Tag Archives: c
Some arg checking and file IO in C
This small program reads a filename from the command line, and reads the file, while printing the line number in front of it.
// Line Number. #include <stdio .h> void printnumber(int number); int main(int argc, char *argv[]) { int c; int lines = 1; FILE *fh; if (argc < 2) { printf("ERROR: Need a filename as argument.\n"); return 1; } fh = fopen(argv[1], "r"); printnumber(lines); while ((c = getc(fh)) != EOF) { if (c == '\n') { lines++; printnumber(lines); } else { printf("%c", c); } } return 0; } void printnumber(int number) { printf("\n%3d|", number); }
Arrays 5[a] == a[5]
First time I saw that I thought it was weird.
Thinking more about it, it actually makes a lot of sense.
Consider the a array, with the 6 elements
The memory address of this array is a.
The first element of the array evaluates to *(a).
So, the 6th element can be written as *(a + 5).
And as *(5 + a), thus 5[a].
C enums
#include <stdio .h> enum months { JAN = 1, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC }; const char *monthName[] = { "", "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" }; int main() { enum months month; for (month = JAN; month < = DEC; month++) { puts(monthName[month]); } return 0; }
C pointers
* Once a variable is declared, we can get its address by preceding its name with the unary & operator, as in &k.
* We can “dereference” a pointer, i.e. refer to the value of that which it points to, by using the unary ‘*’ operator as in *ptr.
* An “lvalue” of a variable is the value of its address, i.e. where it is stored in memory.
The “rvalue” of a variable is the value stored in that variable (at that address).
#include <stdio .h> int main() { int k=3; int *ptr; ptr = &k; printf("The value ptr is pointing to:%d at address:%p\n", *ptr, ptr); } </stdio>
C prinft formatting
Where specifier is the most significant one and defines the type and the interpretation of the value of the coresponding argument:
| specifier | Output | Example |
|---|---|---|
| c | Character | a |
| d or i | Signed decimal integer | 392 |
| e | Scientific notation (mantise/exponent) using e character | 3.9265e+2 |
| E | Scientific notation (mantise/exponent) using E character | 3.9265E+2 |
| f | Decimal floating point | 392.65 |
| g | Use the shorter of %e or %f | 392.65 |
| G | Use the shorter of %E or %f | 392.65 |
| o | Signed octal | 610 |
| s | String of characters | sample |
| u | Unsigned decimal integer | 7235 |
| x | Unsigned hexadecimal integer | 7fa |
| X | Unsigned hexadecimal integer (capital letters) | 7FA |
| p | Pointer address | B800:0000 |
| n | Nothing printed. The argument must be a pointer to a signed int, where the number of characters written so far is stored. | |
| % | A % followed by another % character will write % to stdout. |
Strings in C
C doesn’t know about strings. It knows about characters, and thus, a string is an array of characters like so:
char the_string[] = "this is my fancy string";
Another way to allocate a number of chars is this:
char the_string[50];
To show that its really an array:
#include <stdio .h> int main() { char some_array[] = {'a', 'b', 'c'}; printf("%s\n", some_array); return 0; } </stdio>
Which yields:
abc
Here we create an array of characters named the_string. A string in C is terminated by the “\0″ (NULL) character.
Example of passing a string to a method called print:
#include <stdio .h> void print(char string[]) { printf("%s\n", string); } int main() { print("test 1 2 3"); return 0; } </stdio>
C function pointers
C specifier meanings
%c – Print a character
%d – Print a Integer
%i – Print a Integer
%e – Print float value in exponential form.
%f – Print float value
%g – Print using %e or %f whichever is smaller
%o – Print actual value
%s – Print a string
%x – Print a hexadecimal integer (Unsigned) using lower case a – F
%X – Print a hexadecimal integer (Unsigned) using upper case A – F
%a – Print a unsigned integer.
%p – Print a pointer value
%hx – hex short
%lo – octal long
%ld – long