Basic elements of the C language Basics of Programming 1 G. Horváth, A.B. Nagy, Z. Zsóka, P. Fiala, A. Vitéz 7. September, 2022

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Basic elements of the C language

Basics of Programming 1

DEPARTMENT OF NETWORKED SYSTEMS AND SERVICES

G. Horváth, A.B. Nagy, Z. Zsóka, P. Fiala, A. Vitéz

7. September, 2022

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Chapter 1 Introduction

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Contact

BME Faculty of Electrical Engineering and Informatics Department of Networked Systems and Services Gábor Horváth

email: horvath.gabor@vik.bme.hu Important webpages for the course:

The main webpage (slides, exercises, solutions):

http://www.hit.bme.hu/~ghorvath/bop/

The portal managing the assignments:

https://cprog.eet.bme.hu

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Requirements

1 Laboratory practices

Active participation on at least 70% of the labs Entrance test at the beginning of the labs Max. number of absences/falied entrance tests: 4 Solutions to lab problems must be uploaded to CProg

2 Classroom practices

Participation on at least 70% of the practices 6 mid-term “small tests” in total (10 pts each) The sum of best 4 must be above 20 pts No re-take available

3 “Big test”

There will be 2 mid-term “big tests”

Option 1: sum of the scores≥50%

Option 2: the score of the second one≥50%

The one with lower score can be re-taken

4 Homework project

Milestones will be published, extra score if deadlines are met Face-to-face presentation in mandatory

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Recommended literature

Any book about Standard C programming language in your own language

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Chapter 2 C language basics

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Short history of C programming language

1972: Start of development at AT&T Bell Labs Most of the UNIX kernel was created in C

1978: K&R C – Brian Kernigham, Dennis Ritchie:

The C Programming Language

1989: Standardization: ANSI X3.159-1989 1999: C99-standard:

new data types (complex) international character encoding arrays with variable sizes

. . .

2007–: C1X standard, 2011: C11 standard C++ compatibility

multi-thread programs . . .

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Main features of C

Compiled language program.c source code

program.exe executable file compiler, linker

”small language”: few (10) instructions, a lot of (>50) operators

concise syntax (”zipped”)

hard to read (must pay attention) easy to make a mistake

hard to find a mistake

it gives a code that can be optimized efficiently and runs fast easy to implement for different platforms

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The first C program

The source code of the minimum-program

1 /* f i r s t . c - - The f i r s t p r o g r a m */

2

3 int m a i n ()

4 {

5 r e t u r n 0;

6 } link

The program starts, and after that it ends (finishes its run) between/* and */there are comments: messages for the programmer

int main()– All C programs starts like this

int Main()– and not like this. C is ”case sensitive”

{ } – block, it encloses the program body return 0; – It marks the end of the program

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The first C program

. . . that actually does something

1 /* H e l l o w o r l d . c - - My f i r s t p r o g r a m */

2 # i n c l u d e < s t d i o . h > /* n e e d e d for p r i n t f */

3

4 /* The m a i n p r o g r a m */

5 int m a i n ()

6 {

7 p r i n t f (" H e l l o w o r l d !\ n "); /* P r i n t i n g */

8 r e t u r n 0;

9 } link

After compiling and running it gives the following output:

Hello world!

#include– to insert other C program parts printf– printing, \n– new line (line feed)

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A more complicated one

Instructions in a sequence

1 /* f o o t b a l l . c - - f o o t b a l l f a n s */

2 # i n c l u d e < s t d i o . h >

3 int m a i n ()

4 {

5 p r i n t f (" Are you "); /* no new l i n e h e re */

6 p r i n t f (" b l i n d ?\ n "); /* h er e is new l i n e */

7 p r i n t f (" Go Bayern , go ! ");

8 r e t u r n 0;

9 } link

Are you blind?

Go Bayern, go!

