CS1010: Programming Methodology http://www.comp.nus.edu.sg/~cs1010/
Feb 23, 2016
CS1010: Programming Methodology http://www.comp.nus.edu.sg/~cs1010/
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
CS1010 (AY2012/3 Semester 1) Week5 - 3
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)
CS1010 (AY2012/3 Semester 1) Week5 - 4
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)
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):
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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
Week4 - 7CS1010 (AY2012/3 Semester 1)
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
Day type: 1Boarding hour and minute: 9 20Distance: 6123Total taxi fare is $7.83
First 1km: $3.40Next 5123m: 13 $0.22 = $2.86Basic fare = $6.26Surcharge = 25% $6.26 = $1.57Total fare = $7.83
Day type: 1Boarding hour and minute: 5 59Distance: 9000Total taxi fare is $11.70
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
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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
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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
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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.
Week5 - 12Week5 - 12CS1010 (AY2012/3 Semester 1)
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;
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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;
Week5 - 14Week5 - 14CS1010 (AY2012/3 Semester 1)
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
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Recall: Control Structures
Week5 - 16Week5 - 16
Sequence
Selection
Repetition
if-else, switch
CS1010 (AY2012/3 Semester 1)
2. LOOPS! (1/2)
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“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).
CS1010 (AY2012/3 Semester 1)
2. LOOPS! (2/2)
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cond?
Somestatement(s)
true
false
loop body
Loop condition
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Each round of the loop is called an iteration.
CS1010 (AY2012/3 Semester 1)
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
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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
CS1010 (AY2012/3 Semester 1)
2. Loop: Demo (3/3)
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num >= 0?
printf …printf … scanf …
true
false
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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
CS1010 (AY2012/3 Semester 1) Week5 - 22
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
Week5 - 23Week5 - 23CS1010 (AY2012/3 Semester 1)
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.
3.1 The while Loop: Demo (2/3)
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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;...
Week5 - 24CS1010 (AY2012/3 Semester 1)
3.1 The while Loop: Demo (3/3)
Week5 - 25Week5 - 25CS1010 (AY2012/3 Semester 1)
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.
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Output: ?
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a = 2;b = 7;while (a == b) {
print a;a = a + 2;
}
3.2 while Loop Condition (2/2)
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Output: ?
Week5 - 27CS1010 (AY2012/3 Semester 1)
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)
Week5 - 28CS1010 (AY2012/3 Semester 1)
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:
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Enter a positive integer: 2894334982
CS1010 (AY2012/3 Semester 1)
3.3 Tracing while Loop (3/4) Example: Given a positive integer n, print out
its digits from least significant to most significant.
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// 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)
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n initially 28943n @ point 29843digit @ point ***
What are the values of n and digit after exiting the loop?
CS1010 (AY2012/3 Semester 1)
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
CS1010 (AY2012/3 Semester 1)
4. The do-while Loop (2/2) Example: Count the number
of digits in an integer.
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do { // loop body} while ( condition );
CS1010 (AY2012/3 Semester 1)
// 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)
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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
CS1010 (AY2012/3 Semester 1)
5. The for Loop (2/2) Example: Print numbers 1 to 10
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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
CS1010 (AY2012/3 Semester 1)
5.1 The for Loop: Odd Integers (1/2)
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#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)
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// Precond: n > 0void print_odd_integers(int n) {
int i;for (i=1; i<=n; i++)
if (i%2 != 0)printf("%d ", i);
printf("\n");}
Week5_OddIntegers_v2.c
// Precond: n > 0void print_odd_integers(int n) {
for ( ; n > 0; n--)if (n%2 != 0)
printf("%d ", n);printf("\n");
}
Week5_OddIntegers_v3.c
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:
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Enter a positive integer: 50Sum = 408
CS1010 (AY2012/3 Semester 1)
6. Exercise #1: Sum of Multiples of 3 (2/2) How about using a while loop instead? Pseudo-code using a while loop:
Week5 - 39CS1010 (AY2012/3 Semester 1)
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:
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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 *?
CS1010 (AY2012/3 Semester 1)
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).
Week5 - 41CS1010 (AY2012/3 Semester 1)
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.)
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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)
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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.
Week5 - 44 CS1010 (AY2012/3 Semester 1)
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.
Week5 - 45 CS1010 (AY2012/3 Semester 1)
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
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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
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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’.
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// 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’
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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:
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Enter a positive integer: 131131 is a prime.
Enter a positive integer: 713713 is not a prime.
CS1010 (AY2012/3 Semester 1)
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.
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Summary for Today (1/2)
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Repetition statements (loops) for, while, do-while Nested loops break and continue
CS1010 (AY2012/3 Semester 1)
Summary for Today (2/2)
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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
CS1010 (AY2012/3 Semester 1)
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
Week5 - 54CS1010 (AY2012/3 Semester 1)
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