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Lecture 5 More flow control structures

for do continue break switch

Structured programming Common programming errors and tips Readings:

Chapter 3: Section 10-21

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for statement General form of for-statement

repetition statement

for (expr1; expr2; expr3) statement

The above form is equivalent to the following while statement given that expr2 exists, and that no continue statement exists in statement

expr1;while (expr2) { statement expr3;}

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Example of for statement

/* To sum 1 to 100 */ #include <stdio.h>

int main(void) { int i, sum = 0; for (i = 1; i <= 100; i++) sum + = i; printf(“%d”, sum); return 0;}

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Comma operator (,) General form is

expr1, expr2 It has the lowest precedence of all operators in C

and is evaluated from left to right The comma expression as a whole has the value

and type of its right operand Sometimes used in for statements to allow

multiple initializations and multiple processing of indices

The comma operator is rarely used Not all commas in a C program are comma

operators

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Examples of comma operator

sum=0;

for (j=1; j<=10; j++)

sum += j;

and

for (sum=0,j=1; j<=10; j++)

sum += j;

and

for (sum=0,j=1; j<=10; sum += j, j++)

;

are equivalent

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do statement General form of do statement (repetition

statement)

do { statement

} while (expression);

Semantics: statement is executed first; thus the loop

body is run at least once If the value of expression is non-zero (true),

the loop repeats; otherwise, the loop terminates

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Example of do statementint error; /* no Boolean type support */int n;...do {

printf(“Input a positive integer: ”);scanf(“%d”, &n);

if (n <= 0) { error = 1; /* non-zero value */

printf(“\nError: -ve input!\n”}; } else error = 0;} while (error);...

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Program Remarks The variable error is used as a flag:

When it is non-zero, it indicates that n0 When it is zero, it indicates that n>0 The do-while statement makes use of error to

determine whether to continue or not If we want to execute the loop-body at least

once, it is more convenient to use a do-while statement than a while statement. E.g., compare this loop with that on slide 14 of lecture 4.

While-statements can often be written as for-statements, and vice-versa.

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break statement the break statement causes an exit from the

innermost enclosing loop or switch statement (discussed later)

while (1) {

scanf(“%lf”, &x);

if (x < 0)

break; /* exit loop if x is -ve */

printf(“%f\n”, sqrt(x));

}

/* If break is run, jumps to here */

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continue statement

continue statement causes the current iteration of a loop to stop and the next iteration to begin immediately

It can be applied in a while, do-while or for statement

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Example of continue statement

/* read in and sum 10 not-too-small nos. */

cnt = 0;

while (cnt < 10) {

scanf(“%lf”, &x);

if (x > -0.01 && x < 0.01)

continue; /* discard small values */

++cnt;

sum += x;

/* continue transfer control here */

}

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switch statement General format of switch statement (selection

statement)

switch (expression) { case constant_expr1: statement_seq1

case constant_expr2: statement_seq2

...

...

case constant_exprN: statement_seqN

default: statement_seq

}

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switch statement (cont’d) Semantics

Evaluate the switch expression which results in an integer type (int, long, short, char)

Go to the case label having a constant value that matches the value of the switch expression; if a match is not found, go to the default label; if default label does not exist, terminate the switch

Terminate the switch when a break statement is encountered

If there is no break statement, execution “falls through” to the next statement in the succeeding case

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Example of switch statement

/* To print the no. of days in a month */

scanf(“%d”, &month); /* assume month is integer var.*/

switch(month) {

case 1: case 3: case 5: case 7:

case 8: case 10: case 12:

printf(“31 days”);

break;

case 4: case 6: case 9: case 11:

printf(“30 days”);

break;

case 2:

printf(“28 days “);

break;

default:

print(“input error”);

}

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Another example: switch/* To find out the type of value stored in variable c (a character variable) */switch (c) { case ‘0’: case ‘1’: case ‘2’: case ‘3’: case ‘4’: case ‘5’: case ‘6’: case ‘7’: case ‘8’: case ‘9’: printf(“A digit\n”); /* no braces is needed */ break; case ‘ ’: case ‘\n’: case ‘\t’: printf(“A white-space\n”); break; default: printf(“A letter or special character\n”); break;}

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Another example

Task: write a program that reads in two integers, month and day; and then calculates the number of days from 1/1/2003 to day/month/2003

Idea: Step 1: Calculate the number of days from

January up to month-1 Step 2: Add day to the number of days

calculated in step 1

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conditional (?:) operator

General format of ?: operator is

expr1 ? expr2 : expr3

Semantics expr1 is evaluated If the above result is non-zero, then expr2 is evaluated;

else expr3 is evaluated The value of the whole ?: expression is the value of

expression evaluated at the end

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Structured Programming Structured programming statements are single-entry

and single exit

Structured programming

construct

Goto statements will render some programming constructs to have more than one entry/exit

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Examples: if, switch

E

S1 S2

T F

E

S1 SkS2…..

if-then-elsecase

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Structured Programming Guidelines

The flow of control of a structured program is easier to follow

How? Apply stepwise refinement to decompose a problem

into smaller problems repeatedly until they are simple enough to be coded

In each refinement, apply one of these 7 control structures: compound, if-, if-else, switch, while, do-while, for

For efficiency purpose, may use continue/break statements with great care

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Common programming errors

Mix up = and ==E.g. the expression in the following if-statement is always false:

if (x=0) printf(“Hello\n”);

Use == for equality testing: if (x==0)

...

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Common Programming Errors (cont’d)

Mix up , and ;

E.g. for (j=1, j<=10, j++) /* error */

sum += j;

Put in extra semi-colons, e.g.,

sum=0;

for (j=1; j<=10; j++);

sum += j;

Misuse of relational operators

if (2 < k < 7) // wrong

if (2 < k && k < 7) // correct

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Some Programming Tips

Make sure that the termination condition of a loop is reachable: If possible, use < > <= >= instead of == !=

Don’t use a variable of any floating point type to control a loop because real numbers are represented in their approximate values internally

Infinite loop can be made deliberately: while (1) statement for (;;) statement