4) Intro to C - Computational Methods for Scientists Fall 26

Today¶

Compiling a C program
Variables and Basic Data Types

1 Compiling a C program¶

Contrary to the examples you have seen in the previous lectures in Julia, C programs need to be compiled first, before being executed. This is a drawback of a compiled language (e.g., C, Fortran, Java, etc) Vs an interpreted language (e.g., MATLAB, Python, Julia, etc) in which the interpreter compiles and executes at the same time.

To compile a C program, you need a compiler. This may vary accoriding to the system you use. For:

Example in Linux, using gcc:

gcc hello.c -o hello

The gcc command invokes the compiler, then you pass the source file you want to compile (hello.c in this case), and then after the -o target, you can specify the output file for the compiled program (otherwise, the compiler will automatically create a.out) for you.

Inspect the directory after compiling, and (if the compilation was successful) you will an object output file (by default a.out) or the name of the executable you provided after the -o option.

To run/execute your program, simply type ./name_of_your_program in your terminal:

./hello

2 Variables and Basic Data Types¶

Our first C program: a Hello, World! print statement.

hello.c

#include <stdio.h> // includes the standard input-output header to be able to use standard IO functions, such as printf() and scanf(), to perform input and output operations.

// a "main" program is required in all C programs
int main(void)
{
     printf("Hello, world!\n"); // this printf statement will print formatted output in the standard output (your terminal screen)
}

Displaying Values of Variables:

product.c

#include <stdio.h> // includes the standard input-output header to be able to use standard IO functions, such as printf() and scanf(), to perform input and output operations.

int main(void)
{
    int product; // declaration of an integer variable
    product = 9*2*3;
    printf("The product of 9, 2, and 3 is %i \n", product); // this printf statement will print formatted output in the standard output (your terminal screen)

    return 0;
}

Variables¶

Note:

In C, you need to declare the variable type before the variable is used
Variable names begin with letters, _, followed by combination of letters, _, or digits 0-9
Do not use reserved words
C is case-sensitive!

Global variables¶

They have a place in C programming, but are often used to fix badly written code. If multiple parts of your code (like two functions) need to operate on a variable you should use pointers (we’ll see more on this later) to share this variable rather than make it available to every function.

Basic Data Types¶

int, float, double, char (single character), and _Bool (or bool if you include the header stdbool.h)
Constant: any number, single character or character string
Constant expressions: entirely of constant values

int

Integer constant: consist of one or more digits
Decimal, octal (prefix 0) and hexadecimal (prefix 0x or 0X) notations, with print format specifiers %o for octal and %x and %X for hexadecimal
Range of values of int are associated to the type amount of storage (size). Typically this is machine and architecture dependent

float or double

Floating-point literals (decimal point)
A variable type double can store roughly twice as many significant digits as can a variable type float
Floating and scientific notation (mantissa d. and exponent)
%f, %e, %g (if < 4 or > 5, %e is used)

float length = 190.4;
printf("Length= %f \n",length);

char

A variable of type char can store a single character (enclosed by quotes)
Special characters (e.g., '\n')
char characterVar = 'z';
printf("Character variable %c\n", characterVar);

Bool

A variable of type _Bool (or bool, if you include the header stdbool.h) takes only values 0 [or false] and 1 [or true]
When assigning, 0 stores 0 in the variable whereas any nonzero stores 1
_Bool booleanVar = 9;
printf("Boolean variable %i\n", booleanVar);

Displaying Values of Variables:

print_variables.c

#include <stdio.h>

int main (void)
{
    int integerVar = 230;
    float floatVar = 626.32;
    double doubleVar = 3.24e+3;
    char charVar = 'a';
    _Bool boolVar = 0;

    printf("integer variable= %i \n",integerVar);
    printf("float variable = %f \n", floatVar);
    printf("double variable = %e \n", doubleVar);
    printf("char variable = %c \n", charVar);
    printf("boolean variable = %i \n", boolVar);

    return 0;
}

Output:

integer variable= 230
float variable = 626.320007
double variable = 3.240000e+03
char variable = a
boolean variable = 0

Const variables¶

The const qualifier is used for variables whose value will not change in the program

const int base = 10;

Any attempt to change its value will generate a compiler error message. Useful to avoid bugs: when you define a variable and use it multiple times in your code, it is also easier this way if you want to change its value, to only change it once where it is defined.