CS1010: Programming Methodology comp.nus.sg/~cs1010

CS1010: Programming Methodology http://www.comp.nus.edu.sg/~cs1010/

http://www.comp.nus.edu.sg/~cs1101/

Week 5: Repetition StatementsObjectives:

Understand the program control structure called loops

Compare the different types of repetition structure

CS1010 (AY2012/3 Semester 1) Week5 - 2

References: Chapter 4 Lessons 4.7 – 4.11

Week 5: Outline (1/2)1. Week 4 Exercise #3: NRIC Check Code2. Loops!3. The while Loop

3.1 Demo3.2 Loop condition3.3 Tracing

4. The do-while Loop5. The for Loop

5.1 Odd Integers

6. Exercise #1: Sum of Multiples of 37. Exercise #2: Asterisks


Week 5: Outline (2/2)8. Common Errors

9. Some Notes of Caution

10. Exercise #3: Tracing Nested Loops

11. Using break in Loop

12. Using continue in Loop

13. Exercise #4: Prime Number (take-home)


1. Week 4 Exercise #2: Taxi Fare (1/3) The taxi fare structure in Singapore must be one of the most

complex in the world! See http://www.taxisingapore.com/taxi-fare/ Write a program Week4_TaxiFare.c that reads the following input

data (all are of int type) from the user, and computes the taxi fare: dayType: 0 represents weekends and public holidays (PH for short); 1

represents weekdays and non-PH boardHour, boardMin: the hour and minute the passengers board the

taxi (eg: 14 27 if the passengers board the taxi at 2:27 PM) distance: the distance of the journey, in metres

Your program should have a functionfloat computeFare(int dayType, int boardTime, int distance)

The parameter boardTime is converted from the input data boardHour and boardMin. It is the number of minutes since 0:00hr.

Eg: If boardHour and boardMin are 14 and 27 respectively, then boardTime is 867.

Week4 - 5CS1010 (AY2012/3 Semester 1)

http://www.taxisingapore.com/taxi-fare/

1. Week 4 Exercise #2: Taxi Fare (2/3) To implement the actual taxi fare could be a PE question . In this

exercise, we use a (grossly) simplified fare structure: Basic Fare:

Surcharge (applicable at the time of boarding):


Flag-down (inclusive of 1st km or less) $3.40Every 400m thereafter or less up to 10.2km $0.22Every 350m thereafter or less after 10.2km $0.22

dayType Midnight charge (12am – 5:59am)

Peak hour charge (6am – 9:29am)

Peak hour charge (6pm – 11:59pm)

0: Weekends & PH 50% of metered fare

None 25% of metered fare

1: Weekdays and non-PH

50% of metered fare

25% of metered fare 25% of metered fare

1. Week 4 Exercise #2: Taxi Fare (3/3) You are given an incomplete program Week4_TaxiFarePartial.c.

Complete the program. This exercise is mounted on CodeCrunch. Sample runs below for your checking


Day type: 0Boarding hour and minute: 14 27Distance: 10950Total taxi fare is $9.12

First 1km: $3.40Next 9.2km: 23 $0.22 = $5.06Next 750m: 3$0.22 = $0.66Basic fare = $9.12No surchargeTotal fare = $9.12


First 1km: $3.40Next 5123m: 13 $0.22 = $2.86Basic fare = $6.26Surcharge = 25% $6.26 = $1.57Total fare = $7.83


First 1km: $3.40Next 8km: 20 $0.22 = $4.40Basic fare = $7.80Surcharge = 50% $7.80 = $3.90Total fare = $11.70

1. Week 4 Exercise #2: Taxi Fare


float computeFare(int daytype, int bTime, int dist){ basicFare = calcBasicFare(dist) ; return includeSurcharge(basicFare, daytype, bTime) ;}

float calcBasicFare(int dist){ // Pre-cond: 0 <= dist ; if (dist <= 1000) return 3.40; else if (dist <= 10200 && dist > 1000) return (3.40 + ceil((dist - 1000) /400.0) * INCREMENT); else if (dist > 10200) return (3.40 + 9200 / 400.0 * INCREMENT + (ceil((dist - 10200) / 350.0)) * INCREMENT);}

