updated 21 April 2022
TOC
- Intro
- Intro To C
- Data Types
- Flow Control
- Loops
- Pointers
- Functions
- Structures
- Command Line Arguments
- Dynamically Allocated Arrays
- File Input Output
After this, you can study a good example how these things are being applied in software development:
https://github.com/xkcph2017x/CalC
Thanks for visiting!
We prepared a detailed documentation for you. You don't even need to compile the source codes because there are screenshots of the results!
Much more, we are willing to collaborate and communicate! This is all about learning C, so we might not be able to put every information, but we are willing to answer your queries.
The source codes that were mentioned in this tutorial can be downloaded from this repository:
https://github.com/jdevstatic/C-Language-Tutorial
or by clicking the View on GitHub button.
Also, in order for you to truly collaborate, you need a GitHub account and you need to be at GitHub too. There, you can fork this repository, make some changes then create a pull request and many more features. It's quite strange at first but this is the best way to develop a software as one team. Furthermore, GitHub can be used for blogging and can be a collaborative tool for non-code projects! Also, this can be your portfolio as software developer, so why not start exploring now. It's worth it. This is the link:
Although not all the topics here were covered, but you can easily grasp the idea because of this tutorial. Enjoy life full of coding projects!
It's very important to understand why C language is there and why it is not obsolete, as others believe.
C was created on the premise that it would be the tool to make utilities of the existing Unix. And later, Unix was rewritten using C. But before C, there were problems that could not be addressed by existing languages back then, such as:
-
simplifying the existing assembly languages to write programs but should be almost of the same speed as them
-
each assembler basically responds to a particular machine architecture
-
existing languages such as COBOL, FORTRAN and BASIC were not meant to be for structured programming
When C was introduced, it was the programming language that answered these problems. And then, there came a time, programs were complex enough that structured programming would not suffice. So, object-oriented programming (OOP) came into existence through several programming languages like C++ and Java. But remember, C is still the one for system programming, say, creating a programming language, an operating system, and the like.
And since C language upgraded the world of computer programming, its influence can be felt in almost all programming languages after C. Hence, it will greatly help those who are still learning computer programming in general, as learning C will provide a solid background because most of the programming languages adopted concepts from it.
When using MinGW in Windows OS, in the Command Prompt, change the directory to where you put your source codes, say, on the Desktop.
And using hello_world.c as an example, type:
gcc hello_world.c
hit Enter, then type:
a.exe
You will see the output in your console.
When you want a unique name for your executable, type:
gcc -o hello_world.exe hello_world.c
hit Enter, then type:
hello_world.exe
Make sure that GNU Make is installed in your computer. Windows OS does not have this tool by default. So, visit this site for more information, including how to install it in Windows OS:
http://gnuwin32.sourceforge.net/packages/make.htm
GNU Make is a build tool that can run commands automatically. It is used in large projects, so programmers would not end up manually updating every file that catches a lot of errors.
Here, we want to use Make as part of the demonstration, to prepare you using this build tool.
First, you open the Command Prompt. Change the directory to where you put your codes. Then,
-
to build all executables, type:
make -
to build a specific executable, type:
make name_of_c_program -
to delete all executables, type:
make clean
When you tell the Command Prompt to execute
the command make, it actually executes the
contents of the Makefile. So, the computer and
this tool will automate things for you, particularly,
in large projects to avoid errors and build
the software with ease.
In Windows OS, it is best to use MinGW C compiler. Please refer to http://www.mingw.org/ for full documentation.
Compiler is a software that takes the job of translating source codes into machine language, one way or the other.
An assembly language, even though low-level, will still be needing an assembler to translate everything into an instruction the machine can understand. So, every source code will always be translated to machine language. But, it is not up to us, humans, to do the translation manually. That is the reason why writing compilers takes a lot of skills and intellect.
Technology companies usually offer their own compilers. They created them according to their own designs, to satisfy their needs or maybe to topple every business competitor.
ACCRE licensed this repo under MIT.
xdvrx1 included that license and the changes he made were also licensed under MIT.
