ICS124 Session 10 Creating a C program from Flowcharts & Pseudocode 1.

ICS124

Session 10

Creating a C program from Flowcharts & Pseudocode

1

By the end of this section, the student will be able to:

Appreciate the value of pseudocode as an interim step between flowcharts and a 3GL language

Translate a flowchart into psuedocode Describe the 5 steps of the compile process Use the compiler on Phobos to compile a simple program Recognize a preprocessor directive Incorporate the stdio.h header file into a program through the use of a

preprocessor directive Construct the "main" function of a C program Identify comments within a C program Write comments in a C program List the two symbols that define the beginning and ending of a block of code List the two components of a variable declaration Declare variables in a C program, and assign values to them List the two parts of a printf() function Use the printf() function to display text and values of variables List the two parts of the scanf() function Use the scanf() function to read integers and float variable types List the 6 relational operators Correctly use the relational operators in a C program Determine the numeric and logical result of a relational expression Translate psuedocode to C code

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Flowcharting, review

ReviewFlowcharts are a means to plan when things are done in a program. They can be thought of as a 'roadmap' to the solution. The symbols we have seen are:

Does Something(I/O, Process)

Makes a decision(Based on some information: do I

perform this or that set of steps, do I continue to loop?)

The thread that holds the flowchart together

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Pseudocode

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Pseudocode

PseudocodePseudocode (pseudo = fake, code = computer language), a fake computer language (3GL)

Throughout these discussions we have been using the PDC to move from a high level, general analysis of a problem to a more specific solutionThe IPO Chart gave us a brief description of the inputs, the outputs and a list of processes for transforming the inputs into the outputsThe Flowchart gave us, diagrammatically, the sequence or order in which processes and decisions are made to solve the problem

Pseudocode is a hybrid between the structured computer languages (C, Pascal, BASIC...) and EnglishThe control structures we defined in Flowcharting are represented hereBecause pseudocode is not a real computer language and it has loose rules for implementation, it can be used to develop an algorithm for any structured computer languageFrom the pseudocode (after testing the algorithm) the program can be coded by translating the statements in pseudocode to statements in the programming language (C in our case).

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Pseudocode

Pseudocode, continuedThere is no real standard for pseudocode, however it must have the following characteristics:

Statements are written in simple EnglishEach instruction is written on a separate lineKeywords and indentation are used to signify control structuresEach set of instructions is written from top to bottom, with only one entry point and one exit point

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Pseudocode

Pseudocode, continuedAgain the three basic structures are represented:

simple sequencesimple decisioniteration

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Pseudocode- Simple Sequence, continued

Simple sequence - Assigning Variables

This is where variables are manipulated within your algorithmThe New Webster's Computer Terms dictionary definition:

A variable is a symbolic name representing a value that changes during the program's execution

Back to Secondary School algebra, remember these?

5 = 5x + 4y0 = 5(x + y) - 3x + 5

Solve for x and y.

x and y are the variables.

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Simple sequence - Assigning Variables, continued

When creating a name for a variable, make it meaningfulDon't make it too long (you will end up with writer's ramp now and carpal tunnel syndrome later)

custName may be not as meaningful ascustomers_first_and_last_name but it is easier to write and probably conveys enough information about how the variable is being used. The other extreme cn is obviously meaningless.

Use your judgement. Ask yourself: If I had to re-read this in another year, would the variable name be meaningful?

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There are two types of variable assignments:When you are initializing a variableAs a result of some processing

It will be considered BAD FORM not to initialize a variable before using it

Verbs used: INITIALIZESET

INITIALIZE counters to zeroSET recordSize to 500

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As a result of processing:

Expect to use the symbol "="

totalPrice = basePrice + salesTax

You can use this form for initializing variables as well

counterA = 0counterB = 0recordSize = 500

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Flowchart to Pseudocode:

counterA = 0

SET recordSize to 500

counterA = 0

SET recordSizeto 500

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Simple sequence - Input of Data

Verbs used: READGET

READ usually refers to data being read from a file

GET usually refers to data being entered by the user at a keyboard

READ address FROM customerFileGET newAddress

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Simple sequence - Input of Data, continued

Flowchart to Pseudocode

READ address FROM customerFile

GET newAddress

Read addressFROM

customerFile

Get newAddress

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Simple sequence - Output of Data

Verbs used: PRINTWRITEOUTPUTDISPLAY

PRINT usually refers to data being sent to a printer

WRITE usually refers to data being sent to a file

OUTPUT and DISPLAY usually refer to data being sent to a monitor.