1. Week 4 Exercise #2: Taxi Fare


float computeFare(int daytype, int bTime, int dist){ basicFare = calcBasicFare(dist) ; return includeSurcharge(basicFare, daytype, bTime) ;}

float includeSurcharge(float fare, int dType, int bTime){ // Pre-cond: dType = 0 or 1, 0 <= bTime <= 2359 ; if (bTime < 360 && (dType == 0 || dType == 1)) return fare * 1.50; else if (dType == 1 && (bTime < 600 && dtime >=360)) return fare * 1.25; else if (bTime >= 1080) return fare * 1.25; else return fare ;}

1. Week 4 Ex3: NRIC Check Code (1/3) Algorithm for NRIC check code

NRIC consists of 7 digits. Eg: 8730215

Step 1: Multiply the digits with corresponding weights 2,7,6,5,4,3,2 and add them up. Eg: 82 + 77 + 36 + 05 + 24 + 13 + 52 =

16+49+18+0+8+3+10 = 104

Step 2: Divide step 1 result by 11 to obtain the remainder. Eg: 104 % 11 = 5

Week5 - 10Week5 - 10CS1010 (AY2012/3 Semester 1)

1. Week 4 Ex3: NRIC Check Code (2/3) Algorithm for NRIC check code (cont…)

Step 3: Subtract step 2 result from 11 Eg: 11 – 5 = 6

Step 4: Match step 3 result in this table for the check code

Eg: The check code corresponding to 6 is ‘F’.

Therefore, the check code for 8730215 is ‘F’.

Sample run:

1 2 3 4 5 6 7 8 9 10 11A B C D E F G H I Z J

Week5 - 11Week5 - 11

Enter 7-digit NRIC number: 8730215Check code is F

CS1010 (AY2012/3 Semester 1)

1. Week 4 Ex3: NRIC Check Code (3/3) Write a program Week4_NRIC.c to generate the check code given

a 7-digit NRIC number. Your program should include a function char generateCode(int) that

takes in a single integer (the NRIC number) and returns a character (which is the check code). You need to use the char type. (Explore this on your own.) A character constant is enclosed in single quotes (eg: 'A', 'Z'). The format specifier for char type is %c (to be used in a printf()

statement). Do not use techniques that are not covered in class, such as array.

Your program may be long now. You can write an improved version later.

This is your take-home exercise. This exercise is mounted on CodeCrunch.


1. Week 4 Ex3: NRIC Check code Step 1: Multiply the digits with corresponding weights

2,7,6,5,4,3,2 and add them up. Eg: 82 + 77 + 36 + 05 + 24 + 13 + 52 =

16+49+18+0+8+3+10 = 104 // Extract the digits digit7 = num%10; num /= 10; digit6 = num%10; num /= 10; digit5 = num%10; num /= 10; digit4 = num%10; num /= 10; digit3 = num%10; num /= 10; digit2 = num%10; num /= 10; digit1 = num%10;

step1 = digit1*2 + digit2*7 + digit3*6 + digit4*5 + digit5*4 + digit6*3 + digit7*2;


1. Week 4 Ex3: NRIC Check code Step 2: Divide step 1 result by 11 to obtain the

remainder. Eg: 104 % 11 = 5

step2 = step1 % 11;

Step 3: Subtract step 2 result from 11 Eg: 11 – 5 = 6

step3 = 11 - step2;


1. Week 4 Ex3: NRIC Check Code

Step 4: Match step 3 result in this table for the check code

switch (step3) { case 1: code = 'A'; break; case 2: code = 'B'; break; case 3: code = 'C'; break; case 4: code = 'D'; break; case 5: code = 'E'; break; case 6: code = 'F'; break; case 7: code = 'G'; break; case 8: code = 'H'; break; case 9: code = 'I'; break; case 10: code = 'Z'; break; case 11: code = 'J'; } // end switch

1 2 3 4 5 6 7 8 9 10 11A B C D E F G H I Z J


Recall: Control Structures


Sequence

Selection

Repetition

if-else, switch


2. LOOPS! (1/2)


“A program without a loop and a structure variable isn’t worth writing.”