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Printing the value of a variable

1 # i n c l u d e < s t d i o . h >

2 int m a i n ()

3 {

4 int n ; /* d e c l a r i n g an i n t e g e r var . , c a l l e d n */

5 n = 2; /* n < - 2 a s s i g n e m e n t of v a l u e */

6 p r i n t f (" The v a l u e is : % d \ n ", n ); /* p r i n t i n g */

7 n = -5; /* n < - -5 a s s i g n e m e n t of v a l u e */

9 r e t u r n 0;

10 } link

The value is: 2 The value is: -5

int n – declaration of variable.

int (integer number) is the type,n is the identifier n = 2 – assignement of value, variablen takes value of expression ”2”

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. . . continued

1 # i n c l u d e < s t d i o . h >

2 int m a i n ()

3 {

4 int n ; /* d e c l a r i n g an i n t e g e r var . , c a l l e d n */

5 n = 2; /* n < - 2 a s s i g n e m e n t of v a l u e */

7 n = -5; /* n < - -5 a s s i g n e m e n t of v a l u e */

9 r e t u r n 0;

10 } link

printf(<format>, <what>) –

printing the value of expression <what>in the given <format>

format

%d– decimal (decimal number system)

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Block and declaration

Structure of the block {

}

1 {

2 /* d e c l a r a t i o n s */

3 int n ;

4

5 /* i n s t r u c t i o n s */

6 n = 2;

7 p r i n t f (" % d \ n ", n );

8 }

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Block and declaration

Structure of declaration

<type name> <identifier> [ = <initial value>]opt;

1 int n ; /* not i n i t i a l i z e d */

2 int n u m b e r _ o f _ d o g s = 2; /* i n i t i a l i z e d */

value of n is garbage from memory at the beginning value of number_of_dogs is2 at the beginning

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Inputting data

1 /* s q u a r e . c - - s q u a r e of a n u m b e r */

2 # i n c l u d e < s t d i o . h >

3 int m a i n ()

4 {

5 int num ; /* d e c l a r i n g an i n t e g e r var . */

6 p r i n t f (" P l e a s e g i v e an i n t e g e r v a l u e : "); /* i n f o */

7 s c a n f (" % d ", & num ); /* i n p u t t i n g */

8 /* p r i n t i n g the v a l u e of 2 e x p r e s s i o n s */

9 p r i n t f (" The s q u a r e of % d is : % d \ n ", num , num * num );

10 r e t u r n 0;

11 } link

Please give an integer value: 8 The square of 8 is: 64

scanf(<format>, &<where to>)–

Inputting (scanning) data in<format> format and putting it into <where to>variable

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Inputting data

This is another option, that gives the same result.

1 # include< s t d i o . h >int m a i n (){int num ; p r i n t f

2 (" P l e a s e g i v e an i n t e g e r v a l u e : "); s c a n f (" % d ",

3 & num ); p r i n t f (" The s q u a r e of % d is : % d \ n ",

4 num , num * num );r e t u r n

5 0;} link

Not recommended, unmanagable

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Chapter 3 Structured programming

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Algorithms

Finding zeros of functions

We are searching the zeros of functionf(x), a monotonically increasing function, between pointsn andp, withϵaccuracy.

x f(x)

n k p

1 p - n < eps ?

2 IF TRUE , J U M P TO 10

3 k ← ( n + p ) / 2

4 f ( k ) < 0?

6 p ← k ;

7 J U M P TO 1

8 n ← k ;

9 J U M P TO 1

10 The z e r o is : n

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Algorithms

Finding zeros – a different approach

We are searching the zeros of functionf(x), a monotonically increasing function, between pointsn andp, withϵaccuracy.

x f(x)

n k p

1 W H I L E p - n > eps , r e p e a t

2 k ← ( n + p ) / 2

3 IF f ( k ) > 0

4 p ← k ;

5 O T H E R W I S E

6 n ← k ;

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Structured vs unstructured

Two programs of the same algorithm

2 k ← ( n + p ) / 2

3 IF f ( k ) > 0

4 p ← k ;

5 O T H E R W I S E

6 n ← k ;

1 p - n < eps ?

