Learn About C / C++: November 2015

Kamis,19
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The Explanation About Stdarg in C

#Include <Stdarg.h>

To declare a function that will use a variable number of arguments, you must make sure you have at least one defined argument in the argument list; that is, a specific argument with a name and type associated with it. You can have as many real arguments as you would like; you just have to have at least one.

Following the last defined argument, you should place the C-symbol that indicates your use of a variable number of arguments, the ellipsis, or ... (three periods in a row). An example would look like this:

type FunctionName ( defined_args,... )

You control the access of the variable arguments with the following definitions/functions/macros provided. All of these references must only appear inside the body of a function that will have a variable number of arguments (as denoted by the ellipsis):

va_list varname;

This is the name of a structure that will maintain the information about the variable argument list, and it is therefore used in each of the following functions/macros. NOTE that the va stands for variable arguments. And example might be:

va_list args;

va_start (varname, last_defined_arg;

This function initializes the processing of the variable argument list. The first parameter corresponds to the variable name you used when you defined the va_list structure. The second parameter corresponds to the variable name of the last defined argument in the argument list in the function's definition.

This function must be called before you try to reference any of the variable arguments. If it is called more than one time inside a function, the processing of the list of variable arguments will start over again at the first variable argument.

Kamis,19
0

The Explanation About File in C

To use file I/O in C, you generally have stdio.h included at the top of your file (most C programs include this anyway):

#include <stdio.h>

In C, you declare a file pointer variable for any file you want to use. For example, the following line declares a file pointer variable called "infile" (our standard name for the file to be read from. You can call yours anything you like).

FILE *infile;

To open the file, you call the function "fopen()", passing it a filename and "r" for reading or "w" for writing. For example, to open a file called mydata.txt, you would use:

infile = fopen("mydata.txt","r");

Then to read from the file, you use a variation of scanf() called fscanf() which takes a file as its first parameter. For example, to read an integer from mydata.txt, you would use:

fscanf(infile,"%d",&myint);

Other than the fact that it's reading from a file, fscanf() is identical to fprintf().
Once you've read all the data you want to, close the file using the fclose() function:

fclose(infile);

To write to a file, you do the exact same thing, except you use "w" rather than "r" in the fopen() line and fprintf() rather than fscanf() to write the data out.

Potential Error Conditions:

Whenever you open a file, it's a good idea to check and make sure that there weren't any problems in doing so. Problems can include trying to open a file for reading when it doesn't exist. Or trying to open a file for writing in a directory that you don't have write permissions in. If your C file pointer is NULL after calling fopen() it indicates that there was a problem.
Reading or writing a file that had problems when opening it can lead to error conditions or your program crashing.

Creating Data Files

The data files that contain integers are simply text files that contain values for your array in sorted order. Thus, one way to create them would be to open your favorite editor and start typing numbers in, saving the file as a text file. However, it's likely that to get times that are large enough for you to see asymptotic trends, you'll have to run on problem sizes that are larger than you're willing to type in (1000 values? 10,000 values? 1,000,000 values?)

So... an easy way to create these data files is just to write a small program that creates them (free advice: always make computers do work that you're not willing to do yourself whenever you can). Writing this small program from scratch would be a great chance to practice your file I/O skills. Or if you want to, you can look at the simple ones below. Note, that we don't check to see whether there were problems opening the files. Note also that the consecutive numbers 1 through 10 may not be the best input set to cause the worst case performance for some of the search algorithms.

Example

#include <stdio.h>
void main() {
int i;
FILE *outfile;
outfile = fopen("out.dat","w");
for (i=0;i<10;i++) {
fprintf(outfile,"%d\n",i+1);
}
fclose(outfile);
}

The Explanation About Stdarg in C

The Explanation About File in C

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