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Simple sequence - Output of Data, continued

Example:PRINT "Program Completed"WRITE newAddress TO customerFileOUTPUT name, address, postalCodeDISPLAY "End of Data Reached"

Note that any phrase that is to be output EXACTLY as written is enclosed in quotes

this helps to differentiate between a literal string and a variable name.

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Simple sequence - Output of Data, continued


WRITE address TO customerFile

DISPLAY newAddress

Write addressTO

customerFile

Display newAddress

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Simple sequence - Computation

Some verbs used: ADDCOMPUTECALCULATEMULTIPLYDIVIDESUBTRACT

ADD 1 TO counterCOMPUTE tax = price x 0.15degreesC = (degreesF - 32) x (5/9)

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Simple sequence - Computation, continued


ADD 1 TO counter

degreesC = (degreesF - 32) x (5/9)

Add 1 to counter

degreesC =(degreesF - 32)

x (5/9)

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Pseudocode- Selection

Simple Selection

The basic form looks like:

IF condition THENstatement(s)

ELSEstatement(s)

ENDIF

If the condition is TRUE, then the statements immediately following the THEN keyword are executed, until the ELSE clause. At which point the program resumes after the ENDIF keywordIf the condition is FALSE, then the statements immediately following the ELSE keyword are executedNote the indentation that is being used

Makes it easier to see the components of the selectionProvides for readability

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Pseudocode- Selection, continued

Simple Selection, continued


IF a > b THENc = e + f

ELSEc = t x q

ENDIFa > b c = e + f

c = t x q

TRUE

FALSE

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IF condition THENstatementAstatementBstatementC

ELSEstatementDstatementE

ENDIF

If condition StatementAStatementDTRUEFALSE

StatementB

StatementC

StatementE

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An example of no indentation: with indentation:

IF a = b THEN IF a = b THENDISPLAY "A equals B" DISPLAY "A equals B"c = (23 - d) / 2 c = (23 - d) / 2lifeUniverseEverything = 42 lifeUniverseEverything = 42ELSE ELSEDISPLAY "A does not equal B" DISPLAY "A does not equal B"vogons = 1 vogons = 1earth = 0 earth = 0ENDIF ENDIF

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What if you don't need an ELSE clause?

IF avg < 50 THENDISPLAY "Low average"

END IF

avg < 50DISPLAY

"Low Average"TRUE

FALSE

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Pseudocode- Iteration

Iteration: Pre-test loop

The basic form looks like:

DOWHILE conditionstatement(s)

ENDWHILE

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Pseudocode- Iteration, continued

Iteration: Pre-test loop, continued


DOWHILE i < 10b = b x c;i = i + 1

ENDWHILE

b = b x c

False

True

i < 10

i = i + 1

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Pseudocode, continued

Exercise Convert the following flowchart into pseudocode

hi = 101 resp != 'C'

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Start

lo = 1

DISPLAY"Choose a number"

DISPLAY"I Guess", guess,

"is it Hi, Lo or Correct?"

resp = 'N'

guess =(hi + lo) / 2

GET resp

resp = 'L'

resp = 'H'

hi = guess

lo = guess

A

A

B

B

C

C

Stop

DISPLAY"I found your number",

guess

TRUE

TRUE

TRUEFALSE

FALSE

FALSE

The C Language

A C ProgramA Program, and its flowchart that does nothing:

int main(void){ ;}

The Language C is case-sensitivecase-sensitive. If you use a lower-case character instead of an upper-case character, the compiler will treat the two words DIFFERENTLY!

