Tamás Kozsik and others ELTE Eötvös Loránd Tudományegyetem

Imperative programmingOverview

Tamás Kozsik and others

ELTE Eötvös Loránd Tudományegyetem

Imperative programming

Statements, control structuresReading and writing memoryC programming language (link!)

Tamás Kozsik and others (ELTE) Overview 2 / 31

https://www.youtube.com/watch?v=niO16JEbpWs













Outline

1 Structure of programsCompilation and running of programs

Structure of programs


Keywords, literals, operators, other signs, identifiersExpressionsStatementsSub-programs (functions/procesures, routines, methods)Modules (libraries, classes, packages)



Example

int factorial( int n )

{

int result = 1;

int i;

for(i=2; i<=n; ++i)

{

result *= i;

}

return result;

}



Example

int factorial( int n )

{

int result = 1;

int i;

for(i=2; i<=n; ++i)

{

result *= i;

}

return result;

}



Expressions

n

"Hello world!"

100

n+1

++i

range(2,n+1)

employees[factorial(3)].salary * 100



Statements

result = 1;

result *= i;

return result;

for( i=2; i<=n; ++i ){ result *= i; }

while(1) printf("Gyurrrika szép!\n");



Simple statements

assignment (expression evaluation statement)null statementsub-program callreturning from a function



Control structures

branchesloops etc.

int gcd( int n, int m )

{

while( n != m )

if( n > m )

n -= m;

else

m -= n;

return n;

}



Curly braces in control structures

Omitted curly bracesint gcd( int n, int m )

{

while( n != m )

if( n > m )

n -= m;

else

m -= n;

return n;

}

Foolproof solutionint gcd( int n, int m )

{

while( n != m ){

if( n > m ){

n -= m;

} else {

m -= n;

}

}

return n;

}



Dangling else

I wrote thisif( x > 0 )

if( y != 0 )

y = 0;

else

x = 0;

Means thisif( x > 0 )

if( y != 0 )

y = 0;

else

x = 0;

I wanted thisif( x > 0 ){

if( y != 0 )

y = 0;

} else

x = 0;

See…goto-fail (Apple) link!


https://www.synopsys.com/blogs/software-security/understanding-apple-goto-fail-vulnerability-2/


Printing to the standard output

Print an integer and a new line character

printf("%d\n",factorial(10));



More complex print

printf("10! = %d, ln(10) = %f\n", factorial(10), log(10));

10! = 3628800, ln(10) = 2.302585



More complex print

printf("10! = %d, ln(10) = %f\n", factorial(10), log(10));

10! = 3628800, ln(10) = 2.302585



Types

Express the way bit sequence is interpretedDetermine the possible values of a variableDetermine the possible operations on values

In Cint – an interval of integers, e.g. [−231 .. 231 − 1]float – subset of rational numberschar – characterschar[] – texts, array of charactersint[]– array of integersint* – pointer to an integer

etc.



Values of different type in the memory









Role of types

Protection against programming errorsExpress the intention of programmersHelp forming abstractionsHelp efficient code generations



Type checking

Checks whether variables and functions are used according to theirtypesNon type correct programs are meaningless

Static and dynamic type systemC compiler checkes type correctness in compile time

Strongly and weakly typed languagesIn a weakly typed language values are automatically converted toother types if needed

Seems comfortable for the first sightWe can easily write something else than we wanted

Rules in C are relatively strict (quite strongly typed)



Subprograms

Description of multi-step computationsGeneral, can be parameterized, reusableStructuring programs – handling complexity

Shouldn’t be longer than a page

Has different namesroutine or subroutinefunctions: computes a value and “returns’ ’ itprocedure: changed program statemethod: object oriented terminology



Main program

Where program execution starts

CA subprogram with a given name: main Egy megfelelő nevű alprogram:main

int main()

{

int half = 21;

printf("%d\n",2*half);

return 0; /* sikeres végrehajtás */

}



Comment

int main()

{

int half = 21;

printf("%d\n",2*half);

return 0; /* Here I write a comment */

}



Module

Modularity: encapsulation, independence, tight interface

Reusable librariese.g. the standard library of the language

Bigger program unitsImplementation of abstractions



Breakdown into modules

Reusable factorial

factorial.c – factorial functiontenfactorial.c – main program

tenfactorial.c#include <stdio.h>

int factorial( int n ); /* Declaration of this function */

int main()

{

printf("%d\n", factorial(10));

return 0;

}


Structure of programs Compilation and running of programs

Source code

Code written in a programming languageComputer: machine codeExecution

interpretationcompilation, running

Source file, e.g.: factorial.c



Interpreter

Processes source code by statementsReporting errors in case of erroneous statementsExecution in case of no errors

Execution of a statement: based on built-in machine code

DisadvantagesRun-time error in case of erroneous program (rarely executedstatement???)Slower execution

AdvantagesIntegration of program writing and execution

REPL = Read-Evaluate-Print-LoopFast prototyping

Beginners may learn it easierTamás Kozsik and others (ELTE) Overview 24 / 31


Source code in Cfactorial.c#include <stdio.h>

int factorial( int n ){

int result = 1;

int i;

for(i=2; i<=n; ++i){

result *= i;

}

return result;

}

int main(){

printf("%d\n", factorial(10));

return 0;

}



Separation of compilation and running

A lot of programming errors can be determined without running theprogramWe can examine the program in advanceNeeds to be done only once (during compile-time)Less error during executionGoal: efficient and reliable machine code!

“Compile time” and “runtime”



Compilation and running

Source filefactorial.c

Compilationgcc factorial.c

Compiled programa.out

Execution./a.out



-o flag of gcc

Source filefactorial.c

Compilationgcc -o fact factorial.c

Compiled programfact

Execution./fact



Compilation errors

Violating language rulesDetected by compiler

factorial.c-benint factorial( int n )

{

int result = 1;

for(i=2; i<=n; ++i)

{

result *= i;

}

return result;

}

gcc factorial.c

factorial.c: In function ‘factorial’:

factorial.c:6:9: error: i undeclared (first use in this function)

for(i=2; i<=n; ++i)

^



Compiler warnings

Language rules followedCompiler detects if something issuspiciousProbably we made a mistakeGoal: compilation withoutwarnings

factorial.c-benint factorial( int n )

{

int result = 1, i;

for(i=2; i<=n; ++i)

{

result *= i;

}

}

gcc -W -Wall --pedantic factorial.c

factorial.c: In function ‘factorial’:

factorial.c:10:1: warning: control reaches end of non-void function

[-Wreturn-type]

}

^



Preprocessing

C preprocessor: creates source code (from source code)

Macros#define WIDTH 80

...

char line[WIDTH];

Sharing declarations#include <stdio.h>

...

printf("Hello world!\n");

Conditional compilation#ifdef FRENCH

printf("Salut!\n");

#else

printf("Hello!\n");

#endif


Documents

Tamás Kozsik and others ELTE Eötvös Loránd Tudományegyetem