable where you declare it.
• Use int for counting and double for measurements.
• Use long long whenever a result might pass two billion.

Summary

C variables have fixed types, which set their size and meaning. A local variable starts with garbage until you give it a value, and a char is simply a small number with a matching letter.

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