Alan J.PerlisYale University

The first recipient of ACM Turing Award

A loop is a statement whose job is to repeatedly execute some other statement(s).


2. LOOPS! (2/2)

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cond?

Somestatement(s)

true

false

loop body

Loop condition

Week5 - 18

Each round of the loop is called an iteration.


2. Loop: Demo (1/3) Keep prompting the

user to input a non-negative integer, and output that integer.

Halt the loop when the input is negative.

Enter a number: 12You entered: 12Enter a number: 0You entered: 0Enter a number: 26You entered: 26Enter a number: 5You entered: 5Enter a number: -1


Key observations: You keep repeating a task while certain condition is met,

or alternatively, you repeat until the condition is not met. You do not know beforehand how many iterations there

will be.CS1010 (AY2012/3 Semester 1)

2. Loop: Demo (2/3)int main(void) {

int num;printf("Enter a number: ");scanf("%d", &num);if (num < 0) return 0;printf("You entered: %d\n",

num);printf("Enter a number: ");scanf("%d", &num);if (num < 0) return 0;printf("You entered: %d\n",

num);printf("Enter a number: ");scanf("%d", &num);....

} Week5 - 20Week5 - 20

Loop condition

Enter a number: 12You entered: 12Enter a number: 0You entered: 0Enter a number: 26You entered: 26Enter a number: 5You entered: 5Enter a number: -1

Loop body


2. Loop: Demo (3/3)

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num >= 0?

printf …printf … scanf …

true

false


int main(void) {int num;

printf("Enter a number: ");scanf("%d", &num);while (num >= 0) {

printf("You entered: %d\n", num);

printf("Enter a number: ");

scanf("%d", &num);}

return 0;}

Week5_Read_print.c

3. The while Loop


while ( condition ){ // loop body}

cond?

Loop body

true

false

If condition is true, execute loop body; otherwise, terminate loop.

3.1 The while Loop: Demo (1/3)

Enter a number: 12Enter a number: 0Enter a number: 26Enter a number: 5Enter a number: -1The maximum number is 26


Keep prompting the user to input a non-negative integer, and output that integer.

Halt the loop when the input is negative, and output the maximum integer input so far.


Week5 - 24

maxi = 0;num input;while (num >= 0) {

if (maxi < num) maxi = num;

num input;}

print maxi;

maxi = 0;num input; if (num < 0) {

print maxi; stop; }if (maxi < num)

maxi = num ;num input;if (num < 0) {

print maxi; stop; } if (maxi < num)

maxi = num;num input;...




int main(void) {int num, maxi = 0;

printf("Enter a number: ");scanf("%d", &num);while (num >= 0) {

if (maxi < num) {maxi = num;

}printf("Enter a number: ");scanf("%d", &num);

}prinf("The maximum number is %d\n",

maxi);return 0;

}

Week5_Find_max.c

3.2 while Loop Condition (1/2)

When the loop condition is always false, the loop body is not executed.

Week5 - 26

Output: ?


a = 2;b = 7;while (a == b) {

print a;a = a + 2;

}

3.2 while Loop Condition (2/2)

Week5 - 27

Output: ?


a = 2;b = 7;while (a != b) {

print a;a = a + 2;

}

3.3 Tracing while Loop (1/4) Trace the following codes manually and write out their

outputs (assume all variables are of type int)


a = 1;while (a*a < 100) { printf("%d ", a); a *= 2;}printf("\n");

(a)

b = 0; c = 9;while (b < c) { printf("b=%d, c=%d\n", b, c); b++; c--;}printf("outside: b=%d, c=%d\n", b, c);

(b)

3.3 Tracing while Loop (2/4) Example: Given a positive integer n, print out

its digits from least significant to most significant.

