Chapter 10: Control Structures1 Chapter 10 Control Structures.

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Chapter 2 Chapter 10: Control Structures 1

Chapter 10

Control Structures

Chapter 10 Flow of Control 2

Flow of Control

Sequence

Selection

Repetition

condition

action-1

action-2

condition

action

Selection structure

Repetition structure

Chapter 10 Selection: ‘if’ construct 3

Selection: ‘if’ construct

Syntax: if (expression)statement ;

expression is the condition for the ‘if’ construct.

If expression is evaluated to non-zero (true), statement is executed.

If expression is evaluated to zero (false), statement is skipped.

Chapter 10 'if’ construct 4

‘if’ construct

Syntax: if (expression)statement ;

fine = 20; if (speed > 50) fine += 10; printf("Fine is %d\n", fine);

Chapter 10 Compound-statement Action 5

Compound-statement Action

Syntax: if (expression) { compound-statement ;}

if (a > b) max = a; if (a > b) printf("%d is larger than %d\n", a, b);

if (a > b) { max = a; printf("%d is larger than %d\n", a, b); }

Chapter 10 Flags 6

Flag: an integer variable that simulates a Boolean variable. Contains 0 (false) or 1 (true).

if (attended == 1) attendance++;

if (attended) attendance++;

if (attended == 0) { absentees++; printf("One more absentee.\n"); }

if !(attended) { absentees++; printf("One more absentee.\n"); }

Chapter 10 Naming a Flag 7

Naming a Flag

Appropriate naming is desirable.

Example: a flag ‘attended’ implies ‘attended’ if it contains 1 (true)

‘did not attend’ if it contains 0 (false)

Chapter 10 Nested ‘if’ statement 8

Nested ‘if’ statement

Example:

if (exam >= 80) if (project >= 90) grade = 'A';

Above could be rewritten as:

if (exam >= 80 && project >= 90) grade = 'A';

Chapter 10 Short-circuit evaluation 9

Short-circuit evaluation

Evaluation stops as soon as value of expression is known. Evaluation is from left to right.

For logical AND (&&), if the partial expression is false, the whole expression is false.

For logical OR (||), if the partial expression is true, the whole expression is true.

Chapter 10 Short-circuit evaluation 10

Short-circuit evaluation

Example:

if (bank_bal < 0.0 || expenses > 100.0) printf("Red alert!\n");

Chapter 10 Complementing a Condition 11

Complementing a Condition

Comlementing or negating a logical expression means changing the polarity.

Examples:

!(a == 30) is equivalent to (a != 30)

!(a > b) is equivalent to (a <= b)

Chapter 10 DeMorgan’s Theorem 12

DeMorgan’s Theorem

NOT(a AND b) same as NOT(a) OR NOT (b)

NOT(a OR b) same as NOT(a) AND NOT(b)

!(expr1 && expr2) same as !(expr1) || !(expr2)

!(expr1 || expr2) same as !(expr1) && !(expr2)

Chapter 10 DeMorgan’s Theorem 13

DeMorgan’s Theorem

Example:

!(age > 25 && (status == 'S' || status == 'D'))

(age <= 25) || (status != 'S' && status != 'D'))

Chapter 10 Common Mistakes 14

Common Mistakes

Do not use == and != on floating-point numbers.

Mixing up == with =.

Wrong placament of semi-colon, resulting in empty statement:

printf() is outside ‘if’ construct.

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

Common Mistakes

Translating condition in English to C:

Let lower be 10, and upper be 30.If x is 20, (lower <= x) is true, so it is evaluated to 1. Since (1 <= upper) is also true, condition is true!

if (lower <= x <= upper) ...

Common Mistakes

Wrong condition:

Correct method:

if (lower <= x <= upper) ...

which is equivalent to this (since <= has a higher precedence than &&):

if ((lower <= x) && (x <= upper))

if (lower <= x && x <= upper)

Common Mistakes

Wrong condition:

Correct method:

if (x && y > z)

if (x > z && y > z)

Common Mistakes

Forgetting the braces for compound statements:

Correct method:

if (a > b) max = a; printf("%d is larger than %d\n", a, b);

if (a > b) { max = a; printf("%d is larger than %d\n", a, b); }

Chapter 10 ‘if-else’ construct 19

‘if-else’ construct

Syntax:

if (expression)statement1 ;

elsestatement2 ;

if (expression) { compound-statement1 ;}else { compound- statement2 ;}

May be used to avoid redundant code:

Use ‘if-else’ construct:

if (count == 0) ave = 0.0; if (count != 0) ave = (float) total/count;

if (count == 0) ave = 0.0; else ave = (float) total/count;

Another example: if (score1 < score2) { better_score = score2; printf("score2 is better\n"); } else { better_score = score1; printf("score1 is better\n"); }

