Jason's Notebook

Getting Started with C Programming

5 min read

Python vs. C

'''
The Hello World Program in Python
'''
 
# Python math library
from math import *
 
# Main function definition:
def main():
	# Statements on their own line
	print("Hello World")
	print("sqrt(4) is %f" %(sqrt(4)))
	
# Call the main function
main()
/*
The Hello World Program in C
*/
/* C math and I/O libraries */
#include <math.h>
#include <stdio.h>
 
/*main funciton definition:*/
int main(void) {
	// statements and in a semicolon
	printf("Hello World\n");
	printf("sqrt(4) is %f\n", sqrt(4));
	
	return 0; // main returns value 0
}

Comments

  • Multi-line Comments: /* This is a multi-line comment*/
  • Single-line Comments: // This is a single line comment

Importing from Library

#include <library.h>

  • All include statements appear on top of the program, outside of function bodies

Blocks

  • Starts with { and ends with }
  • includes
    • Functions
    • Loops
    • Conditional Bodies

Main Function

int main(void) { }

  • main function returns a value type of int → C’s name for specifying the signed integer type
    • returns 0: running to completion without error
  • void: does not expect to receive a parameter

IMPORTANT

  • A C program must have a function named main, and its return type must be int (signed integer type)
  • The C main function has an explicit return statement to return an int value
    • return 0 if main function is successfully executed without errors
  • In C, main function is automatically called when the C program executes

Statements

  • Each statement ends with a semicolon ;
  • Must be within the body of some function

Output

  • printf: prints a formatted string
    • Placeholders → additional arguments separated by commas

NOTE

Indentation: Have no meaning in C, but a good programming style to indent statements based on the nested level of their containing block Output: C’s printf function does not automatically print a newline character at the end → need to explicitly specify a new-line character (\n) in the format string when a newline is desired in the output

Compiling and Running C Program

To Run a C Program

IMPORTANT

Any changes made to the C Source File, the code must be recompiled with gcc to produce a new version of the Binary Executable

  1. Using a text editor to write and save the C source code program in a file 
    vim hello.c
  2. Translate source code into a form that a computer system can directly execute
    • Compiler: Program that translates C source code into a binary executable1 form that computer hardware can directly execute
    gcc <input_source_file>
    • No Error Found →
      1. Create a Binary Executable file named a.out
      2. Allows specification of the Binary Executable file to generate using the -o flag 
        gcc -o <output_executable_file> <input_source_file>
    • Error Found → File will not be created / recreated, but an older version of the file from a previous successful compilation might still exist
  3. Execute the program
gcc hello.c # compile the hello.c file from text source to binary executable into default name (a)
./a.out # Run executable

NOTE

Some C compilers might need to be explicitly told to link in the math library: -lm

gcc hello.c -lm

Due to gcc command line can be long, frequently the make utility is used to simplify compiling C programs and for cleaning up files created by gcc

Variables and C Numeric Types

Variables

  • Scope: defines when the variable has meaning2 and its lifetime3
  • Type: the range of values that the variable can represent and how those values will be interpreted when performing operations on its data
  • Can only have only a single type
    • char: a character storing a single-byte integer
      • Often used to store a single ASCII character value (The ASCII numeric encoding of a character)
      • Different type than a string in C
    • int: a whole number
    • float: a decimal number
    • double: a decimal number but with more digits behind the decimal point
  • By convention, C variables should be declared at the beginning of their scope

IMPORTANT

All variables must be declared before they can be used typename variable_name;

{
	// 1. Define variables in this block's scope at the top of the block
	int x;           // Declare x to be an int type variable and allocates space for it
	int i, j, k;     // Can define multiple variables of the same type like this
	char letter;     // A char storea a single ASCII value
	float winpct;    // winpct is declared to be a float type
	double pi;       // the double type is more precise than float
	
	// 2. After defining all variables, you can use them in C statements.
	x = 7;           // x stores 7 (initialize variables before using their value)
	k = x + 2;       // use x's value in an expression
	
	letter = 'A';        // a single quote that is used for single character value
	letter = letter + 1; // letter stores 'B' (ASCII value one more than 'A')
	
	pi = 3.1415926;
	
	winpct = 11/2.0;     // winpct gets 5.5, winpct is a float type
	j = 11 / 2;          // j gets 5: int division truncates after the decimal
	x = k % 2;           // % is C's mod operator, so x gets 9 mod 2 (1)
}

NOTE

C expects a ; after every statement → gcc almost never informs you when missed ; The compiler indicates a syntax error on the line after the one with the missing ; → gcc interprets the current line as part of the previous line

Footnotes

  1. The Binary Executable consists of a series of 0 and 1 in a well-defined format that a computer can run ↩

  2. Where and When can the variable can be used in the program ↩

  3. It can persist for the entire program or only during a function activation ↩

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