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ECE 246(INTRODUCTION TO C

PROGRAMMING)CHAPTER 3:

CONTROL FLOW

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Learning Outcomes Able to understand the assignment variations

Able to understand the usage selection

statements/structures Able to understand the usage of repetition

statements/structures

Able to understand the usage ofcontinue/break

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Lesson Outline Assignment variations

Selection statements: if, if else, switch

Repetition statements: for, while, do while

Break/continue

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Lets Recap Computing problems

All can be solved by executing a series of actions in a

specific order

Algorithm: procedure in terms of

Actions to be executed

The order in which these actions are to be executed

Program control

Specify order in which statements are to be executed

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Lets Recap Pseudocode

Artificial, informal language that helps us

develop algorithms Similar to everyday English

Not actually executed on computers

Helps us think out a program before writing it Easy to convert into a corresponding C++ program

Consists only of executable statements

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More Assignment Operators Assignment operators abbreviate assignment expressions

c = c + 3;

can be abbreviated as c += 3;using the addition assignment operator

Statements of the formvariable=variableoperatorexpression;can be rewritten as

variableoperator=expression;

Examples of other assignment operators:

d -= 4 (d = d - 4)e *= 5 (e = e * 5)

f /= 3 (f = f / 3)

g %= 9 (g = g % 9)

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8

Assignmentoperator

Sampleexpression

Explanation Assigns

Assume:intc = 3,d = 5,e = 4,f = 6,g = 12;+= c += 7 C = c + 7 10to c-= d -= 4 D = d - 4 1to d*= e *= 5 E = e * 5 20to e/= f /= 3 F = f / 3 2to f%= g %= 9 G = g % 9 3to g

More Arithmetic assignment operators.

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Increment and Decrement Operators Increment operator (++)

Can be used instead of c+=1

Decrement operator (--)

Can be used instead of c-=1

Preincrement

Operator is used before the variable (++cor --c)

Variable is changed before the expression it is in is evaluated

Postincrement Operator is used after the variable (c++or c--)

Expression executes before the variable is changed

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Increment and Decrement Operators If cequals 5, then

printf( "%d", ++c );

Prints 6

printf( "%d", c++ );

Prints 5

In either case, cnow has the value of 6

When variable not in an expression

Preincrementing and postincrementing have the same effect

++c;

printf( %d, c );

Has the same effect asc++;

printf( %d, c );

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11

Operator Sample expression Explanation

++ ++a Increment aby 1, then use the new value of ain theexpression in which aresides.

++ a++ Use the current value of ain the expression in which aresides, then increment aby 1.

-- --b Decrement bby 1, then use the new value of bin theexpression in which bresides.

-- b-- Use the current value ofbin the expression in which bresides, then decrement bby 1.

Increment and decrement operators.

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The ifselection statement Selection structure:

Used to choose among alternative courses of action

Pseudocode:

If students grade is greater than or equal to 60Print Passed

If condition true

Print statement executed and program goes on to next

statement If false, print statement is ignored and the program

goes onto the next statement

Indenting makes programs easier to read

C ignores whitespace characters

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The if selection statement

Pseudocode statement in C:if ( grade >= 60 )

printf( "Passed\n" );

C code corresponds closely to the pseudocode

Diamond symbol (decision symbol)

Indicates decision is to be made

Contains an expression that can be trueor false

Test the condition, follow appropriate path

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Flowcharting the single-selection ifstatement.

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The ifelseselection statement if

Only performs an action if the condition is true

ifelse

Specifies an action to be performed both whenthe condition is trueand when it is false

Psuedocode:

If students grade is greater than or equal to 60Print Passed

elsePrint Failed

Note spacing/indentation conventions

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If statement Exercise: Expected Output

/* demonstrates the usage of if */

#include

int main()

{

const float limit = 3000.0;

int id_num;float miles;

printf("Please type in the car number and mileage:\n");

scanf("%d\n %f\n",&id_num,&miles);

if (miles > limit)printf("Car %d is over the limit.\n",id_num);

printf("End of program output");

return 0;

}

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The ifelseselection statement

Sample of C code:

if ( grade >= 60 )

printf( "Passed\n");else

printf( "Failed\n");

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Flowcharting the double-selection if elsestatement.