Chapter 10 Style 22

Two common styles:

if (expression) { compound-statement1 ;}else { compound- statement2 ;}

Chapter 10 Removing common statements 23

Removing common statements

Common statements in the ‘then’ and ‘else’ parts should be moved out of the ‘if’ construct, if appropriate:

if (a < 0) { count++; neg++; printf("Enter an integer: "); scanf("%d", &k); } else { count++; pos++; printf("Enter an integer: "); scanf("%d", &k); }

Chapter 10 Removing common statements 24

Removing common statements

After moving common statements out of ‘if’ construct:

count++; if (a < 0) neg++; else pos++; printf("Enter an integer: "); scanf("%d", &k);

Chapter 10 Logical assignment for Flags 25

Logical assignment for Flags

Example:

if (age >= 65) snr_citizen = 1; else snr_citizen = 0;

‘if-else’ statement may be replaced by an assignment statement.

snr_citizen = (age >= 65);

Chapter 10 Logical assignment for Flags 26

Logical assignment for Flags

Another example:

if (n % 2 == 0) even = 1; else even = 0;

even = (n % 2 == 0);

even = !(n % 2);

Chapter 10 Nested ‘if-else’ statements 27

Nested ‘if-else’ statements

Example:

if (marks < 50) grade = 'F'; else if (marks < 70) grade = 'B'; else grade = 'A’;

Which ‘if’ is the ‘else’ associated with? if (a == 1) if (b == 2) printf("***\n"); else printf("###\n");

‘else’ is associated with nearest ‘if’. if (a == 1) if (b == 2) printf("***\n"); else printf("###\n");

To override default association, use braces to mark out block.

if (a == 1) { if (b == 2) printf("***\n"); } else printf("###\n");

Example:

if (a == 0) if (b == 0) printf("Both a and b are zeros.\n"); else c = b/a;

if (a == 0) { if (b == 0) printf("Both a and b are zeros.\n"); } else c = b/a;

Example: if (marks < 50) grade = 'F'; else if (marks < 60) grade = 'D'; else if (marks < 70) grade = 'C’; else if (marks < 80) grade = 'B'; else grade = 'A';

if (marks < 50) grade = 'F'; else if (marks < 60) grade = 'D'; else if (marks < 70) grade = 'C’; else if (marks < 80) grade = 'B'; else grade = 'A';

Common Mistakes

Wrong matching of ‘else’ with ‘if’.

Wrong placement of semi-colon.

if (a == b) { a++; b--; }; else a = b;

Chapter 10 Conditional Operator (?:) 33

Conditional Operator (?:)

Ternary operator:

condition ? expr1 : expr2

First operand is condition.

If condition is true, take value of expr1; otherwise, take value of expr2.

Example:

max = (a > b ? a : b);

equivalent to:

if (a > b) max = a; else max = b;

Example:

equivalent to:

printf("%s\n", grade < 50 ? "Failed" : "Passed");

if (marks < 50) printf("Failed\n"); else printf("Passed\n");

Chapter 10 ‘switch’ construct 36

‘switch’ construct

Multi-way selection statement:

switch (expression) {case v1:s1 ;

break;case v2:s2 ;

break;. . .default:sn ;

break; / * optional break */}

May only test constant integral expressions, i.e., expressions that evaluate to integers or characters.

The vi’s are integral values; the si’s are compound statements.

After expression is evaluated, control jumps to appropriate ‘case’ label.

‘break’ statements are inserted to avoid falling through.

Example: switch (class) { case 'B': case 'b': printf ("Battleship\n"); break; case 'C': case 'c': printf ("Cruiser\n"); break; case 'D': case 'd': printf ("Destroyer\n"); break; case 'F': case 'f': printf ("Frigate\n"); break; default : printf ("Unknown ship class %c\n", class); }

Chapter 10 Repetition Structure 39

Repetition Structure

Counter-controlled repetiton: number of iterations known.

Sentinel-controlled repetiton: iterate until a sentinel value is entered, or terminating condition is true.

Chapter 10 ‘while’ construct 40

‘while’ construct

Loop structure with pre-test condition.

while (expression)statement ;

expression is loop condition.

If expression is true, statement in loop body is executed, and expression tested again.

If expression is false, loop terminates.

Example: Print n asterisks. count_star = 0; while (count_star < n) { printf("*"); count_star++; }

count_star = 0; while (count_star++ < n) printf("*");

count_star is the loop control variable.

Example: Compute sum of first 100 positive integers.

int num = 1; /* declaration and */int total = 0; /* initialisation */

while (num <= 100) { total += num; num++;}

Which of these is/are same as previous code?

int num = 1;int total = 0;while (num <= 100) { total += num; ++num;}

int num = 1;int total = 0;while (num <= 100) total += num++;

int num = 1;int total = 0;while (num <= 100) total += ++num;

Loop control variable. Initialisation: before the loop is entered, the variable

must be initialised. Testing: condition involving the loop control variable

is tested before the start of each loop iteration; if condition is true, loop body is executed.

Updating: loop control variable is updated during each iteration (usually at the beginning or the end of the loop body).

