1 Flowchart notation and loops Implementation of loops in C –while loops –do-while loops –for loops Auxiliary Statements used inside the loops –break –continue

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• Flowchart notation and loops• Implementation of loops in C

– while loops– do-while loops– for loops

• Auxiliary Statements used inside the loops– break– continue– goto

Today’s Material

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• Often you would need to repeatedly execute some statements as long as a condition is true

• Consider computing “an“, where “a” is a real number and “n” is an integer >= 0 – E.g., Compute 35

– 35 = 3*3*3*3*3

The Need For Loops

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• Keep a running power, initialized to 1– power = 1 (a0=1)

• Multiply power with “a”, and count the number of times we have multiplied “power” with “a”– Initially count = 0

• When “count” reaches “n”, that is, when we have multiplied “power” with “a” “n” times, we are done

How to compute an?

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1. Prompt the user and get “a” and “n”2. Set count to 0 3. Set power to 1 /* power = a0 = 1 */

4. repeat while (count < n)4.1. power = power * a; /* power = acount now */4.2. count++;

5. Print power

Algorithm for Computing an

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Flowchart for Computing an

count = 0

StartStart

power = 1

count < n?

Prompt the user and get “a” and “n”

Print power

no

EndEnd

yes

power *= a;

count++;

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• Clearly, we need to repeatedly execute steps 4.1 and 4.2 until “count” reaches “n”

• That is, we need to loop around steps 4.1, 4.2 as long as a condition is true

• C provides 3 looping constructs– while loops– do-while loops– for loops

C Looping Constructs

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while Statement• The while loop keeps repeating an action until an

associated test returns false • Useful when the programmer does not know in

advance how many times the loop will be iterated • Syntax:

while(expression){

statement1;statement2;...

}

expressionexpression YY

NNstatement 1statement 1

statement 2statement 2

......

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Code for Computing an

int count;int n;double a;double power;

printf(“Enter a: “);scanf(“%lf”, &a);printf(“Enter n: “);scanf(“%d”, &n);

count = 0;power = 1; /* power = a0 */while (count < n){ power *= a; count++;} /* end-while */

printf(“a^n: %lf\n”, power);

count = 0

StartStart

power = 1

count < n?

Prompt the user and get “a” and “n”

Print power

no

EndEnd

yes

power *= a;

count++;

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Trace of the Code for a = 3, n = 5

int count;int n;double a;double power;


count = 0;power = 1;while (count < n){ power *= a; count++;} /* end-while */

printf(“a^n: %lf\n”, power);

0 1

count power3 5a n

Test: 0 < 5? True1 3

Test: 1 < 5? True2 9

Test: 2 < 5? True3 27

Test: 3 < 5? True4 81

Test: 5 < 5? False

Test: 4 < 5? True5 243

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Computing an: Alternative Code

int i;int n;double a;double power;


i = 0;power = 1;while (i < n){ power *= a; i++;} /* end-while */

printf(“a^n is: %lf\n”, power);

int i;int n;double a;double power;


i = 0;power = 1;while (i++ < n) power *= a; printf(“a^n is: %lf\n”, power);

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1. Prompt the user and get “n”2. Set i to 1 /* Iteration variable */ 3. Set sum to 0 /* Running sum */

4. repeat while (i <= n)4.1. sum += i;4.2. i++;

5. Print sum

Computing 1+2+3+..+N

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Flowchart and Code for Computing 1+2+3+..+N

int i;int n;int sum = 0;

printf(“Enter n: “);scanf(“%d”, &n);

i = 1;while (i<= n){ sum += i; i++;} /* end-while */

printf(“Sum is: %d\n”, sum);

i = 1

StartStart

sum = 0

i <= n?