#include <stdio.h>
int main()
{
printf("Hello World!\n");
return 0;
}
Hello World!
In C, including files are common, just like in our example:
#include <stdio.h>
It is just like copying the content of another source file and pasting it to the other one. Why not just copy and paste manually? We usually don't do that, it can cause errors. Hence, the need for this command.
Separating one big source code into small ones is a good practice in software development. Specific parts can be located easily. The logic among them can be inferred without much hassle.
This defines a function called main:
int main()
{
printf("Hello World!\n");
return 0;
}
The return type is an int (integer)
and it takes no arguments.
The main function is required for all
C programs. It's where execution of the
code begins.
I tend to use spaces to make blocks
of code more readable, you can also
use tabs.
This line calls a function called printf:
printf("Hello World!\n");
and passes a "Hello World!" string, along
with a special end of line character \n.
A semi-colon ; is required at the end of
each line. Take note, free-form languages just like C
does not terminate a command by simply pressing
enter or return to create a newline, it will
still look for the semicolon as the indicator that
a command is ended.
printf is actually printing output to stdout.
This topic is quite complex for this introduction,
but in basic terms, printf is one way to output
something. Much of the
hidden process is happening in the processor.
Again, the return type for the main function is an integer.
return 0;
Here, we return the integer zero, which is the convention for signifying that the program exited normally. You actually don't need to include this line at all. You might get a warning from the compiler but the program will compile and run normally.
#include <stdio.h>
int i;
int j = 12;
const int k = 345;
unsigned int l = 6789;
char myletter[] = "myletter";
float x = 1.23;
double y = 45.6789;
int myarray[5] = { 2, 4, 6, 8, 10 };
float farray[100];
int main()
{
i = 1;
printf("int i: %d\n", i);
printf("int j: %d\n", j);
printf("const int k: %d\n", k);
printf("unsigned int l: %d\n", l);
printf("char myletter[]: %s\n", myletter);
printf("float x: %f\n", x);
printf("double y: %f\n", y);
printf("(@index 0) int myarray[0] : %d\n", myarray[0]);
printf("(@index 0) float farray[0] : %1.2f\n", farray[0]);
return 0;
}
int i: 1
int j: 12
const int k: 345
unsigned int l: 6789
char myletter[]: myletter
float x: 1.230000
double y: 45.678900
(@index 0) int myarray[0] : 2
(@index 0) float farray[0] : 0.00
Data types are very important in a programming language. In basic terms, they tell how data should be treated by the compiler. Other programming languages are just loosely type, but still, they have data types.
Declare an integer:
int i;
Declare and initialize an integer:
int j = 12;
Declare and initialize a constant integer:
const int k = 345;
Declare and initialize an unsigned integer:
unsigned int l = 6789;
Declare and initialize a string called "myletter":
char myletter[] = "myletter";
Take note: In C, strings are just one-dimensional array of characters.
Declare and initialize a single-precision floating point number:
float x = 1.23;
Declare and initialize a double-precision floating point number:
double y = 45.6789;
Arrays are contiguous in memory and can be accessed like so:
myarray[0], myarray[1], ...
Note that the first index is always 0 in C!
Declare and initialize a 5-element integer array:
int myarray[5] = { 2, 4, 6, 8, 10 };
Declare a 100-element array of single-precision floating point numbers:
float farray[100];
#include <stdio.h>
int main()
{
int i = 1;
if ( i ) printf("i is true with a value of: %d\n", i);
i = 123;
if ( i ) printf("i is true with a value of: %d\n", i);
i = -321;
if ( i ) printf("i is true with a value of: %d\n", i);
i = 0;
if ( i ) {
printf("i is true with a value of: %d\n", i);
}
else {
printf("i is false with a value of: %d\n", i);
}
int y = 26;
if ( i > 0 && y == 26 ) {
printf("Condition 1 met!\n");
}
else if ( i > 0 || y == 25 ) {
printf("Condition 2 met!\n");
}
else if ( i != 0 ) {
printf("Condition 3 met!\n");
}
else if ( !i && y >= 20 ) {
printf("Condition 4 met!\n");
}
else if ( i == 0 ) {
printf("Condition 5 met!\n");
}
else {
printf("no condition met!\n");
}
if ( y == 26 ) {
if ( i != 0 ) {
printf("i is not 0!\n");
}
else {
printf("i is 0\n");
}
}
return 0;
}
i is true with a value of: 1
i is true with a value of: 123
i is true with a value of: -321
i is false with a value of: 0
Condition 4 met!
i is 0
We stick first to conditional statements, because this can alter the execution of the program by making decisions and skipping some parts of the code.
