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<nowiki>Here we try out a small program that computes the value of Pi using the </nowiki> <nowiki>following formula:</nowiki> <nowiki></nowiki> <nowiki>Pi = 4 * (1 - 1/3 + 1/5 - 1/7 + 1/9 - ... )</nowiki> <nowiki></nowiki> <nowiki>Here is the code:</nowiki> <nowiki></nowiki> <nowiki> 1 #include <stdio.h></nowiki> <nowiki> 2 #include <math.h></nowiki> <nowiki></nowiki> <nowiki> 3 int main(int argc, char *argv[]){</nowiki> <nowiki> 4 double t, k = 3.0, l = -1.0;</nowiki> <nowiki> 5 int i, s;</nowiki> <nowiki></nowiki> <nowiki> 6 if(argc < 2){</nowiki> <nowiki> 7 fprintf(stderr, "%s <number of iterations>\n", argv[0]);</nowiki> <nowiki> 8 exit(1);</nowiki> <nowiki> 9 }</nowiki> <nowiki></nowiki> <nowiki> </nowiki> <nowiki> 10 t = 1.0; </nowiki> <nowiki> 11 for(i = 0, s = atoi(argv[1]); i < s; i++){</nowiki> <nowiki> 12 t += l/k; </nowiki> <nowiki> 13 k += 2.0; </nowiki> <nowiki> 14 l *= -1.0; </nowiki> <nowiki> 15 }</nowiki> <nowiki> 16 t *= 4; </nowiki> <nowiki> 17 printf("My value of Pi: %1.6f, math.h's value of Pi: %.16f\n", t, M_PI); </nowiki> <nowiki> 18 printf("Absolute difference: %.16f\n", fabs(M_PI - t)); </nowiki> <nowiki></nowiki> <nowiki> 19 return 0; </nowiki> <nowiki> 20 }</nowiki> <nowiki></nowiki> <nowiki>To compile the program, remove all lines numberings, those are just there to </nowiki> <nowiki>make it easier to go through the code. Then type in your shell, provided that </nowiki> <nowiki>you save the code in a file called pi.c:</nowiki> <nowiki></nowiki> <nowiki> % gcc -o pi pi.c -lm</nowiki> <nowiki></nowiki> <nowiki>We compile pi.c to a binary called pi. -lm tells the linker (a part of the </nowiki> <nowiki>compiler) that we want to use math libraries. Using the -lm flag is not always </nowiki> <nowiki>neseccary though. Anyway, to run the program type the following in your shell:</nowiki> <nowiki></nowiki> <nowiki> % ./pi 1000</nowiki> <nowiki></nowiki> <nowiki>The number 1000 is the number of iterations we would like to do in the main </nowiki> <nowiki>loop, i.e. how many items we want to use in the formula above. You should get </nowiki> <nowiki>something like this:</nowiki> <nowiki></nowiki> <nowiki> $ ./pi 1000</nowiki> <nowiki> My value of Pi: 3.142592, math.h's value of Pi: 3.1415926535897931</nowiki> <nowiki> Absolute difference: 0.0009990007497471</nowiki> <nowiki></nowiki> <nowiki>Lets dissect the code. On line 1 and 2 we include some standard libraries.</nowiki> <nowiki></nowiki> <nowiki> 1 #include <stdio.h></nowiki> <nowiki> 2 #include <math.h></nowiki> <nowiki></nowiki> <nowiki>stdio.h contains standard input/output functions such as printf and alike. </nowiki> <nowiki>math.h contains math functions such as sin, fabs, etc. What are those .h files </nowiki> <nowiki>anyway? It demands some explanation. Assume you would like to use some </nowiki> <nowiki>function like printf in your program. This function is nothing you have written </nowiki> <nowiki>yourself, the function comes in compiled form in your standard libraries. When </nowiki> <nowiki>the program is compiled a program called the linker (often ld, try man ld) </nowiki> <nowiki>tries to figure out how to call functions in these standard libraries. This is </nowiki> <nowiki>something that more or less just happens, as a new C programmer this is nothing </nowiki> <nowiki>you have to worry about. What is important here is that the compiler wants to </nowiki> <nowiki>know what printf looks like, i.e. how many arguments it wants, return type etc. </nowiki> <nowiki>Information about this is included in the .h files, which often is referred to </nowiki> <nowiki>as a header file (.h for header). We often talk about function prototypes, i.e. </nowiki> <nowiki>a description of the function. But why? So that the compiler can complain when </nowiki> <nowiki>you try to use a function is some way that is considerer incorrect, using the </nowiki> <nowiki>wrong number of arguments for instance. Hope that sorts it out.</nowiki> <nowiki></nowiki> <nowiki>What about this like then?