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If else statement Exercise: Expected Output

/* demonstrates the usage of if else*/

#include

int main()

{

float radius;

printf("Please type in the radius\n");

scanf(%f\n",&radius);

if (radius < 0.0)

printf(A negative radius is invalid.\n);

elseprintf(Thearea of this circle is %f,(3.14*radius*radius));

return 0;

}

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The ifelseselection statement

Nested ifelsestatements

Test for multiple cases by placing ifelse

selection statements inside ifelseselectionstatement

Once condition is met, rest of statements skipped

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The ifelseselection statement Pseudocode for a nested ifelsestatement

If students grade is greater than or equal to 90Print A

elseIf students grade is greater than or equal to 80Print B

elseIf students grade is greater than or equal to 70

Print CelseIf students grade is greater than or equal to 60

Print Delse

Print F

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The ifelseselection statement Compound statement:

Set of statements within a pair of braces

Example:

if ( grade >= 60 )printf( "Passed.\n" );else {

printf( "Failed.\n" );printf( "You must take this course

again.\n" );}

Without the braces, the statement printf( "You must take this course

again.\n" );

would be executed automatically

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Nested If else statement Exercise1: Expected Output

/* demonstrates the usage of nested if else*/

#include

int main()

{

char code;

printf("Enter a specification code\n");scanf("%c\n",&code);

if (code == 'S')

printf("The item is space exploration grade.\n");

else if (code == 'M')

printf("The item is military grade.\n");

else if (code == 'C')printf("The item is commercial grade.\n");

else if (code == 'T')

printf("The item is toy grade.\n");

else

printf("An invalid code was entered.\n");

return 0;

}

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switchMultiple-Selection Statement

switch Useful when a variable or expression is tested for all the values it can

assume and different actions are taken

Format Series of caselabels and an optional defaultcase

switch ( value ){

case '1':

actions

case '2':

actions

default:

actions

}

break;exits from statement

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switch multiple-selection statement withbreaks.

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Nested If else statement Exercise2: Expected Output/* demonstrates the usage of switch*/

#include

int main()

{

int opselect;

float fnum, snum;

printf("Please type in two numbers\n");

scanf("%f\n%f\n",&fnum,&snum);

printf("Enter a select code:\n");printf("1 For addition:\n2 For multiplication:\n3 For division:\n");

scanf("%d\n",&opselect);

switch (opselect)

{

case 1:

printf("The sum of the numbers entered is %f", fnum+snum);

break;case 2:

printf("The product of the numbers entered is %f", fnum*snum);

break;

case 3:

printf("The first number divided by the second is %f", (fnum/snum));

break;

}

return 0;

}

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Good Programming Practice

Provide a defaultcase in switchstatements. Cases not explicitly tested in

a switchare ignored. The defaultcase helps prevent this by focusing the

programmer on the need to process

exceptional conditions. There aresituations in which no defaultprocessing is needed.

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In a switchstatement when thedefaultclause is listed last, the

breakstatement is not required.But some programmers include this

breakfor clarity and symmetry

with other cases.

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Repetition Statement Essentials

Loop

Group of instructions computer executes repeatedly while

some condition remains true

Counter-controlled repetition

Definite repetition: know how many times loop will

execute

Control variable used to count repetitions

Sentinel-controlled repetition

Indefinite repetition

Used when number of repetitions not known

Sentinel value indicates "end of data"

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Counter-Controlled Repetition - while

Counter-controlled repetition requires

The name of a control variable (or loop counter)

The initial value of the control variable An increment (or decrement) by which the

control variable is modified each time through the

loop

A condition that tests for the final value of the

control variable (i.e., whether looping should

continue)

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Counter-Controlled Repetition - while

Example:int counter = 1; // initialization

while ( counter

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2 Counter-controlled repetition */3 #include4

5 /* function main begins program execution */6 intmain( void)7 {8 intcounter = 1; /* initialization */9

10 while( counter

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Counter-Controlled Repetition

Condensed code

C Programmers would make the program more

concise Initialize counterto 0

while ( ++counter

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Good Programming Practices

Control counting loops with integer values.

