Why am I learning C? Check out my blog post and this post. You can create a PDF of this README using pandoc:
pandoc README.md -o readme.pdf
You will need a compiler, such as gcc; I have some notes of my GitHub page on compiling.
To compile a specific example, type:
gcc input_output.c -o choose_number
or to compile all the examples, type:
make
The binaries will be stored in the bin folder.
Here's the source code of a simple C program:
#include <stdio.h>
/*
This is a comment
*/
int main(){
printf("Hello world.\n");
return 0;
}
The include directive tells the compiler to put code from the header called stdio.h into our program before creating the executable. The "main" function is called when the C program is executed, thereby running the printf() function. The return is used to tell the operating system whether the program has succeeded or not; a return value of 0 means success. Now to compile and to execute the program.
gcc hello.c -o say_hello
say_hello
#Hello world.
Data types are used to store different types of data; basic types include char, int, and float. A variable of type char stores a single character; type int stores integers; and type float stores decimal numbers. The keywords char, int, and float are used to declare a variable. For example, in data_type.c:
char my_char;
my_char = 'A';
printf("my_char -> %c\n", my_char);
int my_integer;
my_integer = 5;
printf("my_integer -> %i\n", my_integer);
float my_float;
my_float = 1.200000001;
printf("my_float -> %f\n", my_float);
Use if, else if, and else.
#include <stdio.h>
int main(){
float answer;
printf("What is the square root of 2?\n");
scanf("%f", &answer);
if (answer > 1.4141 && answer < 1.4143){
printf ("Correct!\n" );
} else if (answer > 1 && answer < 2){
printf("You are close!\n");
} else {
printf("Wrong!\n" );
}
return 0;
}
There are three types of loops: for, while, and do..while; see loop.c.
// for loop
int series[10] = {10, 20, 30, 40, 50, 60, 70, 80, 90, 100};
int i;
for (i = 0; i < 10; i++){
printf("%d\n", series[i]);
}
// while loop with break and continue statements
int n = 0;
while (1){
n++;
if (n % 2 == 1){
continue;
}
printf("%d is an even number\n", n);
// exit loop when n = 20
if (n == 20){
printf("Reached 20 loops\n");
break;
}
}
// do..while
do {
printf( "I will run once even when a condition is not met\n" );
} while (0);
Functions need to be declared right at the start or its prototype. Prototypes are declarations of functions; they are only necessary to alert the compiler about the existence of a function if we don't want to go ahead and fully define the function.
/*
The general format for a prototype
return-type function_name (arg_type arg1, ..., arg_type argN);
*/
int add (int a, int b);
// the actual function
int add (int a, int b){
return x + y;
}
See function.c for implementing functions with a variable number of arguments.
Switch case statements are a substitute for long if statements that compare a variable to several values.
#include <stdio.h>
int main(){
int input = 4;
switch (input) {
case 1:
printf("Input is %i\n", input);
break;
case 2:
printf("Input is %i\n", input);
break;
case 3:
printf("Input is %i\n", input);
break;
case 4:
printf("Input is %i\n", input);
break;
default:
printf("Input was not 1, 2, 3, or 4!\n");
break;
}
return 0;
}
Pointers point to specific memory locations.
/*
The pointer declaration looks like this:
<variable_type> *<name>;
For example, you could declare a pointer that stores the
address of an integer with the following syntax:
*/
int *points_to_integer;
int *pointer1, *pointer2;
Below is an example of obtaining the memory location and how to dereference a pointer.
#include <stdio.h>
int main(){
int x = 31;
int *p;
p = &x;
printf("The memory location of x is %p\n", p );
printf("The pointer, p, points to the value %d\n", *p );
return(0);
}
The function malloc, which resides in the stdlib.h header file, is used to initialise pointers with memory from free store. The argument to malloc is the amount of memory requested (in bytes), and malloc obtains a block of memory of that size and then returns a pointer to the block of memory allocated.
#include <stdlib.h>
float *pointer;
// the sizeof function takes an expression and returns its size
pointer = malloc(sizeof(*pointer));
// the free function returns memory to the operating system
free(pointer);
Structures provide a way of storing many different values in variables of potentially different types under the same name.
/*
The format for defining a structure is:
struct Tag {
Members
};
*/
Example code in structure.c:
#include <stdio.h>
struct gene {
int entrez;
int hgnc;
int omim;
};
int main(){
struct gene HBA1;
HBA1.entrez = 3039;
HBA1.hgnc = 4823;
HBA1.omim = 141800;
printf("The Entrez ID for HBA1 is %i\n", HBA1.entrez);
printf("The HGNC ID for HBA1 is %i\n", HBA1.hgnc);
struct gene *pointer;
pointer = &HBA1;
printf("The OMIM ID for HBA1 is %i\n", pointer->omim);
return 0;
}
C has no data type for strings; instead an array of characters is used: C-style strings are always terminated with a null character, a '\0' character (with the value of 0). The string below can store 99 letters.
char string[100];
Use fgets to read in a string from stdin:
#include <stdio.h>
int main(){
char string[256];
printf("Please a string: ");
fgets(string, 256, stdin);
printf( "You this string, %s", string );
return 0;
}
string.h is a header file that contains many functions for manipulating strings. Some examples are below.
// string comparison
int strcmp (const char *s1, const char *s2);
// string concatenation
char *strcat (char *dest, const char *src);
// string copying
char *strcpy (char *dest, const char *src);
// length of string
size_t strlen (const char *s);
For C File I/O you need to use a FILE pointer; you can think of it as the memory address of the file or the location of the file. Use fopen to obtain the FILE pointer, which allows you to perform functions on the file.
FILE *fopen(const char *filename, const char *mode);
Code to open and read a file (see read_file.c):
#include <stdio.h>
int main(){
FILE *fr;
char buff[255];
fr = fopen("etc/transcript.txt", "r");
while(fgets(buff, 255, fr) != NULL){
printf ("%s", buff);
}
fclose(fr);
return 0;
}
I am attempting to solve Rosalind problems using C. The solutions will be stored in the rosalind folder.
Some notes on my Wiki.