Basic Common Unix commands: Change to directory d

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1 Basic Common Unix commands: cd d Change to directory d mkdir d rmdir d mv f1 [f2...] d mv d1 d2 ls [d] [f...] ls -1 [f...] vi [f] emacs [f] more f cp f1 f2 mv f1 f2 rm f gcc [-o f1] f2 gnuplot Create new directory d Remove directory d Move file f to directory d Rename directory d1 as d2 List files in directory List files in detail Vi fullscreen editor Emacs fullscreen editor List file contents by screen Copy file f1 into f2 Rename file f1 as f2 Delete (remove) file f GNU C compiler Run plotting program gnuplot C Language A First For any C programming, it involves the following four steps. Saluting the World. Step 1. Create the source file Use your favorite editor (if you are working in cygwin environment, you even can use notepad as the editor) to type the following lines (you may skip the comments): #include <stdio.h> // main function. All the c programs must have a main function. // begin of main function printf("hello world!\n"); //call printf which is an outpuf function from stdio.h library. //the \n is the new line character // end of main function. Save the file with a name of your choice, for example, hellowrold.c. Make sure that the file has an extension c. Unix and C Page 1

2 Step 2. Compile the source file Under unix environment, go to the directory where the hellowrld.c is saved, type: gcc helloworld.c o hellworld If there is no error message reported by the compiler, then the source code has passed the compiling and an executable file helloworld should be created under the same directory. Step 3. Debug the source file If you get error or complain in the compiling process, you need go back to step 1 and fix it; then compile the source file until no error is reported. Step 4. Run the executable file: At the working directory, type the name of executable file, in this case, helloworld, you will see a line of Hello World! printed out on your screen. Example 2 Add two numbers A C program can have multiple functions. The following example defines a function called add_two_numbers which is then twice called by the main function. #include <stdio.h> add_two_numbers(2000, 8); // function call add_two_numbers(2000,- 8); // function call add_two_numbers(int a, int b) // a function which prints the sum of two integers a and b int c; // define c as integer type c = a + b; printf( The sum of %d and %d is %d.\n, a, b, c); Compile the program and run it, you should see the following line on your screen: The sum of 2000 and -8 is The sum of 2000 and 8 is A Quick Tutorial on C Variables in C A variable must be declared before it can be sued. A declaration begins with the type, followed by the name of one or more variables. int high, low, results[20]; Declarations can also be spread out. int high; int low; int results[20]; The most common types are: int an integer float a floating point number (real number) double a double precision floating point number char a single byte of memory to hold a character Unix and C Page 2

3 Local variables and global variables Local variables are declared within the body of a function and can only be used within that function; Global variables are located outside any of the program s function; usually this is done at the beginning of the program. Constants Number constants, examples: 1, 0.5, 12.e9. Character constansts, examples: a, b, \n Arrays An array is a collection of variables of the same type. Array elements are identified by an integer index with zero for the first element. int results[20]; float data[200]; For higher dimensions; int results[20][10]; As an example, here is a simple function to add up all of the integers in a single dimensioned array. int add_array(int array[], int size) // the size of array is determined when this function is called int i; int total = 0; for(i = 0; i < size; i++) total += array[i]; return(total); Expressions and operators Arithmetic operators: +, -, *, /, % (remainder from integer division) Assignment statement y=(m*x)+c ; which will save the value of the expression in variable y. Shorthand for some expressions i++; equivalent to: i=i+1; i--; equivalent to: i=i-1; i += 10; i=i+10; i -=10; i=i-10; i *=10; i=i*10; i /=10; i=i/10; Type conversion You can mix the types of values in your arithmetic expressions. But sometime you need apply type conversion to avoid trouble. int a=12, b=24; float x; x=(float)a/(float)b; Unix and C Page 3

4 The cast is made by putting the bracketed name of the required type just before the value. Comparison C notation Meaning == Equal to > Greater than < Less than >= Greater than or equal to <= Less than or equal to!= Not equal to Note that == is used in comparisons and = is used in assignments. float max(float a, float b) float max; if (a>=b) max=a; else max=b; return max; Logical oconnectors: && and or! not Check if a number is even or odd. If a number is even, the function returns 1; otherwise, it returns 0. int even_odd(int a) If(a!=0 && (a%2==0)) return 1; else return 0; Control statements If else statement Example. if(result >=60) printf( Pass\n ); else printf( Fail\n ); The loop statement Unix and C Page 4

5 While, do while and for Add the first 100 positive integers: int sum_of_1_100() int i,sum; i=1; sum=0; while (i<=100) sum=sum+i; i=i+1; return sum; Or int sum_of_1_100() int i,sum; sum=0; for(i=1;i<=100;i++) sum=sum+i; return sum; or int sum_of_1_100() int i,sum; i=1; sum=0; do sum=sum+i; i=i+1; while(i<=100); return sum; Functions in c A c program can have multiple functions besides the main function. Let s use the following example to explain all the elements of a function. a function which converts the temperture from of to oc: double convert(double Tf) double Tc; Tc=(Tf-32)*5./9.; Return Tc; Let us examine the details of this function: double convert(double Tf) Unix and C Page 5

6 This line is the function definition. It tells us the type of the return value is double; the name of the function is convert; the argument used by the function is Tf. Another example which return noting (void): void error_message(int line) printf( Error in line %d\n, line); Return value in function using a pointer There is another way to return a value through function, which is using a pointer. A pointer points to the address of a piece of memory. int *res; this tells us that res is a pointer to a address which stores an integer, and the value stored in the memory is *res. void convert (double Tf, double *Tc) *Tc=(Tf-32)*5./9.; To call the above function, double a=73, b; convert(a,&b); // we created a pointer to double by using &b which represents the address of b. Test the following program to get a better understanding. #include <stdio.h> void convert(double a, double *b); double a=73.,b; convert(a,&b); printf("tem is %lf \n",b); printf("address of b is %d\n",&b); void convert(double a, double *b) *b=(a-32.)*5./9.; printf("inside address is %d\n",b); Input and output: scanf and printf For formatted input or output, Control string entry What gets printed %d A decimal integer %f A floating value Unix and C Page 6

7 %lf A double precision floating number %e Exponential representation %c A character %s A character string float x; scanf( %f\n,&x); // scanf needs to know the address of the items to be read! printf( the input data is %f\n,x); You can also insert numbers into the control strong to control field widths for values to be printed. printf( %10.5\n,x); This will display x in a number of a filed 10 spaces wide with room to show 5 decimal places. File Handling C communicates with files using a file pointer, FILE *. Example, a file pointer fp is declared below: FILE *fp; Open a file: fp=fopen( output_file, w ); It takes two string araguments, the first is the name of the file to be opened, the second is an access character which can be one of r, w or a (for reading, writing or appending, respectively). Close a file: fclose(fp); this releases the file pointer fp. C language keywords: Auto break case char continue default do double else enum extern float for goto if int long register return short sizeof static struct switch typedef union unsigned void while. These keywords have already defined in the C language and cannot be re-defined. Guidelines for Writing a Code 1. Write all code so that anybody unfamiliar with the work can understand it as quickly as possible. a. Start all subprograms with a header block of comments, clearly defining the purpose of the subprogram, defining and delineating all input arguments and output arguments, and giving your name and date the subprogram was created. b. Define all significant variables c. Add comments describing the purpose of each key subsection of the subprogram. 2. Structure code to make it as readable as possible a. Use indentation b. The main program should act as an outline with calls to subprograms which actually perform desired operations Unix and C Page 7