Indent the statements in the body of each controlstatement.

Put a blank line before and after each control statement tomake it stand out in a program.

Too many levels of nesting can make a program difficultto understand. As a general rule, try to avoid using morethan three levels of nesting.

The combination of vertical spacing before and aftercontrol statements and indentation of the bodies of controlstatements within the control-statement headers gives

programs a two-dimensional appearance that greatly

improves program readability.

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forstatement header components.

1 /* Fig. 4.2: fig04_02.c

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2 Counter-controlled repetition with the for statement */3 #include4

5 /* function main begins program execution */6 intmain( void)7 {8 intcounter; /* define counter */9

10 /* initialization, repetition condition, and increment11 are all included in the for statement header. */12 for( counter = 1; counter

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Error-Prevention Tip

Using the final value in the condition of a

whileor forstatement and using the

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forRepetition Statement

Format when using forloopsfor( initialization; loopContinuationTest; increment)

statement

Example:for( int counter = 1; counter

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forRepetition Statement For loops can usually be rewritten as while loops:

initialization;while( loopContinuationTest) {statement;increment;

}

Initialization and increment

Can be comma-separated lists

Example:for (int i = 0, j = 0; j + i

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Common Programming Errors

Using commas instead of semicolons

in a forheader is a syntax error.

Placing a semicolon immediately to

the right of a forheader makes thebody of that forstatement an empty

statement. This is normally a logicerror.

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forStatement :Notes and Observations

Arithmetic expressions

Initialization, loop-continuation, and increment can contain arithmeticexpressions. If xequals 2and yequals 10

for ( j = x; j

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Although the value of the control

variable can be changed in the body

of a forloop, this can lead to subtleerrors. It is best not to change it.

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Flowcharting a typical forrepetition statement.

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Exercise: Expected Output

/* by using for */#include

/* function main begins program execution */

int main( void )

{

int counter; /* declaration */

/* for statement header */

for ( counter = 2; counter

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Good Programming Practices

Although statements preceding a forandstatements in the body of a forcan often bemerged into the forheader, avoid doing sobecause it makes the program more difficult to

read.

Limit the size of control-statement headers to a

single line if possible.

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/* by using for */

#include

/* function main begins program execution */int main( void )

{

const int MAXNUM = 10; /* declaration of constant value*/

int num;

printf("NUMBER\tSQUARE\tCUBE\n------\t------\t----\n");

for ( num = 1; num

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Do not use variables of type floatordoubleto perform monetary

calculations. The impreciseness offloating-point numbers can cause errors

that will result in incorrect monetary

values.

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dowhileRepetition Statement

The dowhilerepetition statement

Similar to the whilestructure

Condition for repetition only tested after the bodyof the loop is performed

All actions are performed at least once

Format:

do {

statement;

} while (condition);

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dowhileRepetition Statement

Example (letting counter = 1):do {

printf( "%d ", counter );

} while (++counter

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Some programmers always include

braces in a do...whilestatement

even if the braces are not necessary.This helps eliminate ambiguity between

the do...whilestatement

containing one statement and thewhilestatement.

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Common Programming Error

Infinite loops are caused when the loop-continuation condition in a while, forordo...whilestatement never becomesfalse. To prevent this, make sure there is nota semicolon immediately after the header ofa whileor forstatement. In a counter-controlled loop, make sure the control

variable is incremented (or decremented) inthe loop. In a sentinel-controlled loop, makesure the sentinel value is eventually input.

1 /* Fig. 4.9: fig04_09.c2

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2 Using the do/while repetition statement */3 #include4

5 /* function main begins program execution */6 intmain( void)7 {8 intcounter = 1; /* initialize counter */9

10 do{11 printf( %d , counter ); /* display counter */12 } while( ++counter

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Flowcharting the do whilerepetition statement.