Native Boolean data type does not exist in C.
Integer is used instead, with a
value of zero evaluated as FALSE and a non-zero
value evaluated as TRUE.
This will resut to TRUE because i is TRUE
with a value of 1:
int i = 1;
if ( i ) printf("i is true with a value of: %d\n", i);
This will result to TRUE because i is TRUE:
it is not zero, it is 123.
i = 123;
if ( i ) printf("i is true with a value of: %d\n", i);
This will still result to TRUE because i is
still TRUE: it is not zero, it is -321.
i = -321;
if ( i ) printf("i is true with a value of: %d\n", i);
This will result to FALSE because i is now zero.
i = 0;
if ( i ) {
printf("i is true with a value of: %d\n", i);
}
else {
printf("i is false with a value of: %d\n", i);
}
Note that in a large if-else block like the one
below, program flow jumps out of the block after
the first match is found. Its contents are
executed, then this if-else block is done.
int y = 26;
if ( i > 0 && y == 26 ) {
printf("Condition 1 met!\n");
}
else if ( i > 0 || y == 25 ) {
printf("Condition 2 met!\n");
}
else if ( i != 0 ) {
printf("Condition 3 met!\n");
}
else if ( !i && y >= 20 ) {
printf("Condition 4 met!\n");
}
else if ( i == 0 ) {
printf("Condition 5 met!\n");
}
else {
printf("no condition met!\n");
}
In our case here, Condition 4 is the first
match, even if Condition 5 is also true,
it will not continue
seeking for the other matches.
This is why Condition 5 is met! is not
printed.
Nested if and if-else blocks are also allowed:
if ( y == 26 ) {
if ( i != 0 ) {
printf("i is not 0!\n");
}
else {
printf("i is 0\n");
}
}
As we see here, there are two if conditions,
so the program flow will be like:
Is it really true that y is equal to 26?
If yes, proceed to the second if.
If not, this if block is done, cancel
everything inside this block, including
the if-else.
In our case here, y is really equal to 26,
so it goes down to the second if
that tests whether i is not equal to zero,
but according to the last value of i,
it is zero, so it will execute
the else block, so it prints i is 0.
After this, you can now easily grasp the idea of loops, where it automates things for you.
#include <stdio.h>
int main()
{
int n = 10;
int i;
int x[n];
for ( i=0; i<n; i++ ) {
x[i] = i * 10;
printf("i: %d x[i]: %d\n", i, x[i]);
}
int y = 0;
while ( y < 100 ) {
y += 12;
printf("y: %d\n", y);
}
y = 0;
do {
y += 12;
printf("y: %d\n", y);
}
while ( y < 100 );
return 0;
}
i: 0 x[i]: 0
i: 1 x[i]: 10
i: 2 x[i]: 20
i: 3 x[i]: 30
i: 4 x[i]: 40
i: 5 x[i]: 50
i: 6 x[i]: 60
i: 7 x[i]: 70
i: 8 x[i]: 80
i: 9 x[i]: 90
y: 12
y: 24
y: 36
y: 48
y: 60
y: 72
y: 84
y: 96
y: 108
y: 12
y: 24
y: 36
y: 48
y: 60
y: 72
y: 84
y: 96
y: 108
We will use i as an index within for loop:
int i;
int x[n];
for ( i=0; i<n; i++ ) {
x[i] = i * 10;
printf("i: %d x[i]: %d\n", i, x[i]);
}
Here, i will start at 0. Then, it will test whether
i is less than n which is 10. If this test evaluates to true,
it will execute the commands. That range will be from 0 to 9.
