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High-level survey Problems, algorithms, and programs

Problem solving and program design

Compiling and running a C program

Errors and debugging

Focus on the big ideas Many details to cover in future lectures

Reading Sec. 1.3-1.5; start reading Ch. 2.

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ProblemDefinition of task to be performed (often by a computer)

Algorithm

A particular sequence of steps that will solve a problemSteps must be precise and mechanicalThe notion of an algorithm is a (the?) fundamentalintellectual concept associated with computing

ProgramAn algorithm expressed in a specific computer

programming language (C, C++, Java, Perl, )

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Programming Cooking

Problem Make fudge brownies

Algorithm Recipe

Program Recipe written in a specificlanguage (English, Russian,

Chinese, Latvian, etc.)

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Clearly specify the problemAnalyze the problem

Design an algorithm to solve theproblem

Implement the algorithm (write theprogram)

Test and verify the completed program

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Is a given number even or odd?

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What numbers are allowed?Where does the number comefrom?

What do even and odd mean?

How is the answer to be reported?

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Given an integer number typed in fromthe keyboard, If it is even, write even on the screen If it is odd, write odd on the screen

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Read in the number Divide the number by 2 If the remainder is 0, write even Otherwise, write odd

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Read in the number Divide the number by 2 If the remainder is 0, write even

Otherwise, write odd

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Test: 234784832792543

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Read in the number Divide the number by 2 If the remainder is 0, write even Otherwise, write odd

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Test: 234784832792543

An alternate algorithm:If the rightmost digit is 0, 2, 4, 6, or 8, write even

Otherwise, write odd

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Now that have an algorithm, we wouldlike to write a C program to carry it out.

But first, what is a program? In fact,what is a computer?

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CPU or processor: executes simple instructions manipulatingvalues in memory

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CentralProcessingUnit (CPU)

MainMemory

Monitor

Network

Disk

Keyboardmouse

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The CPU executes instructions one after theother.

Such a sequence of instructions is called aprogram

Without a program, the computer is just

useless hardware

Complex programs may contain millions ofinstructions

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Memory is a collection of locations calledvariables

Each variable has

Aname(anidentifier)Atype(the kind of information it can contain)

Basic types includeint (integers whole numbers: 17, -42)

double(floating-point numbers with optional fraction

and/or exponent: 3.14159, 6.02e23)char(character data: a, ?, N, , 9)

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Ask the user to enter a number Read the number and call it num Divide num by 2 and call the remainder rem

Ifrem is 0 write even otherwise write odd

The actual program has LOTS ofdetails IGNORE THEM FOR NOW Pay attention to the main ideas

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/* read a number and report whether it is even or odd */ #include

intmain (void) { intnum; /* input number */ intrem; /* remainder after division by 2 */

/* get number from user */ printf(Please enter a number: ); scanf(%d, &num);

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/* calculate remainder and report even or odd */ rem = num % 2; if(rem == 0) { printf(even\n); }else{ printf(odd\n); } /* terminate program */

return0; }

Remember: Dont sweat the details!!! (for now)

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Text surrounded by/*and*/arecommentsUsed to help the reader understandthe programIgnored during program execution

Programs change over time. Itsimportant that programmers beable to understand old code - good

comments are essential.

/* read a number */#includeintmain (void) { intnum; /* input number */ intrem; /* remainder */ /* get number from user */ printf(Please enter a number: ); scanf(%d, &num); /* calculate remainder */ rem = num % 2; if(rem == 0) { printf(even\n); }else{ printf(odd\n); } /* terminate program */ return0;}

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Variabledeclarations createnew variables and specify theirnames and types.

/* read a number */#includeint main(void) { int num; /* input number */ int rem; /* remainder */ /* get number from user */ printf(Please enter a number: ); scanf(%d, &num); /* calculate remainder */ rem = num % 2;

if(rem == 0) { printf(even\n); }else{ printf(odd\n); } /* terminate program */ return0;}

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Following the declarations arestatementsthat specify the operations the program isto carry outLots of different kindsSome (if, else, return) are part of the Clanguage properOthers (scanf, printf) are contained inlibrariesof routines that are available for use in ourprogramsFor now, dont worry too much about the

distinction

/* read a number */#includeintmain (void) { intnum; /* input number */ intrem; /* remainder */ /* get number from user */ printf(Please enter a number: ); scanf(%d, &num); /* calculate remainder */ rem = num % 2;

if (rem == 0) { printf(even\n); } else { printf(odd\n); } /* terminate program */ return 0;}

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Functions are sequences ofstatements defined elsewhere. Somefunctions (such as printf and scanfhere) are provided with the system.We will also learn how to write anduse our own functions.

/* read a number */#includeintmain (void) { intnum; /* input number */ intrem; /* remainder */ /* get number from user */ printf(Please enter a number: ); scanf(%d, &num); /* calculate remainder */ rem = num % 2;

if(rem == 0) { printf(even\n); } else { printf(odd\n); } /* terminate program */ return 0;}

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Some parts of the program arestandard utterances that need tobe included at the beginning andend.

Well explain all of thiseventuallyJust copy it for now in each ofyour programs

/* read a number */#include int main (void) { intnum; /* input number */ intrem; /* remainder */ /* get number from user */ printf(Please enter a number: ); scanf(%d, &num); /* calculate remainder */ rem = num % 2;

if(rem == 0) { printf(even\n); }else{ printf(odd\n); } /* terminate program */ return 0;}

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The computers processor only understandsprograms written in its ownmachinelanguage

Sequences of 1s and 0s

Different for each processor family (x86, PowerPC,SPARC, ARM, )

How is it that it can obey instructionswritten in C?

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There are two steps in creating an executableprogram starting from C source code

A program called the Ccompilercompilertranslates the C code intoan equivalent program in the processors machinelanguage (1s and 0s)

A program called thelinkerlinkercombines this translatedprogram with any library files it references (printf, scanf,etc.) to produce an executable machine language program(.exe file)

Environments like Visual Studio do both steps whenyou build the program

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library(ANSI)header

(stdio.h)

executableprogram

com

piler

l

i

nk

e

r

sourcecode

objectcode

.c file 011010001101

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Lots!Things are rarely perfect on the first attempt

Both the compiler and linker could detect errors

Even if no errors are are detected, logic errors(bugs) could be lurking in the code

Getting the bugs out is a challenge even forprofessional software developers

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Syntax:Syntax:the required form of the programpunctuation, keywords (int, if, return, ), wordorder, etc.

The C compiler always catches these syntaxerrors or compiler errors

Semantics (logic):Semantics (logic):what the program meanswhat you want it to do

The C compiler cannot catch these kinds of errors!

They can be extremely difficult to find

They may not show up right away

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Type in the even/odd program and seewhat happens when you:

Leave off a few semicolons or misspellsomething (syntax)

In the last printf statements, change odd toeven. Run the program. What happens if youenter 17? (semantics)

Experiment and see what happens

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Weve covered a lotof new ideasAlgorithms and programsComputer organization and memoryThe basic components of C programsComments, declarations, statementsCompilers, linkers, libraries, and programexecutionErrors

Lots of terminology, too

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Upcoming lectures: review what weveseen today and fill in details

Meanwhile, get started reading andtrying things on the computer!

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