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breakand continue

Statements break

Causes immediate exit from a while, for,

dowhileor switchstatement

Program execution continues with the first

statement after the structure

Common uses of the breakstatement

Escape early from a loop

Skip the remainder of a switchstatement

1 /* Fig. 4.11: fig04_11.c2 Using the break statement in a for statement */

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3 #include4

5 /* function main begins program execution */6 intmain( void)7 {8 intx; /* counter */9

10 /* loop 10 times */11 for( x = 1; x

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breakand continue

Statements continue

Skips the remaining statements in the body of a

while, foror dowhilestatement

Proceeds with the next iteration of the loop

whileand dowhile

Loop-continuation test is evaluated immediately after

the continuestatement is executed for

Increment expression is executed, then the loop-

continuation test is evaluated

1 /* Fig. 4.12: fig04_12.c2 Using the continue statement in a for statement */

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3 #include4

5 /* function main begins program execution */6 intmain( void)7 {8 intx; /* counter */9

10 /* loop 10 times */11 for( x = 1; x

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Logical Operators

&&( logical AND )

Returns trueif both conditions are true

||( logical OR )

Returns trueif either of its conditions are true

!( logical NOT, logical negation )

Reverses the truth/falsity of its condition

Unary operator, has one operand

Useful as conditions in loopsExpression Result

true && false falsetrue || false true

!false true

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expression1 expression2 expression1 &&expression20 0 0

0 nonzero 0

nonzero 0 0

nonzero nonzero 1

Truth table for the (logical AND)

operator.

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expression !expression

0 1

nonzero 0

Truth table for operator !

(logical negation).

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Performance Tip

In expressions using operator &&, make

the condition that is most likely to be false

the leftmost condition. In expressionsusing operator ||, make the condition thatis most likely to be true the leftmost

condition. This can reduce a programsexecution time.

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Operators Associativity Type

++ (postfix) -- (postfix) right to left postfix+ - ++(prefix) -- (prefix) (type) right to left unary* / % left to right multiplicative+ - left to right additive< >= left to right relational== = left to right equality&& left to right logical AND|| left to right logical OR?: right to left conditional= += -= *= /= %= right to left assignment, left to right comma

Operator precedence and

associativity.

f i li ( ) d

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Confusing Equality (==) andAssignment (=) Operators

Dangerous error

Does not ordinarily cause syntax errors

Any expression that produces a value can be usedin control structures

Nonzero values are true, zero values are false

Example using ==:if ( payCode == 4 )

printf( "You get a bonus!\n" );

Checks payCode, if it is 4then a bonus is awarded

C f i li ( ) d

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Confusing Equality (==) andAssignment (=) Operators

Example, replacing ==with =:if ( payCode = 4 )

printf( "You get a bonus!\n" );

This sets payCodeto 4

4is nonzero, so expression is true, and bonus awarded no

matter what the payCodewas

Logic error, not a syntax error

C f i E li ( ) d

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Confusing Equality (==) andAssignment (=) Operators lvalues

Expressions that can appear on the left side of an equation

Their values can be changed, such as variable names

x = 4;

rvalues

Expressions that can only appear on the right side of an equation

Constants, such as numbers

Cannot write 4 = x;

Must write x = 4; lvalues can be used as rvalues, but not vice versa

y = x;

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Structured Programming Summary

All programs can be broken down into 3 controls

Sequencehandled automatically by compiler

Selectionif, ifelseor switch

Repetitionwhile, dowhileor for Any selection can be rewritten as an ifstatement, and

any repetition can be rewritten as a whilestatement

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Review ExercisesWrite individual for statements for the following cases:

Use a counter named i that has an initial value

of 1, a final value of 20, and an increment of 1

Use a counter named icount that has an initial

value of 1, a final value of 20, and an

increment of 2

Use a counter namedj that has an initial value

of 1, a final value of 100, and an increment of

5

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Review Exercises continueWrite individual for statements for the following cases:



increment of -1



increment of -2

Use a counter named count that has an initial

value of 1.0, a final value of 16.2, and an

increment of 0.2

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Review Exercises continue

Finally, determine the number of times that each

for loop is executed for the previous for

statements.

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References

Burgess, M. (1999), C Programming Tutorial

(K&R version 4)

Deitel, P.J. & Deitel, H.M. (2007), C How ToProgram, Pearson International Edition

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Ece246 Chapter 3