Prompt the user and get “n”

Print sum

no

EndEnd

yes

sum += i;

i++;

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Trace of the Code for n=5





1 0

i sum

Test: 1 <= 5? True2 1

Test: 2 <= 5? True3 3

Test: 3 <= 5? True4 6

Test: 4 <= 5? True5 10

Test: 6 <= 5? False

Test: 5 <= 5? True6 15

5

n

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Computing 1+2+3+..+N: Alternative Code







i = 1;while (i<= n) sum += i++; printf(“Sum is: %d\n”, sum);

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Printing a Table of Squares

+-----+-----+| i | i*i |+-----+-----+| 1| 1|| 2| 4|| 3| 9|| 4| 16|| 5| 25|| 6| 36|+-----+-----+

• We want to print a table of squares for numbers 1, 2, 3, 4, .., n for some number “n”

• Here is how the table should look like for n = 6

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Code for Printing a Table of Squares int i;int n;


printf(“+-----+-----+\n”);printf(“| i | i*i |\n”);printf(“+-----+-----+\n”);

i = 1;while (i <= n){ printf(“|%5d|%5d|\n”, i, i*i); i++;} /* end-while */

printf(“+-----+-----+\n”);

i = 1

StartStart

i <= n?

Prompt the user and get and “n”

Print bottom lineof the table

no

EndEnd

yes

print (i, i*i)

i++;

Print the heading

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Trace of the code for n=4printf(“+-----+-----+\n”);printf(“| i | i*i |\n”);printf(“+-----+-----+\n”);

i = 1;while (i <= n){ printf(“|%5d|%5d|\n”, i, i*i); i++;} /* end-while */

printf(“+-----+-----+\n”);

1

i4

n

Test: 1 <= 4? True

+-----+-----+| i | i*i |+-----+-----+| 1| 1|| 2| 4|| 3| 9|| 4| 16|+-----+-----+

2

Test: 2 <= 4? True3

Test: 3 <= 4? True4

Test: 4 <= 4? True5

Test: 5 <= 4? False

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Another while Example

int i = 0;

printf(“How do you like C programming?\n”);while(i < 10){ printf(“Programming is fun!\n”); i++;} /* end-while */

• Repeats 10 times (for i from 0 to 9)• Prints the same message 10 times

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Yet Another while Example

int i = 20;

printf(“How do you like C programming?\n”);while(i < 10){ printf(“Programming is fun!\n”); i++;} /* end-while */

• Repeats 0 times (i is 20, not less than 10)

• Does not print any "... is fun" messages.

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do while Statement

do{


} while(expression);expressionexpression

YY

NN


statement 2statement 2......

• While loop tests the loop condition at the beginning of the loop, before the first iteration begins

• Sometimes you want to test the loop condition at the end of the loop. In such cases do-while loops are used– This ensures that the loop body is run at least

once

• Syntax:

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Asking for a Password(1)• Assume you want to repeatedly ask the

user for the password until the user enters the password correctly

/* Implementation using while */#define PASSWORD 123456int passwd;

printf(“Enter the password: “);scanf(“%d”, &passwd);

while (passwd != PASSWD){ printf(“Enter the password: “); scanf(“%d”, &passwd);} /* end-while */

printf(“Password is OK\n”);

• Clearly you want to ask for the password at least once!– Using a while

loop, we have to repeat printf/scanf statements

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Asking for a Password(2)• Here is the same loop with do-while

#define PASSWORD 123456int passwd;

do { printf(“Enter the password: “); scanf(“%d”, &passwd);} while(passwd != PASSWD);

printf(“Password is OK\n”);

• This is cleaner compared to while loop

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do while Example• Asking for a positive integer

int no;

do { printf(“Enter a positive integer: “); scanf(“%d”, &no);} while(no <= 0);

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Another do while Example

char option = 'x';do{

printf("Select an option: \n");printf("(a) Calculate grades \n");printf("(b) Calculate class average \n");printf("(c) Print grades \n");printf("(q) Quit \n");

option = getchar(); getchar(); /* Skip ‘\n’ */

if (option == 'a') ...else if (option == 'b') ...else if (option == 'c') ...