</nowiki> <nowiki></nowiki> <nowiki> 3 int main(int argc, char *argv[]){</nowiki> <nowiki></nowiki> <nowiki>All C programs are composed of functions more or less. The compiler looks for </nowiki> <nowiki>a function called main. When the program is executed, that is by your shell, </nowiki> <nowiki>control is transfered from the shell to the place where main starts. I will </nowiki> <nowiki>later try (supposed I have some spare time) to explain how to write a shell, </nowiki> <nowiki>something that is easier than it might seem. Also there are two arguments to </nowiki> <nowiki>this function. int argc, char *argv[]. int argc is the number of arguments to </nowiki> <nowiki>the program and char *argv[] are those arguments. Arguments? These are the ones </nowiki> <nowiki>you provided via the command line when you started the program, "1000" as an </nowiki> <nowiki>example. However the programs name is always the first argument. If we look at </nowiki> <nowiki>the example where we did:</nowiki> <nowiki></nowiki> <nowiki> % ./pi 1000</nowiki> <nowiki></nowiki> <nowiki>Then *argv[] looks like {"./pi", "1000"}. That also means that argc = 2, NOT 1 </nowiki> <nowiki>as you might expect. That means that the first argument that you are likely to </nowiki> <nowiki>be interested in is argv[1]. As an exercise you could try to write a program </nowiki> <nowiki>that prints it's own name.</nowiki> <nowiki></nowiki> <nowiki>Here we declare some variables that we use later on:</nowiki> <nowiki></nowiki> <nowiki> 4 double t, k = 3.0, l = -1.0;</nowiki> <nowiki> 5 int i, s;</nowiki> <nowiki></nowiki> <nowiki>Nothing much about these really, just keep in mind that variables should be </nowiki> <nowiki>declared in the beginning of each code block. That is the beginning of </nowiki> <nowiki>functions, loops, if statements etc. Quiet frankly everything that looks like </nowiki> <nowiki>this is valid syntax:</nowiki> <nowiki></nowiki> <nowiki> {</nowiki> <nowiki> int a, ...</nowiki> <nowiki> }</nowiki> <nowiki></nowiki> <nowiki>Newer versions of gcc allows you to declare variables everywhere, but try to </nowiki> <nowiki>avoid that. You probably want your program to be compatible with older versions </nowiki> <nowiki>of gcc and other compilers.</nowiki> <nowiki></nowiki> <nowiki>Here we check that the user have supplied the correct number of arguments to </nowiki> <nowiki>the program:</nowiki> <nowiki></nowiki> <nowiki> 6 if(argc < 2){</nowiki> <nowiki> 7 fprintf(stderr, "%s <number of iterations>\n", argv[0]);</nowiki> <nowiki> 8 exit(1);</nowiki> <nowiki> 9 }</nowiki> <nowiki></nowiki> <nowiki> </nowiki> <nowiki></nowiki> <nowiki>Recall that argc is the number of arguments to the program. If these are less </nowiki> <nowiki>than 2 we have a problem. We print out a message, on line 7, that tells the </nowiki> <nowiki>user that he/she tried to use the program in an incorrect way. Then we exit the </nowiki> <nowiki>program with the value 1. Why 1? I'll explain this at the end of this example, </nowiki> <nowiki>bare with me for a while. We use the function fprintf which could be </nowiki> <nowiki>interpreted as: File-PRINT-Format. FILE here refers to the file stderr, which </nowiki> <nowiki>is a standard file of type FILE*. See the example ConnectingASocketInC for a </nowiki> <nowiki>better explanation about this. PRINT simply refers to printing, that is writing </nowiki> <nowiki>something to the shell. Format means that want to format the output in some </nowiki> <nowiki>sense. The %s in "%s <number of iterations>" refers to a string, that is the </nowiki> <nowiki>second argument given to fprintf, namely argv[0]. Check your manpages for </nowiki> <nowiki>fprintf (that is man fprintf) for a lot more details.</nowiki> <nowiki></nowiki> <nowiki>Simply set t to 1.0.</nowiki> <nowiki></nowiki> <nowiki> </nowiki> <nowiki> 10 t = 1.0; </nowiki> <nowiki></nowiki> <nowiki>This for-loop can be interpreted as follows. First set i to 0, then s to </nowiki> <nowiki>atoi(argv[0]). Observe that we do both these things before the first ;. This </nowiki> <nowiki>part is just done once, that is when we enter the loop. What is this atoi? The </nowiki> <nowiki>argument "1000" in the example above, i.e. the number of iterations we want to </nowiki> <nowiki>do, comes as a string, not a number. Therefor we have to convert this string </nowiki> <nowiki>to a number. atoi does this, (man atoi).