C programming language → Basic Elements of the C Language

Basic Elements of the C Language

Structure of a C program

The smallest complete program has a main function – the place where execution starts:

#include <stdio.h>            /* library with printf and scanf */

int main(void) {              /* execution starts here */
    printf("Hello, world!\n");
    return 0;                 /* 0 means: the program finished correctly */
}

Tokens: the words of the language

The compiler first splits the source code into tokens – the smallest meaningful units. There are five kinds:

In the statement sum = a + 5; the tokens are: sum, =, a, +, 5 and ;.

Rules for identifiers

• they may contain letters of the English alphabet, digits and the underscore _;
• they must not start with a digit (2sum is wrong, sum2 is fine);
• a keyword cannot be used as a name (int for; is an error);
• C is case sensitive: sum, Sum and SUM are three different names.

Choose names that say what the variable holds: numberOfStudents is better than n2.

Whitespace and comments

Spaces, tabs and new lines (whitespace) separate tokens; the compiler ignores extra whitespace, so we use it to make code readable. Comments are text for people – the compiler skips them:

#include <stdio.h>

int main(void) {
    // a one-line comment lasts until the end of the line
    int a = 7, b = 5;

    /* a multi-line comment
       can span several lines */
    printf("%d\n", a * b);    /* a comment can follow code */
    return 0;
}

Statements and blocks

A statement is an instruction for the computer: a declaration (int x;), an assignment (x = 5;), a function call (printf(...);) or a control statement (if, for, while). Curly braces join several statements into a block, which is treated as one statement:

#include <stdio.h>

int main(void) {
    int x;
    scanf("%d", &x);
    if (x > 0) {                 /* block: two statements run together */
        printf("positive\n");
        printf("square: %d\n", x * x);
    } else
        printf("not positive\n"); /* a single statement needs no braces */
    return 0;
}