} while(option != ‘q’);

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Another do while Example

int i = 0;

printf(“How do you like C programming?\n”);do {

printf(“Programming is fun!\n”);i++;

} while(i < 10);

• Repeats 10 times (for i from 0 to 9)• Prints the same message 10 times

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Yet Another do while Example

int i = 20;

printf(“How do you like C programming?\n”);do {

printf(“Programming is fun!\n”);i++;

} while(i < 10);

• Repeats once (for i == 20)• Prints the same message once

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What are the differences between the while and the do

while statements?

while do whileEntry control structure Exit control structure

Loop may or may not be executed

Loop is executed at least once

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for Statement• More frequently used• Ideal for loops that have a “counting”

variable– i.e., We need to loop a fixed number of times

• Is general enough to be used by other kinds of loops as well

• Syntax:for (initializing list; expression; altering list){


}

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for Statement Flowchart, and Equivalent while Statement

for(initialize; check; modify){


}

modifymodify

checkcheck YY

NN

initializeinitialize


statement 2statement 2......initialize;

while(check){

statement1;statement2;...modify;

}

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Another for Example

int count;for(count = 1; count <= 10; count++){

printf("%d ",count);} /* end-for */

printf("\n");

1 2 3 4 5 6 7 8 9 10

Printed output:Printed output:

StartStart

count++count++

EndEnd

DisplayDisplaycountcount

count <= 10count <= 10YY

NN

count = 1count = 1

• Problem: Print numbers from 1 to 10

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Using for Statement

for (i=0; i<N; i++) …

• For statement is usually the best choice for loops that “count up” (increment a variable) or “count down” (decrement a variable)

• Counting up from 0 to N-1

for (i=1; i<=N; i++) … • Counting up from 1 to N

for (i=N-1; i>=0; i--) … • Counting down from N-1 to 0

for (i=N; i>=1; i--) … • Counting down from N to 1

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for Example: Compute an

power = 1;for(i=1; i<=N; i++){ power *= a;} /* end-for */

StartStart

i++i++

EndEnd

DisplayDisplaypowerpower

i <= Ni <= N YY

NN

i = 1i = 1

power *=a;power *=a;

1 1

i power3 5a n

Test: 1 <= 5? True2 3

Test: 2 <= 5? True3 9

Test: 3 <= 5? True4 27

Test: 4 <= 5? True5 81

Test: 6 <= 5? False

Test: 5 <= 5? True6 243

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Printing a Table of Squares

printf(“+-----+-----+\n”);printf(“| i | i*i |\n”);printf(“+-----+-----+\n”);

for (i=1; i <= n; i++){ printf(“|%5d|%5d|\n”, i, i*i); } /* end-while */

printf(“+-----+-----+\n”);

+-----+-----+| i | i*i |+-----+-----+| 1| 1|| 2| 4|| 3| 9|| 4| 16|| 5| 25|| 6| 36|+-----+-----+

Output for n=6

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Another for Example

sum = 0;for(i=1; i<=N; i++){ sum += i;}

• Problem: Compute 1+2+3+4+…+N

sum=0;i=1;for(; i<=N; i++){ sum += i;}

sum=0;for(i=1; i<=N;){ sum += i++;}

• Initialize (i=1), check(i<=N) and modify(i++) statements are all optional and can be omitted

sum=0;i=1;for(; i<=N;){ sum += i++;}

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• It is possible to nest loops inside each other– Many applications require nesting of loops

• Example: Print the multiplication table

Nested Loops

+---+---+---+---+---+---+---+---+---+---+---+| * | 1| 2| 3| 4| 5| 6| 7| 8| 9| 10|+---+---+---+---+---+---+---+---+---+---+---+| 1| 1| 2| 3| 4| 5| 6| 7| 8| 9| 10|| 2| 2| 4| 6| 8| 10| 12| 14| 16| 18| 20|| 3| 3| 6| 9| 12| 15| 18| 21| 24| 27| 30|| 4| 4| 8| 12| 16| 20| 24| 28| 32| 36| 40|| 5| 5| 10| 15| 20| 25| 30| 35| 40| 45| 50|+---+---+---+---+---+---+---+---+---+---+---+

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Printing the Multiplication Tableint i, j;