I will use both flowcharts and pseudocode to define the C language.

Start

Stop

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The C Language, continued

C Program structureWhat about the structure of that useless program?

{};

A block of statements that are to be executed together are surrounded by the { and } characters.

For example: All the statements and structures in an IF-THEN-ELSE. All the statements and structures in a DOWHILE.

The semi-colon (;) is used to show the end of a statement.

A semi-colon by itself is a statement that does nothing. Not much use, nor very interesting

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C Program structure, continuedIn the C language (as in most languages) there is a means to imbed comments. Comments are a form of documentation and are a way to leaves notes to yourself or other programmers as to what is happening in your program.

All comments begin with '/*' and end with '*/'

/* this is a comment */

/* this is also a comment */

/***************************************\* ** Is this a comment ? ** *\***************************************/

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C Program structure, continuedHere is another program that is perhaps a little more interesting:

#include <stdio.h>int main(void){ printf("A program is born.\n");}

What is that #include <stdio.h>, and what about that printf()

Recall the compile process...

Start

Display "Aprogram is

born."”

Stop

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C Program structure, continued

That #include <stdio.h> is a preprocessor directive

It tells the compiler that there is a file called stdio.h that contains additional information that needs to be INCLUDED in your program.

header files have an extension of .hsource code files have an extension of .c

Source file

Library files (for O/S)

Object filePreprocessor and translator

Executable file

Linker

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PreprocessorAnatomy of the "include" preprocessor directive:

#include <stdio.h>

These angle brackets tell the preprocessor to look in the system library directory This preprocessor directive is used to include other files into your file

All preprocessor directives begin with a '#'

When you write your own header files, you will use the " and " characters around the filename.

stdio.h is the Standard-Input-Output library file. It is THE most common header file.

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printf() statementNow about the printf() statement:

This is the equivalent of the DISPLAY statement in pseudocode.

Whatever formatted data is output by the printf() statement is sent to the standard output device of the computer - in our case the monitor.

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printf() statement, continuedAnatomy of a simple printf() statement:

printf("A program is born.\n");

An "escape sequence" that is used to tell the computer you want to go to a NEWLINE

The beginning of a string of data to be printed EXACTLY as written

The library function being called

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printf() statement, continuedAnother, slightly more interesting program:

#include <stdio.h>int main(void){ printf("A program is born.\n"); printf("\nOh, sure you think it's cute and cuddly now\n"); printf("but just wait!\nAs it grows up it will become a "); printf("monster before you know it!\n");}

What is the output of this program?

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VariablesWe talked briefly about variables when discussing flowcharts and pseudocode. When using variables in C, there are rules that are followed.

When defining a variable to be used by the program, there are two parts: data type variable name

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Variables, data typesWe listed the data types for C in an early class when discussing data representation. Here they are again, with their typical sizes:

char 0 ... 255 (the numbers are usually used to represent ASCII or EBCDIC characters)

short int -32,768 ... 32,767int -2,147,483,648 ... 2,147,483,647long int -9,223,372,036,854,775,808 ... 9,223,372,036,854,775,807float 0.11754943508222875x10-39 ... 0.34028234663852886x10+39

double 0.2225073858507201x10-309 ... 0.17976931348623158x10+309

Any of the integer data types cannot store fractional information. To assign the calculation "5/2" to an integer would result in '2' being stored, not '2.5'.

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Variables, namingHere are some rules for picking variable names:

first character must be an alphabetic character (a-z A-Z) or an underscore (_)Must contain ONLY alphabetic characters, digits (0 - 9) and the underscoreOnly the first 31 characters are significant- the compiler only looks at the first 31 characters and ignores the rest

abcdefghijklmnopqrstuvwxyz01234467890 abcdefghijklmnopqrstuvwxyz01234567890 Are considered the same

variableCannot be the same as a 'reserved' word - those words that are known to the compiler (like "if" or "while")

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Variables, declaring Variables are declared at the beginning of the main() function (after the first

'{')

To declare a variable, the format is:

dataType variable1 [=value] [,variable2 [=value]]... ;

For example:

int counter;

int counter, classCount, courseCount;

int x = 0;

int x = 1, y = 2, z = 3;

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printf() funtion revisitedNow that we have defined variables in our C program, how do we display their contents?