Chapter 10 Counter-control repetition 45

Counter-control repetition

A counter is used to keep track of number of iterations. #define N 10

total = 0; count = 1;

while (count++ <= N) { printf("Enter score: "); scanf("%d", &score); total += score; }

avg = (float) total / N; printf("Average is %.2f\n", avg);

Chapter 10 Sentinel-control repetition 46

Sentinel-control repetition

A sentinel is used to denote end of data.

#define SENTINEL –1 . . .

total = 0; count = 0;

printf("Enter score, –1 to end: "); scanf("%d", &score);

while (score != SENTINEL) { total += score; count++; printf("Enter score, –1 to end: "); scanf("%d", &score); }

if (count) { avg = (float) total/count; printf("Average is %.2f\n", avg); } else printf("No scores were entered\n");

Chapter 10 ‘do-while’ construct 47

‘do-while’ construct

Loop structure with post-test condition.

do statement ;while (expression);

Loop body is executed at least once.

Chapter 10 ‘do-while’ construct 48

‘do-while’ construct

Examples:

count = 1; do { printf ("%d ", count); } while (++count <= 10);

do { printf("Enter a letter A thru E: "); scanf("%c", &letter); } while (letter < 'A' || letter > 'E');

Chapter 10 Flag-controlled loops 49

Flag-controlled loops

When loop condition is complex, flags may be used.

valid = 1; while (valid) { printf("Enter a letter A thru E: "); scanf("%c", &letter); valid = (letter >= 'A' && letter <= 'E'); }

Chapter 10 ‘for’ construct 50

‘for’ construct

Another pre-test loop structure.Provides more compact form for counter-

controlled loops.

for ( initialisation-expression;loop-condition;update-expression )

statement;

‘for’ constructThe ‘for’ construct is similar to this ‘while’ construct.

initialisation-expression;while ( loop-condition ) { statement; update-expression;}

‘for’ construct

Example:

for (count_star = 0; /* init */ count_star < N; /* condition */ count_star++) /* update */ printf ("*");

‘for’ construct

The initialisation-expression and update-expression are often comma-separated lists of expressions.

The comma operator evaluates the list from left to right.

for (x = 1, total = 0; x <= 100; x++) total += x;

‘for’ construct

Any of the three expressions in the ‘for’ header may be omitted, but the semi-colons must stay.

If initialisation-expression is omitted, you must perform necessary initialisation before loop.

x = 1; total = 0; for (; x <= 100; x++) total += x;

‘for’ construct

If update-expression is omitted, you must ensure that necessary update operations are done in loop body.

for (x = 1, total = 0; x <= 100;) { total += x; x++; }

‘for’ construct

If loop-condition is omitted, then the test is always true.

This loop is infinite:

for (x = 1, total = 0; ; x++) total += x;

Common Mistakes

Wrong placement of semi-colon.

for (x = 1; x <= 10; x++); printf("%d\n", x);

x = 1; while (x <= 10); printf("%d\n", x++);

Common Mistakes

Omitting semi-colons in ‘for’ header.Mixing up semi-colons with commas in ‘for’

header.Off-by-one error, where the loop executes one

more or one fewer iteration than intended. How many iterations does this loop execute?

for (count = 0; count <= n; ++count) sum += count;

Chapter 10 ‘break’ statement 59

‘break’ statement

Used in loops, ‘break’ causes execution to break out of the loop that contains the statement.

#include <stdio.h> main() { int x; for (x = 1; x <= 10; x++) { if (x == 5) break; /* break loop only if x == 5 */ printf("%d ", x); } printf("\nBroke out of loop at x == %d\n", x); return 0; }

1 2 3 4 Broke out of loop at x == 5

Chapter 10 'continue’ statement 60

‘continue’ statement

The ‘continue’ statement causes execution to skip remaining loop body and proceed to next iteration. #include <stdio.h>

main() { int x; for (x = 1; x <= 10; x++) { if (x == 5) continue; /* skip remaining code in loop only if x == 5 */ printf("%d ", x); } printf("\nUsed 'continue' to "); printf("skip printing the value 5\n"); return 0; }

1 2 3 4 6 7 8 9 10 Used 'continue' to skip printing the value 5

Chapter 10 Nested loops & combined structures

Nested loops & combined structures

An example of nested ‘for’ loops.

for (i = 1; i <= 4; ++i) { for (j = 1; j <= 6; ++j) printf("*"); printf("\n"); }

****** ****** ****** ******

Example 2:

for (i = 1; i <= 4; ++i) { for (j = 1; j <= i; ++j) printf("*"); printf("\n"); }

* ** *** ****

Example 3:

for (i = 1; i <= 6; ++i) { if (i <= 3) printf("%d", i); else for (j = 1; j <= i; ++j) printf("*"); printf("\n"); }

1 2 3 **** ***** ******

Chapter 10 Homework 64

Homework

Try exercises behind chapter 10.

Chapter 10: Control Structures1 Chapter 10 Control Structures.

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