Then, when the value of i is 10,
it stops looping because
i < n evaluates to false. Then, this for loop is done.
while loop is useful in certain situations as well:
int y = 0;
while ( y < 100 ) {
y += 12;
printf("y: %d\n", y);
}
In while loop, it will first test the condition.
If the condition evaluates to false, it will be skipped,
so, while loop
may not execute the commands at all.
But in the example here, while loop succeeded.
do-while loop is used instead of while loop
if you want the code block executed before evaluation
of a condition, at least once.
y = 0;
do {
y += 12;
printf("y: %d\n", y);
}
while ( y < 100 );
Now, here is the tricky part: in our example,
you will notice that both
do-while and while loops ended at
the value of 108, but the problem
is that, in the given condition, y should be
less than 100 to continue looping
or else it should stop looping. Then why these loops ended at 108?
#include <stdio.h>
int main()
{
int i = 1;
int *iPtr;
iPtr = &i;
printf("Memory address of i: %p\n", &i);
printf("iPtr: %p\n", iPtr);
printf("But, iPtr has its own address: %p\n", &iPtr);
printf("Getting the value the pointer points to using * :\n");
printf("i = %d, *iPtr = %d\n", i, *iPtr);
printf("Changing the value directly using a pointer:\n");
*iPtr = 2;
printf("After using \"*iPtr = 2;\", i is now %d\n", i);
return 0;
}
Memory address of i: 0x7ffce7495e0c
iPtr: 0x7ffce7495e0c
But, iPtr has its own address: 0x7ffce7495e10
Getting the value the pointer points to using * :
i = 1, *iPtr = 1
Changing the value directly using a pointer:
After using "*iPtr = 2;", i is now 2
A normal integer is declared like the one below.
This assigns a piece of memory for i,
and stores the value 1 in this memory.
int i = 1;
On the other hand, pointers were introduced as a way to access a memory location directly and the value stored there.
Declare a pointer with the * character:
int *iPtr;
A pointer is a variable that contains memory address. But that pointer has its own memory address too.
printf("Memory address of i: %p\n", &i);
printf("iPtr: %p\n", iPtr);
printf("But, iPtr has its own address: %p\n", &iPtr);
You can reference a variable's memory using the & symbol.
Then, after getting that memory address, you can store it in
a pointer variable, in our case here, it is iPtr.
iPtr = &i;
You can also access the value in memory that a
pointer is pointing to using the * symbol,
in our case here, *iPtr. This
is called dereferencing.
printf("i: %d, *iPtr: %d\n", i, *iPtr);
There are several benefits of using pointers, one
is passing function arguments by reference, which
is illustrated in the functions.c example.
With pointers, you have the ability to alter directly the value stored in a memory location:
*iPtr = 2;
printf("After using \"*iPtr = 2;\", i is now %d\n", i);
#include <stdio.h>
void printFunctionTest()
{
printf("This function 'printFunctionTest()' only prints, nothing else.\n");
}
float mySimpleFuncByValue(int iloc, float xloc)
{
iloc += 101;
xloc *= 14.6;
float yloc = (float)iloc * xloc;
return yloc;
}
float mySimpleFuncByReference(int *iPtr, float *xPtr)
{
*iPtr += 101;
*xPtr *= 14.6;
int iloc = *iPtr;
float xloc = *xPtr;
float yloc = (float)iloc * xloc;
return yloc;
}
void modifyArray( int *array )
{
array[0] = 5;
array[1] = 12;
}
int main()
{
printFunctionTest();
int i = 22;
float x = 234.23;
float y = mySimpleFuncByValue(i, x);
printf("i: %d x: %10.5f y: %10.5f\n", i, x, y);
y = mySimpleFuncByReference(&i, &x);
printf("i: %d x: %10.5f y: %10.5f\n", i, x, y);
int myArray[2] = { 0, 0 };
printf("Before function call, myArray[0]: %d myArray[1]: %d\n", myArray[0], myArray[1]);
printf("array mem address: %d\n", myArray);
modifyArray(myArray);
printf("After function call, myArray[0]: %d myArray[1]: %d\n", myArray[0], myArray[1]);
return 0;
}
This function 'printFunctionTest()' only prints, nothing else.
i: 22 x: 234.23000 y: 420630.25000
i: 123 x: 3419.75806 y: 420630.25000
Before function call, myArray[0]: 0 myArray[1]: 0
array mem address: -214060608
After function call, myArray[0]: 5 myArray[1]: 12
A declaration of a void function:
void printFunctionTest()
{
printf("This function 'printFunctionTest()' only prints, nothing else.\n");
}
A void function does not return any value. A void function will just do the commands inside this block.