3 k ← ( n + p ) / 2

4 f ( k ) < 0?

6 p ← k ;

7 J U M P TO 1

8 n ← k ;

9 J U M P TO 1

Structured program easy to maintain complex control higher level

Unstructured program spaghetti-code easy control

”hardware-level”

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Structured vs unstructured

Hardware level languages Lot of simple instructions

Easy control (JUMP;IF TRUE, JUMP) Unstructured layout

The processor can interpret only this Higher level languages

Rather few, but complex instructions More difficult control (WHILE...REPEAT...;

IF...THEN...ELSE...) Structured layout

The processor is unable to interpret it.

The compiler transforms a high level structured program into a hardware level program, that is equivalent to the original one.

We create a high level structured program, we use the compiler to translate it,

and we execute the hardware level code.

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Elements of structured programs

All structured programs follow this simple scheme:

START Operation

STOP

The structure of the program is determined by the inner structure (layout)of Operation.

Operation can be:

Elementary operation (action) Sequence

Loop or repetition Selection

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Elements of structured programs

Elementary operation

that cannot be further expanded

Operation Elementary

op. E l e m e n t a r y op .

The empty operation (don’t do anything) is also an elementary operation

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Elements of structured programs

Sequence

Execution of two operations after eachother, in the given order

Operation

Op. 1 Op. 2

Op . 1 Op . 2

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Elements of structured programs

Each element of the sequence itself is an operation, so they can be expanded into a sequence

Op. 1 Op. 2

Op. 2a Op. 2b Op. 1

The expansion can be continued, so a sequence can be an arbitrary long (finite) series of operations.

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Elements of structured programs

Condition-based selection

Execution of one of two operations, depending on the logical value of a condition (true or false)

Operation

Condition Op.

if true

Op.

if false

T F

IF C o n d i t i o n Op . if t r u e E L S E

Op . if f a l s e

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Elements of structured programs

One of the branches can also be empty.

Operation

Condition Op. if true

T

F

IF C o n d i t i o n Op . if t r u e

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Elements of structured programs

Top-test loop

Repetition of an operation as long as a condition is true.

Operation

Condition

Body T

F

W H I L E C o n d i t i o n B o d y of l o o p

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Elements of structured programming

Theorem of structured programming By using only

elementary operation, sequence,

selection, and loop

ALLalgorithms can be constructed.

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The structogram

The flowchart

a tool for describing unstrutured programs

can ba translated (compiled) into an unstructured program immediately (IF TRUE, JUMP)

structued elements (esp. loops) are hard to recognize within it The structogram

a tool for representing structured programs only a structured program can be represented by it it is easily translated into a structured program

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The structogram

The program is a rectangle

Operation

it can be expanded into more rectangles with the elements below

Sequence Op. 1 Op. 2

Top-test loop Condition Body of the loop Selection

AA

Condition

Op. if true Op. if false

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The structogram

Finding zeros – flowchart, structogram, structured pseudo-code

p ← k n ← x

f(k)>0

T F

k ← ^n+p₂ p−n> ϵ

T START

STOP F

p−n> ϵ k← ^n+p₂

AA

f(k) > 0 p←k n←k

2 k ← ( n + p ) / 2

3 IF f ( k ) > 0

4 p ← k ;

5 O T H E R W I S E

6 n ← k ;

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Chapter 4 Structured programming in C

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Sequence in C

Forming a sequence is listing instructions one after eachother

1 /* f o o t b a l l . c - - f o o t b a l l f a n s */

2 # i n c l u d e < s t d i o . h >

3 int m a i n ()

4 {

5 p r i n t f (" Are you "); /* no new l i n e h e re */

6 p r i n t f (" b l i n d ?\ n "); /* h er e is new l i n e */

7 p r i n t f (" Go Bayern , go ! ");

8 r e t u r n 0;

9 } link

Are you blind?

