Lec 03 - Basic C Programming
Last updated
Last updated
Slides:
A statement is a unit in a programming language that expresses either a command to be executed or declares a new variable or function.
main() functionIn C, main() is the entry point to the program. It is where the operating system will begin to execute the program.
main returns an integer of type int to the operating system, to signal to the operating system whether the program exits successfully or not. In this case, we always return 0 (success) assuming that nothing goes wrong for simplicity.
In modern C, the main function always returns 0 when it exits. So, we will skip this statement return 0; from the main function from now on.
To declare a vairable, we use a declaration statement looks like as follows:
A variable is only visible within the innermost block that encloses the declaration, as specified by the { and }.
Errors that occur during compiling is called "compilation error". In constrast, errors that occured during execution of a program is called a "run-time error".
However, in real life in C, you may encounter the following three errors/warnings:
A compilation warning is a message provided by the compiler that indicates potential issues in the code, but does not prevent the code from compiling or running.
Example: Unused variables or deprecated functions. While the program can still run, warnings suggest areas that might cause problems or be improved in the future.
Impact: The code still compiles and runs, but you should address warnings for better code quality.
A compilation error is a mistake in the code that prevents the program from being compiled into an executable.
Example: Syntax errors (like missing semicolons, undeclared variables, or incorrect function calls). The compiler cannot generate machine code due to these errors.
Impact: The program doesn't compile at all. You need to fix the error before the code can be run.
A run-time error occurs while the program is running and leads to abnormal behavior or crashes.
Example: A segmentation fault occurs when the program tries to access an illegal memory location, such as dereferencing a null pointer or accessing out-of-bounds array indices.
Impact: The program crashes or behaves unexpectedly while running, and the error needs to be fixed to ensure stability.
In Unix-flavored operating systems, the input is read from an abstract channel called the standard input or stdin for short, and an output is sent to an abstract channel called the standard output or stdout for short.
The fact that these channels are abstract is a powerful concept -- when we write our code, we do not have to worry about where the inputs come from and where the outputs go to. It will depend on how the user runs our program. Thus, it allows the users of our program the flexibility to control where the data comes from or goes.
For instance, the standard input, by default, reads from the keyboard. But the user can choose to read from a file, using the redirection < operator from the command line or the output of another process, using the pipe | operator from the command line. Similarly, the standard output, by default, writes to the terminal. But the user can choose to write to a file using the redirection > operator on the command line or to the input of another process, using the pipe | operator, again, on the command line when invoking the program. You will see how cool these are later. But for C programming, it suffices to know for now that we only need to read from stdin and write to stdout in our code, and we let the users decide where they come from / go to.
In C, The module remainder operator % works only on integer types.
It is commonly misunderstood that the % operator is equivalent to the modulo operation in number theory, where the operator always gives a positive remainder.
This equivalence holds when the two operands are positive. E.g., 9 % 4 gives 1 as the answer. But, when one of the operands is negative, the % operator may return a negative number.
The % operator in C is defined as follows: x % n is equivalent to x - ((x / n) * n) (where / is the integer division operator).
For instance, -9 % 4 is evaluated as -9 - ((-9 / 4) * 4) which is -9 - (-8), and gives -1.
On the other hand, 9 % -4 is evaluated as 9 - ((9 / -4) * -4), which gives 9 - 8, or 1.
To avoid confusion between the % operator in C and the modulo operation in number theory, we will call the % as the remainder operator.
All variables must be declared with its corresponding type before used in C. Indicating the type during declaration lets the computing device running the program knows how much memory should be reserved for the value of this variable, and subsequently, how to interpret the binary sequence stored associated with this variable. (See for deeper understanding)