C introduction: part 1

Transcription

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2 What is C? C is a compiled language that gives the programmer maximum control and efficiency / 26

3 3 / 26 Outline Basic file structure Main function Compilation Declaration of variables Print Simple mathematical operations While and for loops Pointers Functions

4 4 / 26 Basic file structure These are the constituents of a minimum C program: Text file with C code, known as the main file (often called main.c) C-compiler (gcc, icc, etc) Executable (here called program)

5 5 / 26 Main function main.c: # include <stdio.h> int main () { printf (" Hello World!"); return 0; } The main file must contain the main function which calls other functions in main.c and in other C-files. The main function is declared as an integer and should therefore return an integer. If return is left out there will be an implicit return 0.

6 5 / 26 Main function main.c: # include <stdio.h> int main () { printf (" Hello World!"); return 0; } printf prints to standard output (terminal) The header file stdio.h is included to get the printf function. Header files contain instructions on how to use functions found in corresponding C files (stdio.c). Lines are terminated with semicolon (;)

7 5 / 26 Main function main.c: # include <stdio.h> int main () { printf (" Hello World!"); return 0; } In this presentation the rest of the code is placed in the main function if not stated elsewhere.

8 Compilation The program main.c is compiled from the command line by typing: gcc -o program main.c This creates an executable file called program which can be run by typing:./program The compilation also creates an object file called main.o 6 / 26

9 7 / 26 Compilation Flags can be added to tell the compiler to optimize: -03, -02, inhibit warnings: -W include libraries: -lm to include math library (always start with -l followed by library name)... gcc -o program main.c -O3 -W -lm

10 8 / 26 Declaration of variables In C, you need to declare all variables by type. int: integer double: floating point number with double precision FILE: file char: character double x; /* Declaration */ const double y =5; double z = 30; char B = 'B'; x = 10; /* Initialization */ const means that the value of the variable is not allowed to be changed.

11 9 / 26 Declaration of arrays Smaller arrays are most easily declared by adding its size in the declaration: double data [10]; /* array of doubles of length 10 */ int indices [25]; /* array of ints of length 25 */ double A [3][3]; /* 3x3 matrix of doubles */ The data in the arrays are accessed as follows: a = data [5]; /* sixth element in array */ i0 = indices [0]; /* first element */ a23 = A [1][2]; /* second row, third column */ Important! In C indices start at 0.

12 10 / 26 Macro definitions Constants can also be defined in the form of macros in the header: # define PI # define n 250 The value of these macros have no type, instead they are simple text substitutions, e.g. whenever n appears in the code 250 will be inserted as text. Important! Do not put a semicolon at the end of #define statements.

13 11 / 26 Print to standard output (stdout) To print to file or stdout functions in stdio are needed and they are loaded in the header of the main file: # include <stdio.h> To print to stdout the function printf is used: printf (" Hello World!"); double pi = 3.14; printf ("pi =%.2 f",pi); %.2f gives a double with 2 decimal places. %e gives an engineering string. Tip of the day! Google is your best friend for help with formatting in printf.

14 12 / 26 Print to file To print to file the functions fprintf, fopen, and fclose are used: FILE * outputfile ; /* Declare FILE variable */ outputfile = fopen (" ToDoList. txt "," w"); /* Open file with write option */ fprintf ( outputfile, " Eat \n"); /* Write to file */ fprintf ( outputfile, " Drink \n"); fprintf ( outputfile, " Sleep "); fclose ( outputfile ); /* Close the file */ \n gives a new line, \t gives a tab. The asterix (*) sign indicates a pointer variable which refers to an address in memory rather than the value of the variable.

15 13 / 26 Simple mathematical operations Simple mathematical operations are of usual syntax: x = y + z; x = (y/z -1) *5; x = 7/5; /* = 1, Integer division */ x = ( double ) 7 / ( double ) 5; /* = 1.4, float division */ However, be careful when mixing variables of different type: double x = 5.5; int i = 5; int u = x - i; /* i will be converted to double, but the result 0.5 will be converted ( truncated ) to int. */ Note! Please note the integer division!

16 Simple mathematical operations Special functions can be used if math.h is included in the header and -lm is added when compiling: # include <math.h> Example of functions: r = sqrt (y); x = cos (2* PI*f*dt*i); x = exp (y); Useful link math_h.htm 14 / 26

17 15 / 26 if-else statements The if-else statement has the following syntax if ( Boolean ) { Do something ; } else { Do something ; } Example: if (x >= 0) { printf ("X is positive "); } else { printf ("X is negative "); }

18 16 / 26 Boolean operators Boolean operators have a usual syntax: == : equal >= : larger or equal!= : not equal...

19 17 / 26 While loop The syntax of the while loop is as follows: while ( condition ) { Do something ; } Example of usage: int x = 0; while (x <4) { printf ("%d\n",x); x ++; /* x = x+1 */ } This will print integers from 0 to 3 on new lines.

20 18 / 26 for loop The syntax of the for loop is as follows: for ( int i = start_value ; i <= end_value ; i ++) { Do something ; } Example of usage: for ( int i = 0; i < 5; i ++) { printf (" %.3 f",cos (2* PI*f*dt*i)); } The declaration of the loop variable can also be declared before the loop making it accessible after the loop.

21 19 / 26 Pointers The address in memory of a variable is accessed by the ampersand (&) sign: int i = 5; printf (" Value of i variable : %d\n", i ); printf (" Address of i variable : %x\n", &i ); Output: Value of i variable: 5 Address of i variable: bffd8b3c A pointer is a variable that stores the address of a variable.

22 20 / 26 Pointers A pointer is declared using the asterix (*) symbol: int i = 1; /* actual variable declaration */ int * ipointer ; /* pointer variable declaration */ ipointer = & i /* Store address of variable i in pointer variable */ The variable value that the pointer points to is accessed with the asterix (*) symbol: /* address stored in pointer variable */ printf (" Address stored in ipointer variable : % x\ n", ip ); /* access the value using the pointer */ printf (" Value of * ipointer variable : %d\n", *ip );

23 21 / 26 Pointers Why pointers? The amount of data passed during function calls is lowered if pointers are passed instead of variables. When doing dynamic memory allocation, pointers are required. (next lecture)

24 22 / 26 Functions Functions are placed outside the main function. You have already seen the main function: int main () { Some code ; return 0; } The int declaration means that the function should return an integer.

25 23 / 26 Functions General form of a function: functype funcname ( type1 * var1, type2 var2,...) { Some code ; return output ; /* output of type functype */ /* If functype is void nothing should be returned */ } If a variable is passed to a function a copy is created and sent, however, when passing pointers only the address is sent. Note! The asterix (*) is also used to tell the function to take a pointer instead of variable as input.

26 24 / 26 Functions To tell the main function what kind of input and output it should expect from the new function a prototype with declarations is added in the header of the main file. functype funcname ( type1 * var1, type2 var2,...) ; (In the prototype the variable names may be dropped but the types must be kept.) Important! It is crucial to pass variables of correct type to the function.

27 25 / 26 Functions Example function: void add ( double * var1, double var2 ) { * var1 += var2 ; /* value of pointer accessed by * */ /* Using pointers we can change the value of variables using functions */ } double x = 5; add (&x,5) ; /* &x is the pointer to x variable */ /* now x will have the value 10 */ Prototype: void add ( double * var1, double var2 );

28 Good luck with the exercises! 26 / 26