There are two parts to the printf() function: The format string, which contains:

literal text to be output exactly as written

special control characters (such as the \n)

field specifications

A list of variables that correspond the field specifications in the format string

The format string is like a template, showing you where the text is (and what the text is) as well as where within the text the values of the variables are to be inserted

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printf() funtion revisited

printf("Total = %d Average = %f\n", ttl, avg);

The field specifications are "place holders" for the variable data that the program is calculating

Where-ever you place a field specification, the compiler will look for a corresponding variable in the list of variables

The first field specification refers to the first variable in the list, the second field specification refers the second variable in the list and so on...

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printf() funtion revisitedThe field specifications that correspond to the data types we will use in this course:

%c - char

%d - int (probably used 'd' for decimal or digits)

%f - float (by default 6 digits are shown to right of the decimal point)

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Mathematical operatorsThe following symbols are used for mathematical operations in the C language:

+ Addition

- Subtraction

* Multiplication

/ Division

%Modulo

= Assignment

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PracticeConvert the following flowchart into C:

Start

num = 5

num2 = 5

num3 =num x num2

Display "the productof ", num, "and”,num2, "is”, num3

Stop

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PracticeWhat would the print statement required to get this output:

Hi H0, H1 Ho, its off to work I go (at $1.500000 per day)

assuming the following C statements precede your printf() function

char dwarf;int valueA;int valueB;float wage;

dwarf = 'H' /* to be used in the 'Hi' and 'H1' words only */valueA = 0; /* hint: the second word needs this */valueB = 1; /* to be used in the third word */wage = 1.5;

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ExampleDevelop a program to take three numbers, add them together, then print their total and average (the numbers are 10, 20 and 40, and are initialized within the program).

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scanf() statementA computer program is fairly limited if it cannot get input data from a user as it is executing

There exists a function that is very similar to the printf() function that is used for formatted input

Anatomy of the scanf() statement:

scanf("format string", v1, v2, ...);

A list of variables that are referenced in theformat string. Each variable of type char, int or float is proceeded by a '&' character

A list of field specifications for the data being read

The function being called

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scanf() statement, continued

There is one major difference with the scanf() function when dealing with the variables:

scanf("%d", &i);

Notice the "&" symbol before the variable name

This is called the "address-of" operator

For now, it means that when you place the value read from the user into the variable, you can only store the data if you know the address of the variable within the computer's memory

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scanf() statement, continued

Due to the way that the scanf() function operates, include the following statements after every scanf() statement:

fflush(stdin);

This will tell the operating system to flush any characters that are in the input queue.

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Relational operatorsA decision structure within a programming language is fairly useless without a means to compare two or more variables

There are only a few types of Relational Operators

When a relation is expressed in C, the result is either TRUE or FALSE

A relation is also referred to as an Expression

These Expressions can be used in the decision structure- specifically the IF-THEN-ELSE and DOWHILE that we have seen in pseudocode

In C any non-zero value is considered TRUE, zero is considered FALSE

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Relational operators, continuedThe Relational Operators are:

Equal == (NOT "=" by itself)

Not Equal != (NOT "!==")

Less than <

Greater than >

Less than or equal <=

Greater than or equal >=

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Relational operators, continuedExample Expressions based on a relation:

a < b5 > t7 < 0(a + b) > (g - f)

When evaluating expressions that contain relational operators, a TRUE condition is represented by the value 1, a FALSE condition is represented by the value 0

For example:

The expression (7 > 8) would be represented by 0

The expression (7 < 8) would be represented by 1

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Relational operators, continuedWhat is the result of these expressions, and are they TRUE or FALSE?