Sample run:

Week5 - 29

Enter a positive integer: 2894334982


3.3 Tracing while Loop (3/4) Example: Given a positive integer n, print out

its digits from least significant to most significant.


// Precond: n > 0void print_digits(int n) {

int digit;

while (n > 0) {digit = n%10;printf("%d\n",

digit);n /= 10;

}}

Week5_Print_digits.c

// Precond: n > 0void print_digits(int n) {

int digit;

while (n > 0) {digit = n%10;printf("%d\n",

digit);n /= 10;

}}

Week5_Print_digits.c

3.3 Tracing while Loop (4/4)

Week5 - 31

n initially 28943n @ point 29843digit @ point ***

What are the values of n and digit after exiting the loop?


4. The do-while Loop (1/2)

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do { // loop body} while ( condition );

Execute loop body at least once.

cond?

Loopbody

truefalse


4. The do-while Loop (2/2) Example: Count the number

of digits in an integer.

Week5 - 33

do { // loop body} while ( condition );


// Precond: n > 0int count_digits(int n) {

int counter = 0;

do {counter++;n /= 10;

} while (n > 0);

return counter;}

Week5_Count_digits.c

5. The for Loop (1/2)

Week5 - 34

for ( initialization; condition; update ){ // loop body}

Initialization: initialize the loop variable

Condition: repeat loop while the condition on loop variable is true Update: change

value of loop variable


5. The for Loop (2/2) Example: Print numbers 1 to 10

Week5 - 35

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

printf("%3d", n);}

Steps:

1.n=1;2.if (n<=10) { printf(…); n++;

Go to step 2}

3. Exit the loop


5.1 The for Loop: Odd Integers (1/2)


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

int num;printf("Enter a positive integer:

");scanf("%d", &num);print_odd_integers(num);return 0;

}

// Precond: n > 0void print_odd_integers(int n) {

int i;for (i=1; i<=n; i+=2)

printf("%d ", i);printf("\n");

}

Week5_OddIntegers_v1.c

5.1 The for Loop: Odd Integers (2/2)



int i;for (i=1; i<=n; i++)

if (i%2 != 0)printf("%d ", i);

printf("\n");}



for ( ; n > 0; n--)if (n%2 != 0)

printf("%d ", n);printf("\n");

}


Values printed from largest to smallest.Empty

statement

6. Exercise #1: Sum of Multiples of 3 (1/2) Modify the program Week5_OddIntegers_v1.c to read

a positive integer n and then compute the sum of all integers which are multiples of 3 between 1 and n using a ‘for’ loop. Write a function called sum_multiples_of_3(int). This problem can be solved with a formula, but we will use

the ‘while’ loop just for exercise.

Call this program Week5_SumMultiples3.c Sample run:

Week5 - 38

Enter a positive integer: 50Sum = 408


6. Exercise #1: Sum of Multiples of 3 (2/2) How about using a while loop instead? Pseudo-code using a while loop:


precondition: n > 0 sum 0i n while i > 0

if i is multiple of 3 thensum sum + i

i i - 1return sum

7. Exercise #2: Asterisks (1/2) Write a program Week5_Asterisks.c to read an

integer n and print a certain number of asterisks on a single line. Write a function print_asterisks(int).

If n is non-positive, then no asterisk should be printed. Sample runs:

Week5 - 40

Enter n: 3*****Done!

Enter n: 6***********Done!

Enter n: 10*******************Done!

Enter n: -2Done!

Think! What is the relationship between n and the number of *?


7. Exercise #2: Asterisks (2/2) Write a program Week5_Asterisks.c to read an

integer n and print a certain number of asterisks on a single line. Write a function print_asterisks(int).


Pseudo-code:read input n ;if n is non-positive

print “done” and end program ;m compute the number of asterisks given nprint_asterisks(m)end program;

8. Common Errors (1/2) What are the outputs for the following programs? (Do

not code and run them. Trace the programs manually.)