Go Bayern, go!

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Selection control in C – the if statement

Let’s write a program, that decides if the inputted integer number is small (<10) or big (≥10)!

OUT: info IN:x AA

x < 10 OUT:small OUT: big Let x be an i n t e g e r OUT : i n f o

IN : x IF x < 10

OUT : s m a l l O T H E R W I S E

OUT : big

1 # i n c l u d e < s t d i o . h >

2 int m a i n ()

3 {

4 int x ;

5 p r i n t f (" P l e a s e e n t e r a n u m b e r : ");

6 s c a n f (" % d ", & x );

7 if ( x < 10) /* c o n d i t i o n */

8 p r i n t f (" s m a l l "); /* t r u e b r a n c h */

9 e l s e

10 p r i n t f (" big "); /* f a l s e b r a n c h */

11 r e t u r n 0;

12 } link

Please give an integer number: 5 small

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Selection control – the if statement

Syntax of theif statement

if (<condition expression>) <statement if true>

[ else <statement if false> ]opt

1 if ( x < 10 ) /* c o n d i t i o n */

2 p r i n t f (" s m a l l "); /* t r u e b r a n c h */

3 e l s e

4 p r i n t f (" big "); /* f a l s e b r a n c h */

1 if ( a < 0) /* c r e a t i n g a b s o l u t e v a l u e */

2 a = - a ;

3 /* no f a l s e b r a n c h */

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Top-test loop in C – the while statement

Let’s print the square of the integer numbers between 1 and 10!

n←1 n≤10

OUT:n·n n←n+1

Let n be an i n t e g e r

n ← 1

W H I L E n <= 10 OUT : n * n n ← n +1

1 # i n c l u d e < s t d i o . h >

2 int m a i n ()

3 {

4 int n ;

5 n = 1; /* i n i t i a l i z a t i o n */

6 w h i l e ( n <= 10) /* c o n d i t i o n */

7 {

8 p r i n t f (" % d ", n * n );/* p r i n t i n g */

9 n = n +1;

/* i n c r e m e n t */

10 }

11 r e t u r n 0;

12 } link

1 4 9 16 25 36 49 64 81 100

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Top-testing loop – the while statement

Syntax of thewhile statement

while (<condition expression>) <instruction>

If <instruction> is a sequence, we enclose it in a{block}:

1 w h i l e ( n <= 10 )

2 {

3 p r i n t f (" % d ", n * n );

4 n = n +1;

5 }

In language C an instruction always can be replaced with a block.

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A complex application

By using sequence, loop and selection, we can construct everything!

We know enough to construct the algorithm of finding the zeros in C!

A new element: a type for storing real numbers is called doubletype (to be learned later)

1 d o u b l e a ; /* the r e al n u m b e r */

2 a = 2 . 0 ; /* a s s i g n e m e n t of v a l u e */

3 p r i n t f (" % f ", a ); /* p r i n t i n g */

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Finding zero of a function

We are searching the zeros of function f(x) =x²−2, between points n=0 and p =2, with ϵ=0,001 accuracy.

n←0 p←2 p−n> ϵ

k← ^n+p₂

AA

k²−2>0 p←k n←k

OUT:n

1 # i n c l u d e < s t d i o . h >

2

3 int m a i n ()

4 {

5 d o u b l e n = 0.0 , p = 2 . 0 ;

6 w h i l e ( p - n > 0 . 0 0 1 )

7 {

8 d o u b l e k = ( n + p ) / 2 . 0 ;

9 if ( k * k -2 . 0 > 0 . 0 )

10 p = k ;

11 e l s e

12 n = k ;

13 }

14 p r i n t f (" The z e r o is : % f ", n );

15

16 r e t u r n 0;

17 } link

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Thank you for your attention.

Basic elements of the C language Basics of Programming 1 G. Horváth, A.B. Nagy, Z. Zsóka, P. Fiala, A. Vitéz 7. September, 2022