First Lesson in C - Washington State University · 2016. 6. 6. · First Lesson in C History, Basic...

First Lesson in C

History, Basic Data Types, Language Elements

CptS 121 – Summer 2016 – Armen Abnousi

Lecture 2

History

• 1972 by Dennis Ritchie at AT&T Bell Labs

• CPL -> BCPL -> B -> C

• Designed to implement UNIX operating system with it

• Many other languages somehow derived from C: C++, Java, PHP, Python

• Is an imperative procedural language

• Largely used for embedded programming but also for “low level” and general

purpose programming

Let’s write our first C Program!

• HelloWorld Program:

/* This program prints “Hello World!” on the screen! */

/* This is my first program */

#include <stdio.h> /* needed for call to printf */

int main()

{

printf(“Hello World!\n”);

return 0;

}

Analyzing HelloWorld…

• main is where your program starts the execution

• Every C program must have a main

• main should always be preceded by int

• Returning 0 at the indicates normal (without errors)

exit

• printf refers to some operation not defined in basic C,

hence we need to #include the file that contains

instructions about printf.

• Comments make your code more readable and

maintainable!

More programming!

• Time to home calculator: /* this computes the time required to walk to home*/

#include <stdio.h> /* This is required for printf statement */

#define average_walking_speed 3.31

int main() {

/* variable declaration */

double distance;

double required_time;

/*Get the distance from home */

printf(“Enter distance from home”);

scanf(“%lf”, &distance);

/* Compute the time required to walk home */

required_time = distance / average_walking_speed;

/* output the results */

printf(“It will take an estimated %f hours to get home”, required_time);

return 0;

}

Build process in C

C

preprocessor

Source

file (.c)C compiler

( + assembler)

Machine

lang. file (.o)C linkerExecutable

file (.exe)

• Replaces macros in code

• Finds the called functions

in header files

• Translates the code to

machine language

• Brings the .o files required in your code

(e.g. for called functions) and attaches the

instruction for them to your code

Build process in C

C

preprocessor

Source

file (.c)C compiler

( + assembler)

Machine

lang. file (.o)C linkerExecutable

file (.exe)

• Replaces macros in code

• Finds the called functions

in header files

• Translates the code to

machine language

• Brings the .o files required in your code

(e.g. for called functions) and attaches the

instruction for them to your code

#define avg_wlk_spd 3.31#include <stdio.h>

More programming!




int main() {


double distance;









return 0;

}

Memory (Remember from yesterday?)

Address: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Content: ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?

• When we write a program, the OS assigns a section of those cells to our program:

a = 3;

b = 5;

c = a + b;

Name:

Address: 1 2 3 4 5 6 7

b

8

c

9

a

10 11 12 13 14 15 16 17 18

Content: ? ? ? ? ? ? ? 5 8 3 ? ? ? ? ? ? ? ?

My program’s scope assigned by the OS

Variable declaration

Address: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Content: ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?

double distance; /* line 6 */

double required_time; /* line 7 */

• Our program needs to store values for “distance” and “required_time”:

Name:

Address: 1 2 3 4 5 6 7

distance

8 9

required_

time

10 11 12 13 14 15 16 17 18

Content: ? ? ? ? ? ? ? 5 8 3 ? ? ? ? ? ? ? ?

My program’s scope assigned by the OS

Variable Declaration

double distance; /* line 6 */

double required_time; /* line 7 */

• Declaring a variable reserves space for a value in the memory

• Associates a name with that space/value, so the programmer can call the

space/value using that name

• To declare a variable we mention it’s data type and the name we want to call it!

• Basic data types:

• int (for integer numbers) => int my_integer_value;

• double (for real numbers) => double required_time;

• char (for characters) => char your_first_initial;

Variable Declaration

• Define variables by: variable_type variable_name;

• We need to identify the type of data because (1) there are specific operators for

each data type, (2) each data type requires a different amount of space in memory.

• Variables must be declared before executable statements.

• Variable names can include letters, numbers and underscore. But MUST start with a

letter. (use lower case letters and separate name parts with _ for a good style!)