When a function is invoked, you think of the computer finding the declaration of that function, then, when found, will execute everything inside that function.
In our case here, when printFunctionTest() function is called
in main function, the computer is tasked to look
for the declaration of this function. It was declared then defined,
so when the computer finds it, will execute everything in
that function.
There are instances that the computer is instructed to look outside
a source code, maybe in another external library
with the include command. As was mentioned,
this is a good practice in software development.
Parameters will enable you to pass variables of the same type or even pointers, to be processed inside that function. Why not just directly do that inside a function? One main reason is reusability. Other variables of the same type can be passed to these functions instantly when needed.
First of all, we need to distinguish the two for you not to be confused.
When declaring a function, the term is parameter.
When passing variables or pointers to a parameter, the term is argument.
So, you say: "I want to declare a function with two
parameters of integer type, then in main function,
I will insert x and y variables as arguments."
Suppose you need to calculate the distance travelled from the input of the distance sensor and you want to get that distance value, a void function cannot do that. You need to declare a function with a data type and a return value.
The keyword is return. This command tells the compiler
to terminate the current function with or without
a return value, or pause as this
current function will call another function. return can be used
in void functions also when needed.
A declaration of a function
with parameters int iloc, float xloc
and a return value named yloc.
float mySimpleFuncByValue(int iloc, float xloc)
{
iloc += 101;
xloc *= 14.6;
float yloc = (float)iloc * xloc;
return yloc;
}
Take note, inside this function,
int iloc and float xloc are local variables,
so, these variables cannot be
called outside this function. Also, do not be
confused of the * in iloc * xloc:
it's a multiplication operator in this context.
float yloc = (float)iloc * xloc
At this line, since iloc is an integer
and yloc is a float, we need to cast
iloc so the compiler will not return an error.
(float)iloc command will cast iloc integer type to float.
Now, in main function,
i and x are both passed by value,
meaning a copy of each of these variables
is passed to mySimpleFuncByValue(int iloc, float xloc).
So, any changes made in
this function will not reflect
outside this function.
int i = 22;
float x = 234.23;
float y = mySimpleFuncByValue(i, x);
printf("i: %d x: %10.5f y: %10.5f\n", i, x, y);
Another declaration of a function
with pointers as the parameters:
int *iPtr, float *xPtr.
float mySimpleFuncByReference(int *iPtr, float *xPtr)
{
*iPtr += 101;
*xPtr *= 14.6;
int iloc = *iPtr;
float xloc = *xPtr;
float yloc = (float)iloc * xloc;
return yloc;
}
Here, we want to dereference
the pointer and modify the value it points to:
*iPtr += 101,
*xPtr *= 14.6.
Changes made inside
this function will now affect i and x after
this function terminates. That's the reason why,
when you call the second print command,
printf("i: %d x: %10.5f y: %10.5f\n", i, x, y);
i now contains the value of 123 and x contains
3419.75806.
Now, in main function, memory addresses can be inserted as an argument for this function, like this:
mySimpleFuncByReference(&i, &x)
That is the essence of parameter/s of pointer type. You can easily manage variables and their contents.
Another instance of passing pointers to function is as follows:
void modifyArray( int *array )
{
array[0] = 5;
array[1] = 12;
}
This function can modify an array whenever this is called,
int *array is the parameter. Take note, the parameter
is of pointer type again.
Now, in main function,
we just declare myArray[2]. This will be modified
by this function.