/* Print the header */printf(“+---+---+---+---+---+---+---+---+---+---+---+\n”);printf(“| * |”);for (j=1; j<=10; j++) printf(“%3d|”, j);printf(“\n+---+---+---+---+---+---+---+---+---+---+---+\n”);

/* Print the table. i goes over the rows, j over the columns */for (i=1; i <= 10; i++){ printf(“|%3d:|“, i); /* Print 1 row of the table for i */ for (j=1; j <= 10; j++){ printf(“%3d|”, i*j); } /* end-for-inner */ printf(“\n”);} /* end-for-outer */

/* Print the bottom line of the table */printf(“+---+---+---+---+---+---+---+---+---+---+---+\n”);

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• We have seen that “break” transfers the control out of switch statements

• Similarly, when used within loops, “break” transfers the control out of the current loop code block

break & continue in loops

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Code for Computing the sum of a series of numbers

int sum = 0;int n;

while (1){ /* Infinite loop */ printf("Enter an int or -1 to stop: "); scanf("%d", &n"); if (n == -1) break; /* Get out of the loop */ sum += n;} /* end-while */

printf("Sum is %d\n", sum);

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Code for Checking if a number “n” is prime or not

int d;int n;

printf(“Enter an integer: “);scanf(“%d”, &n);

for (d=2; d<n; d++){ if (n%d == 0) break;} /* end-for */

if (d<n) printf(“n=%d is divisible by %d\n”, n, d);else printf(“n=%d is prime\n”, n);

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• break is particularly useful if the escape point is somewhere in the middle of the loop rather than at the beginning or at the end

break (cont)

while (1){ printf(“Enter a number or 0 to stop: “); scanf(“%d”, &n); if (n == 0) break; printf(“n=%d, n*n*n*=%d\n”, n, n*n*n);} /* end-while */

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• break transfers the control out of the innermost enclosing code block.– Thus when you have a nested loop, break only

escapes one level of nesting

break (cont)

while (…){ … switch(…){ … … break; /* takes us out of switch to (A) */ … } (A) }(B)

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• Transfers the control to the end of the loop• Note that we are still inside the loop• “continue” simply skips the rest of the

statements in the loop body, and moves the control to the end

continue

int i;int n = 0; int sum = 0;

while (n<10){ scanf(“%d”, &i); if (i == 0) continue; /* takes us to (B) */ n++; sum += i;

/*(B)*/} /* end-while */

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• Transfers the control to an arbitrary point in the code designated by a label– goto is strictly discouraged as it leads to spaghetti

code, which is hard to understand and maintain– But it may be useful in certain situations

goto

while (…){ … switch(…){ … … goto loop_done; /* break won’t work here, as it takes */ … /* us out of switch only */ } /* end-switch */} /* end-while */loop_done:

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Infinite Loops• A loop that iterates forever

while (1){ …}

do{ …} while (1);

• How do we get out of these loops then?– Simply have a “break” somewhere within the loop

for (;;;){ …}

while (1){ printf(“Enter a number or 0 to stop: “); scanf(“%d”, &n); if (n == 0) break; printf(“n=%d, n*n*n*=%d\n”, n, n*n*n);} /* end-while */

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• Occasionally we may want to combine several expressions together into a single statement– This is where we use the comma operator

• Syntax

Comma Operator

expr1, expr2, …, exprN;

• expr1 is evaluated, its value discarded• expr2 is evaluated, its value discarded• …• exprN is evaluated, its value becomes the value

of the statement

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Comma Operator Example (1)

i=1, j=2, k=i+j; is equivalent to

((i=1), (j=2), (k=i+j));

• Evaluation proceeds left to right• The result of the statement is k=i+j; which

is 3

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Comma Operator Example (2)

sum=0;for(i=1; i<=N; i++) sum += i;

• Problem: Compute 1+2+3+4+…+N

for(sum=0, i=1; i<=N; i++) sum += i;

for(sum=0, i=1; i<=N; sum+=i, i++) ;

Empty statement: Loop body is empty

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1 Flowchart notation and loops Implementation of loops in C –while loops –do-while loops –for loops Auxiliary Statements used inside the loops –break –continue