7 - 2

5 < 3

i = 25

i = 0

((5 >= 3) + 2) < 3

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Relational operators, continuedWe have, in some of our examples, used such words as AND and OR when writing the condition in a Decision Box (for Selection as well as Iteration)

These are referred to as Logical Operators or Boolean operators

In C the AND logical operator is represented by &&In C the OR logical operator is represented by || (two vertical bars)In C the NOT logical operator is represented by !

Example:

PseudocodeIF a > b AND c < d OR g = k THEN

C

if (a > b && c < d || g == k)...

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Selection

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SelectionFlowcharts and Pseudocode:

expressionDo true

stmt

Do falsestmt

T

F

IF expression THEN true statementELSE false statementENDIF

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Selection

A couple of things to note: There is no THEN keyword The expression is enclosed in parenthesis There is no ENDIF keyword Only ONE statement is associated with each part of the IF statement


IF expression THEN true statementELSE false statementENDIF

if (expression) trueStatement;else falseStatement;

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Selection, continuedIn the previous example there was only one statement in each the true branch and one statement in the false branch

How do you create a block of statements in C?


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Selection, continuedHow do you create a block of statements in C?

Use the { and } symbols:

if (expression){ statement1; statement2; statement3;}else{ statement4; statement5;}

Note that there is NO semicolon after the closing brace }


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Selection, continuedAs in pseudocode and flowcharts, the ELSE clause can be omitted:

if (expression) IF expression THEN statement1; statement1 ENDIF

if (expression) IF expression THEN{ statement2 statement2; statement3 statement3; statement4 statement4; ENDIF}


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Selection, continuedAs you might surmise, it may be confusing when reading code that might contain braces for multiple statements or no braces for a single statement:

if (a > b) printf("a is greater than b\n"); if (a > c) printf("a is also greater than c\n");else printf("a is less than or equal to b\n");

Athough it looks OK, in reality it is nothing more than a badly indented version of:

if (a > b) printf("a is greater than b\n");

if (a > c) printf("a is also greater than c\n");else printf("a is less than or equal to b\n");

Be on the lookout for this pitfall- what is the correct code?


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Selection, continued

if (a > b) printf("a is greater than b\n”); if (a > c) printf("a is also greater than c\n”);else printf("a is less than or equal to b\n”);

Should be:

if (a > b){ printf("a is greater than b\n”); if (a > c) printf("a is also greater than c\n”);}else printf("a is less than or equal to b\n”);


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Iteration

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Iteration

We discussed loops previously: pre-test loops


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Iteration, continued

Flowcharts and Pseudocode:


DOWHILE expression repeated expression update statementENDWHILE

expression

repeatedexpression

updatestatement

True

False

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Iteration, continued

A couple of things to note: The expression is enclosed in parenthesis, just like the if There is no ENDWHILE keyword Only ONE statement is associated with the loop, just like the if


DOWHILE expression repeated expressionENDWHILE

while (expression) repeatedExpression;

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Iteration, continuedHow do you create a block of statements in C?

Use the { and } symbols:

while (expression){ statement1; statement2; statement3;}


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Iteration, continuedA comparative example, pseudocode versus C:


SET i = 0WHILE i <= 10 DISPLAY "i = ", i i = i + 1ENDWHILE

DISPLAY "Done Count"”

#include <stdio.h>int main (void){ int i;

i = 0; while (i <= 10) { printf("i = %d\n", i); i = i + 1; } printf("Done Count\n");}

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Exercise

Write a C program that will prompt the user for three pieces of data. The program will add them together and print the result. The prompts and output formats are your choice. After the result has been printed, ask the user for another set of three numbers. Keep repeating until the user enters zero for all three numbers.


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ICS124 Session 10 Creating a C program from Flowcharts & Pseudocode 1.

Documents

c code2flowcharting

c programdetermine

c programlist

programming language

c programidentify comments

c programwrite comments

fake computer language

real computer language