When you just call an array without a specific index, the compiler will return the memory address of this array. By declaring a pointer as a parameter in this function, we can insert a memory address as an argument. You can even access and manipulate every element using only the memory address of an array.
int myArray[2] = { 0, 0 };
printf("Before function call, myArray[0]: %d myArray[1]: %d\n", myArray[0], myArray[1]);
modifyArray(myArray);
printf("After function call, myArray[0]: %d myArray[1]: %d\n", myArray[0], myArray[1]);
So, pointers are truly essential to be an excellent C programmer.
#include <stdio.h>
typedef struct
{
char *name;
int age;
float height;
float grades[10];
} student;
void printStruct( student oneStudent )
{
int i;
printf("=====================\n");
printf("Student name: %s\n", oneStudent.name);
printf(" age: %d\n", oneStudent.age);
printf(" height: %6.3f\n", oneStudent.height);
printf(" grades: ");
for (i=0; i<10; i++) printf("%4.1f ", oneStudent.grades[i]);
printf("\n");
printf("=====================\n");
}
void modifyStruct( student *oneStudent )
{
oneStudent->grades[0] = 72.0;
oneStudent->grades[4] = 81.0;
oneStudent->grades[6] = 85.0;
}
int main()
{
student Nikki;
Nikki.name = "Nikki";
Nikki.age = 19;
Nikki.height = 64.75;
int i;
for (i=0; i<10; i++) {
Nikki.grades[i] = 98.0;
}
printStruct(Nikki);
modifyStruct(&Nikki);
printStruct(Nikki);
return 0;
}
=====================
Student name: Nikki
age: 19
height: 64.750
grades: 98.0 98.0 98.0 98.0 98.0 98.0 98.0 98.0 98.0 98.0
=====================
=====================
Student name: Nikki
age: 19
height: 64.750
grades: 72.0 98.0 98.0 98.0 81.0 98.0 85.0 98.0 98.0 98.0
=====================
This is an example of a structure in C:
typedef struct
{
char *name;
int age;
float height;
float grades[10];
} student;
where int age is in years,
float height in inches
and float grades[10] are the grades
for the 10 assignments.
Structure is a user-defined data type in C. It is really needed to mix different data types in one declaration.
typedef is a keyword which will enable
you to give an existing type a new name.
In our case here, it is just like saying:
"Let us define this structure by
calling student."
student is a structure
variable that can access the members of
this structure.
In order to access a member of an
structure we use . symbol, just like
oneStudent.name.
A function to print the details:
void printStruct( student oneStudent )
{
int i;
printf("=====================\n");
printf("Student name: %s\n", oneStudent.name);
printf(" age: %d\n", oneStudent.age);
printf(" height: %6.3f\n", oneStudent.height);
printf(" grades: ");
for (i=0; i<10; i++) printf("%4.1f ", oneStudent.grades[i]);
printf("\n");
printf("=====================\n");
}
Take note of this parameter: student oneStudent.
oneStudent is a variable of type student.
That is the type we defined in the structure.
Again, you might be overwhelmed by the printf function that
contains strange things like this: %6.3f.
Don't worry, they are just formatting commands like
whether how many decimal places should be there, or
you want to pad those numbers with zero.
In the case of %6.3f, it means, format the floating
type number 6 digit wide, 3 decimal places.
A function to modify the structure:
void modifyStruct( student *oneStudent )
{
oneStudent->grades[0] = 72.0;
oneStudent->grades[4] = 81.0;
oneStudent->grades[6] = 85.0;
}
In order to modify the declared structure, we still use pointer as parameter to accept a memory location as argument: that is, to easily locate it and modify everything using the index of each data member.
Pointers to structs have special -> symbol
for dereferencing the pointer and accessing a
data member.
And what does it do?
oneStudent->grades[0] = 72.0 is equivalent to
(*oneStudent).grades[0] = 72.0.
In main function, student
data type is used: student Nikki
is just like any other variable
declaration, like int myInteger.
Coming from the structure we defined,
we can put data in student Nikki.
student Nikki;
Nikki.name = "Nikki";
Nikki.age = 19;
Nikki.height = 64.75;
int i;
for (i=0; i<10 ; i++) {
Nikki.grades[i] = 98.0;
}
The functions we created, we call them in main function to act upon the structure:
printStruct(Nikki);
modifyStruct(&Nikki);
printStruct(Nikki);
We first check whether Nikki was successfully created by
calling the command printStruct(Nikki).