Week5 - 42 CS1010 (AY2012/3 Semester 1)

int i;

for (i=0; i<10; i++);printf("%d\n", i);

Week5_CommonErrors1.c

int i = 0;

while (i<10);{

printf("%d\n", i);i++;

} Week5_CommonErrors2.c

8. Common Errors (2/2)

Off-by-one error; make sure the loop repeats exactly the correct number of iterations.

Make sure the loop body contains a statement that will eventually cause the loop to terminate.

Using ‘=’ where it should be ‘==’ Putting ‘;’ where it should not be (just like for the ‘if’

statement)


int z = 3;while (z = 1) {

printf("z = %d\n", z);z = 99;

}Week5_CommonErrors3.c

9. Some Notes of Caution (1/2) Involving real numbers

Trace the program manually without running it.


double one_seventh = 1.0/7.0;double f = 0.0;

while (f != 1.0) {printf("%f\n", f);f += one_seventh;

}Week5_Caution1.c

9. Some Notes of Caution (2/2) Involving ‘wrap-around’

Trace the program manually without running it.


int a = 2147483646;int i;

for (i=1; i<=5; i++) {printf("%d\n", a);a++;

}Week5_Caution2.c

10. Exercise #3: Tracing Nested Loops You are given Week5_NestedLoop1.c,

Week5_NestedLoop2.c and Week5_NestedLoop3.c Hand trace the programs and write out the outputs

without running the programs Verify your answers by running the programs


11. Using break in Loop (1/2) You have seen ‘break’ in switch statement ‘break’ can also be used in a loop Test out Week5_BreakInLoop.c


11. Using break in Loop (2/2) Use ‘break’ sparingly, because it violates the one-entry-

one-exit control flow. A loop with ‘break’ can be rewritten into one without ‘break’.


// with breakint n, i = 1, sum = 0;

while (i <= 5) {scanf("%d", &n);if (n < 0)

break;sum += n;i++;

}

// without breakint n, i = 1, sum = 0;int isValid = 1;while ((i <= 5) && isValid){

scanf("%d", &n);if (n < 0)

isValid = 0;else {

sum += n;i++;

}}

12. Using continue in Loop Test out Week5_ContinueInLoop.c ‘continue’ is used even less often than ‘break’


13. Exercise #4: Prime Number (1/2) Primality test is a classic programming problem

Given a positive integer, determine whether it is a prime A prime number has two distinct factors (divisors): 1 and

itself. Examples: 2, 3, 5, 7, 11, ... (Note: 1 is not a prime!)

Write a program Week5_PrimeTest.c. You should include a function is_prime(int). (What does it return?)

Sample runs:

Week5 - 50

Enter a positive integer: 131131 is a prime.

Enter a positive integer: 713713 is not a prime.


13. Exercise #4: Prime Number (2/2) This is your take-home exercise. This exercise is mounted on CodeCrunch. We will discuss this in the next lecture.


Summary for Today (1/2)

Week5 - 52

Repetition statements (loops) for, while, do-while Nested loops break and continue


Summary for Today (2/2)

Week5 - 53

You have learned the 3 control structures: Sequence, Selection, Repetition

With these, you are able to solve just any computing problem!

However, writing good programs is more than just learning the syntax: Logic should be clear Variables should be descriptive Algorithm should be efficient


Announcements/Things-to-do Revise Chapter 4 (Lessons 4.7 – 4.11) Deadline for Lab #2

Deadline: 15th September 2012, Saturday, 12noon

Practical Exam 1 (PE1) 22nd September 2012, Saturday See web page for details:

http://www.comp.nus.edu.sg/~cs1010/3_ca/pe.html

To prepare for next week’s lecture: Read Chapter 5 Functions Bring along your Week5_PrimeTest.c program


http://www.comp.nus.edu.sg/~cs1010/3_ca/pe.html

End of File

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