• int

• integers (at least -32767 to 32767)

• Operations: +, -, *, /, % (mod), >, <, <=, >=, ==, !=

Basic Data Types

• double

• real numbers (must include a decimal point) (at least ~10e-307 – 10e308)

• not all real numbers can be modeled because of space limitations (64 bits)

• Operations: +, -, *, /, >, <, <=, >=, ==, !=

• char

• Characters (ASCII codes)

• Each char is a single character.

• Operations: >, <, <=, >=, ==, !=

• ASCII includes all lower and upper case letters, white spaces (new line, space, tab,

etc., some special characters) (refer to http://www.asciitable.com/)

• Chars are identified by ‘’. You need to enter a character in between single quotes.

• float

• unsigned int

• …

http://www.asciitable.com/

More programming!




int main() {


double distance;









return 0;

}

Executable Statements

• Executable statements follow variable declaration

• These include statements that do some computation, assignment,

input/output statements and other “function calls”.

Assignment Statements

• Variable_name = expression;

Variable_name is the name of a variable defined earlier

Expression can be a combination of variable names and operators


• C computes the left hand side of an assignment statement and stores

the result in the memory space reserved for the variable on the right

hand side.

double a, b, average, average_copy;

average = (a + b) / 2;

average_copy = average;

Input/Output Statements

• Used in interactive programming

• scanf and printf : standard console input I/O functions

• Scanf and printf come from stdio.h (needs to be included)

printf(“ “, … ) scanf(“ “,…)

printf(“estimated %f hours to get home”, required_time);

placeholder Print list


• For basic data types (int, char, double, float, etc) use ‘&’

before variable names in scanf


Placeholders*:

*Hanly & Koffman book, Problem solving and program design in C, 8th ed, Pearson pub.


• Make sure every scanf is following a printf, asking the user to

input data:

printf(“please enter your age:>”);

scanf(“%d”, &age);

• When there is a scanf program pauses until values are input

• Numeric values can be separated by spaces or enter.

• When scanf looks for char (%c), whitespaces are also treated

as input values

• If there are more valus entered than expected, they will be

stored in computer buffer and used in the next scanf

statement! (or can cause errors later!)

Parts of Speech in C

• HelloWorld Program:

/* This program prints “Hello World!” on the screen! */

/* This is my first program */

#include <stdio.h> /* needed for call to printf */

int main()

{

printf(“Hello World!\n”);

return 0;

}

comment

Preprocessor directive

Standard header file

Reserved

words

Standard identifier


• Time to home calculator:



int main() {

double distance;






return 0;

}

Reserved word

Preprocessor

Directive

Comment

Variable

Standard identifiers

Constant macro

Special symbol / operator


• Preprocessor direvctive start with # and are replaced by

C preprocessor during the build process

• Reserved words cannot be used for user-defined

identifiers (variables, constants or user functions).

• Standard identifiers are the names used in standard C

but are not reserved. Programmer can redefine them,

but is discouraged!

Programming Style - comments

• Always start your code with a comment box, including

your name, date (of version), purpose and a description

of the program.

/*

* Programmer:

* (Class: CptS 121, Summer 2016)

* (Programming Assignment #)

* Date:

*

* Description:

*/

• Use comments to document your code.

Programming Style

• Use meaningful names for your variables

• Use underscore if there are multiple words in the

variable name

• Use lower case letters for variable and function names

• Use indentation appropriately

• Use spaces in between operators (+, /, *, etc.) and

operands and after commas.

Programming Style

• C ignores white spaces

• You can write multiple statements in one line

• Or can extend a statement over multiple lines

• But having one statement per line makes it more

readable and more professional!

• Using good indentation also helps the programmer

himself in locating what he needs when the code is long

enough for him to be lost!

References

• J.R. Hanly & E.B. Koffman, Problem Solving and Program

Design in C (8thed.), Pearson, 2016

• Andy O’Fallon’s lecture notes for CptS121

(http://eecs.wsu.edu/~aofallon/cpts121)

First Lesson in C - Washington State University · 2016. 6. 6. · First Lesson in C History, Basic...

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