Then, we modify the initial contents of this structure
by calling the command modifyStruct(&Nikki). Remember that placing &
before a variable will return the address of that variable.
And finally, we check again whether the changes were
saved by calling the command printStruct(Nikki).
There are confusing terms that you might be thinking: data type, database and data structure.
As was demonstrated, a structure in C is there to mix different data types in one declaration. Every member can be accessed easily. In general, data structure is data that can be accessed piece by piece. For example, an XML file is considered semi-structured, for elements can be parsed but from top to bottom while a database file is a structured data, for pieces of information can be accessed randomly without parsing the entire database.
Data type is declared to easily tell the compiler how a particular data should be treated.
Database is an organized collection of data usually expressed as tables. It is commonly built upon a filesystem. Data will persist after the computer is turned off because data resides on persistent storage.
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv)
{
printf("argc: %d\n", argc);
if ( argc != 3 ) {
printf("Usage: ./pass_command_line_options arg1 arg2\n");
printf("where arg1 is an integer and arg2 is a floating point number\n");
exit(0);
}
int i;
for (i=0; i<argc; i++) {
printf("i: %d argv[i]: %s\n", i, argv[i]);
}
i = 1;
int myarg_int = 0;
float myarg_float = 0.0;
if ( argc > 1 ) myarg_int = atoi(argv[i]);
++i;
if ( argc > 2 ) myarg_float = atof(argv[i]);
printf("myarg_int: %d myarg_float: %10.5f\n", myarg_int, myarg_float);
return 0;
}
#include <stdio.h>, as we all know, is for printf.
#include <stdlib.h> is for atoi and atof.
atoi() function converts string into
an integer and returns that integer.
atof() converts string into a double,
then returns that value.
argc captures the number of command line arguments.
printf("argc: %d\n",argc);
if ( argc != 3 ) {
printf("Usage: ./pass_command_line_options arg1 arg2\n");
printf("where arg1 is an integer and arg2 is a floating point number\n");
exit(0);
}
Take note: the executable is
always the first argument and the remaining arguments
are separated by spaces.
Since we want to require a user to supply two arguments,
there will be a total of three arguments including the required
executable file. This is inside the if block so as to make
sure there will be three arguments inserted or else
the user will be instructed to supply three arguments.
argv captures each argument in an array of characters.
Here, we loop over all arguments and print the value:
int i;
for (i=0; i<argc; i++) {
printf("i: %d argv[i]: %s\n", i, argv[i]);
}
Since command line arguments are read in as an array of characters, they must be converted to the appropriate type (e.g. int, float), for further processing:
i = 1;
int myarg_int = 0;
float myarg_float = 0.0;
if ( argc > 1 ) myarg_int = atoi(argv[i]);
++i;
if ( argc > 2 ) myarg_float = atof(argv[i]);
We make i with a value of 1. This will be later
used for the increment. Then, myarg_int and
myarg_float were declared with an initial
value of 0.
Then, there are two if blocks. We did not use
the conventional { } since we only have
one command in each if block.
The first test says if argc
is greater than 1, which means the user entered
two or more arguments, myarg_int will get
the value from the second argument.
That is, at index 1 and is an integer type,
so, conversion will be done through atoi.
Then, i will be incremented but using
++i. There are several discussions out there for this
increment first then assign or assign first then increment.
But, it depends on the situation yet in this example
where it was used just an index,
it is almost neglible.
Before the second if is tested, i now has a value of 2.
If ever the user entered the third argument, if ( argc > 2 )
will result to true and myarg_float will get the value
from the third argument, that is at index 2, which is a float.
Conversion will be done through atof.