</nowiki> <nowiki></nowiki> <nowiki>This loop will loop as long as i is less than s. That is i < s. This test is </nowiki> <nowiki>done at the beginning of the loop. In every iteration of the loop i will be </nowiki> <nowiki>incremented by one. This is done at the end of the loop.</nowiki> <nowiki></nowiki> <nowiki> 11 for(i = 0, s = atoi(argv[1]); i < s; i++){</nowiki> <nowiki></nowiki> <nowiki>Here, compute t += l/k. This corresponds to some random part of the formula, </nowiki> <nowiki>for example: -1/3 or 1/5.</nowiki> <nowiki></nowiki> <nowiki> 12 t += l/k; </nowiki> <nowiki></nowiki> <nowiki>Increment k by two, so if k = 3 before this, then k = 5 after this.</nowiki> <nowiki></nowiki> <nowiki> 13 k += 2.0; </nowiki> <nowiki></nowiki> <nowiki>Multiply l by -1.0, something that simply changes the sign of l. -1*-1 = 1, </nowiki> <nowiki>math you know.</nowiki> <nowiki></nowiki> <nowiki> 14 l *= -1.0; </nowiki> <nowiki></nowiki> <nowiki>This loop is as said repeated say 1000 times, after that we multiply the result </nowiki> <nowiki>by 4, according to the formula.</nowiki> <nowiki></nowiki> <nowiki> 16 t *= 4; </nowiki> <nowiki></nowiki> <nowiki>Now lets print the result. Here again we use formated printing. But where is </nowiki> <nowiki>the stderr thing? Shouldn't we write to a file? Well this is done implicitly. </nowiki> <nowiki>Since C programmers like to print a lot, there is a standard function printf </nowiki> <nowiki>that is the equivalent of fprintf(stdout, ..., ...). We could as well have </nowiki> <nowiki>written fprintf(stdout, "My value of Pi ... ", ...);</nowiki> <nowiki></nowiki> <nowiki> 17 printf("My value of Pi: %.16f, math.h's value of Pi: %.16f\n", t, M_PI); </nowiki> <nowiki> 18 printf("Absolute difference: %.16f\n", fabs(M_PI - t)); </nowiki> <nowiki></nowiki> <nowiki>What is this format %.16f? f is for float, we want to print a floating point </nowiki> <nowiki>number, also we want 16 significant decimals. This .16 thing is just a way to </nowiki> <nowiki>tell the how many decimals se want, nice huh? M_PI is a constant from math.h </nowiki> <nowiki>that simply is set to the value of Pi, using as high accuracy as the computer </nowiki> <nowiki>allows. We also use the function fabs that calculates the absolute value of a </nowiki> <nowiki>value. fabs could well be something like:</nowiki> <nowiki></nowiki> <nowiki> double fabs(double a){</nowiki> <nowiki> if(a >= 0.0){</nowiki> <nowiki> return a; </nowiki> <nowiki> }</nowiki> <nowiki> return -a; </nowiki> <nowiki> }</nowiki> <nowiki></nowiki> <nowiki>At last we return 0. What is the deal with returning things? Is there any </nowiki> <nowiki>function that checks the value? I haven't written anyone anyway. Simply put, if </nowiki> <nowiki>you return anything from main or use the function exit(value); This value is </nowiki> <nowiki>returned to the shell. The return value is normally interpreted in fashion that </nowiki> <nowiki>0 means, "everything is alright, program did NOT crash", everything else means </nowiki> <nowiki>"The program crashed.". The return value is a way to tell the shell how things </nowiki> <nowiki>went wrong. What a value of 23 would mean is quiet undefined, it's more or less </nowiki> <nowiki>up to the program to decide that it means. But it's quiet useful when a </nowiki> <nowiki>shellscript executes a program. For example, shellscript start_daemon.sh tries </nowiki> <nowiki>to start a random daemon, like a mail server, if something goes wrong the error </nowiki> <nowiki>code can be used to print a message "/var/tmp is full, free at least 10 Mb </nowiki> <nowiki>space.".</nowiki> <nowiki></nowiki> <nowiki> 19 return 0;</nowiki> <nowiki></nowiki> <nowiki></nowiki> <nowiki></nowiki> [[Category:Computer_Technology]] [[Category:Programming]] [[Category:C]]

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