#include <stdio.h> // printf()
#include <stdlib.h> // exit(), atoi(), atof()
int main(int argc, char **argv)
{
if ( argc != 2 ) {
printf("Usage: dynamically_allocated_arrays arraySize\n");
exit(0);
}
int n = atoi(argv[1]);
int *myDynamicArray = malloc( n * sizeof(int) );
if ( myDynamicArray == NULL ) {
printf("Allocation failed, exiting!\n");
exit(0);
}
int i;
for ( i=0; i<n ; i++ ) {
myDynamicArray[i] = i * 9;
printf("i: %d, myDynamicArray[i]: %d\n", i, myDynamicArray[i]);
}
free(myDynamicArray);
return 0;
}
Sometimes you can't predict the size of an array at compile time, or you need to grow or shrink the size of an array. Dynamically-allocated arrays are great for each of these purposes.
Here, we first test whether a user entered exactly two arguments, the first argument being the executable file and the second argument, the size of an array:
if ( argc != 2 ) {
printf("Usage: dynamically_allocated_arrays arraySize\n");
exit(0);
}
Then,
initialize an allocatable array of size n.
By standard definition,
malloc() returns a pointer to a chunk
of memory of size size, or NULL if there is an error.
The memory pointed to will be on the heap, not on the stack,
so make sure to free it when you are done with it.
In our case here,
malloc reserves a block of memory for the array.
malloc needs to know the number of bytes you'd
like to reserve, which you can get by multiplying
the desired size of the array times sizeof(type).
int n = atoi(argv[1]);
int *myDynamicArray = malloc( n * sizeof(int) );
Also, it's good practice to check that the array was allocated correctly. You do this by checking for a NULL pointer.
if ( myDynamicArray == NULL ) {
printf("Allocation failed, exiting!\n");
exit(0);
}
Then, fill and access your dynamic array like any other "normal" array:
int i;
for ( i=0; i<n ; i++ ) {
myDynamicArray[i] = i * 9;
printf("i: %d, myDynamicArray[i]: %d\n", i, myDynamicArray[i]);
}
As was mentioned, the memory for dynamically allocated arrays must be manually released before a program terminates. Failure to do so can lead to memory leaks.
free(myDynamicArray);
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE *fp_in;
fp_in = fopen("input_file.txt", "r");
if ( fp_in == NULL ) {
printf("input_file.txt not opened, exiting...\n");
exit(0);
}
int x, y, z;
int i;
for (i=0; i<3; i++) {
fscanf(fp_in, "%d %d %d", &x, &y, &z);
printf("x: %d y: %d z: %d\n", x, y, z);
}
fclose(fp_in);
FILE *fp_out;
fp_out = fopen("output_file.txt", "w");
if ( fp_out == NULL ) {
printf("output_file.txt not opened, exiting...\n");
exit(0);
}
for ( i=0 ; i < 50 ; i++ ) {
fprintf(fp_out, "%d %d\n", i, i*10);
}
fclose(fp_out);
printf("`output_file.txt` created successfully.\n");
return 0;
}
An example of reading from a file:
FILE *fp_in;
fp_in = fopen("input_file.txt","r");
It is good practice to check whether the file
was opened successfully. fopen() will
fail if the file does not exist:
if ( fp_in == NULL ) {
printf("input_file.txt not opened, exiting...\n");
exit(0);
}
And finally, getting the contents of that file through
fscanf and closing it right after reading.
int x, y, z;
int i;
for (i=0; i<3; i++) {
fscanf(fp_in,"%d %d %d", &x, &y, &z);
printf("x: %d y: %d z: %d\n", x, y, z);
}
fclose(fp_in);
Now, an example of writing to a file:
FILE *fp_out;
fp_out = fopen("output_file.txt", "w");
Then, checking whether it was successfully created:
if ( fp_out == NULL ) {
printf("output_file.txt not opened, exiting...\n");
exit(0);
}
Then, writing data to this file and closing it after:
for ( i=0 ; i < 50 ; i++ ) {
fprintf(fp_out,"%d %d\n", i, i*10);
}
fclose(fp_out);
You have reached the end of this tutorial, and we are glad you did. Not all the topics were covered here but the important thing is you get the idea what C is all about.
To demonstrate how C is used in writing computer programs, please visit this repository:
https://github.com/jdevstatic/CalC
Thanks a lot!