1. CS31 Discussion 1D Winter19: week 3
TA: Behnam Shahbazi
Credit to former TAs: Chelsea Ju and Bo-Jhang Ho

2. for ( ; ; ) { } Syntax of for loop Execution flow: Initial value
Execution condition
Increment value
Line 1
Line 2
Line 3
Line 4
Line 5 }
Execution flow:
Initial
Condition
Block
Increment
Condition
Block
Increment  
Condition
Block
Increment
Condition
Block
Increment
Condition

3. Exercise: summation program
int main() { int i; int sum = 0; for (i = 1; i <= 5; i++) sum += i; cout << sum << endl; cout << i << endl; return 0; }
Initial value
Execution condition
Increment value
Block
Result?

4. while ( ) { } Syntax of while loop Execution flow: Execution condition
Line 1
Line 2
Line 3
Line 4
Line 5 }
Execution flow:
Condition
Block
Condition
Block
Condition
Block
Condition

5. Exercise: Counting number of trailing zeros in an integer
For example:
Input 27000, output 3
Input 290, output 1
Input , output 7
Input 123, output 0

6. Exercise: Counting number of trailing zeros in an integer
int main() { int num; cout << "Enter a number: "; cin >> num; int count = 0; while (num % 10 == 0) { num /= 10; count++; } cout << count << " trailing zeros"; return 0; }
Execution condition
Block
What’s the result when input is 27000?
What’s the result when input is 290?

7. Syntax of do-while loop
Line 1
Line 2
Line 3
Line 4
Line 5
} while ( );
Execution condition
Important reminder: Don’t forget the semicolon in the end of while clause!
Execution flow:
Block
Condition
Block
Condition
Block
Condition

8. Exercise: Guessing number game
int main() { int target = 15; int num; do { cout << "Enter a number: "; cin >> num; } while (num != target); cout << "You guess is correct!"; return 0; }
Execution condition
Block
Discussion:
Why do I put != instead of == in the while condition?

9. Comparison of 3 types of loop
for ( ; ; ) {
Initial
Condition
Increment
For
When the task is sequential
A typical usage is for counting
Line 1
Line 2 }
while ( ) {
Condition
While
We don’t know how many times we have to repeat the block, but know when to repeat (or stop)
Line 1
Line 2 }
do {
Do-while
Similar to while, but if we must execute the block at least once
Don’t forget the semicolon!
Line 1
Line 2
} while ( );
Condition

10. Is it possible to replace one type of loop to another?
Technically we can
But it doesn’t make any sense
The 3 kinds of loop fit into different scenario

11. Loop replacement For -> While for ( ; ; ) { while ( ) { } } Initial
Condition
Increment
Line 1
Line 2 } }

12. Loop replacement For -> While for ( ; ; ) { while ( ) { } } Initial
Condition
Increment
Condition
Line 1
Line 2
Line 1
Line 2 } }

13. Loop replacement For -> While for ( ; ; ) { while ( ) { } } Initial
Condition
Increment
Condition
Line 1
Line 2
Line 1
Line 2 }
Increment }

14. Loop replacement While -> For for ( ; ; ) { while ( ) { } }
Condition
Line 1
Line 2 } }

15. Loop replacement While -> For for ( ; ; ) { while ( ) { } }
Condition
Condition
Line 1
Line 2
Line 1
Line 2 } }

16. Nested loops: Printing a rectangle
int main() { for (int r = 1; r <= 3; r++) { for (int c = 1; c <= 4; c++) { cout << "*"; } cout << endl; } return 0; }

17. Nested loops: Printing a triangle
int main() { int size; cout << "Enter size: "; cin >> size; for (int r = 1; r <= size; r++) { for (int c = 1; c <= r; c++) { cout << "*"; } cout << endl; } return 0; }

18. Take-home exercise: Can you print triangles in different angle?
* ** *** **** *****
* ** *** **** *****
Done!
***** **** *** ** *
***** **** *** ** *

19. String demystify: (1) Get the length
int main() { string s = "Hello"; cout << "Length: " << s.size() << endl; return 0; }
Result?
Length: 5

20. String demystify: (2) Dump all the characters
int main() { string s = "Hello"; for (int i = 0; i < s.size(); i++) cout << "index " << i << ": " << s[i] << endl; return 0; }
Result?
index 0: H index 1: e index 2: l index 3: l index 4: o
Variable s is a string
s[<a number>]
Get the character with the corresponding index
The type is a character

21. String demystify: (3) Dump all the characters
int main() { string s = "Hello"; int count = 0; for (int i = 0; i < s.size(); i++) if (s[i] == 'e') count++; cout << "Number of Es: " << count << endl; return 0; }
Result?
Number of Es: 1
‘e’ is a character literal

22. More exercises Compute factorization, e.g., 7!
Compute combination number, e.g., 10 choose 6
List all the prime numbers up to 100
Great common divisor of two numbers

23. Puzzle time: What does the code print?
int main() { int n = 5; for (int i = 0; i < n; i++) { for (int j = 0; j < n; j++) cout << (i / j) * (j / i); cout << endl; } return 0; }

24. Puzzle time: What does the code print?
int main() { int n = 6; for (int i = 1; i < n; i++) { for (int j = 1; j < n; j++) cout << (i / j) * (j / i); cout << endl; } return 0; }

25. What is a function Take some numbers, produce a result value int
double
double
string
Take some numbers, produce a result value

26. What is a function
f(x) = 2x + 3
What is f(5)?

27. What is a function
f(x, y) = 3x + 2y - xy
What is f(4, 1)?

28. What is a function What is the output?
int square(int n) { return n * n; } int main() { int a = 3; int b = 6; a = a * b + square(a + b); cout << a << endl; return 0; }
What is the output?

29. What is a function
int square(int n); int main() { int a = 3; int b = 6; a = a * b + square(a + b); cout << a << endl; return 0; } int square(int n) { return n * n; }
When a function is called, it always look back and try to find the function.
If you want to declare it before the callee, you have to declare it first.

30. Don’t memorize them! ASCII Characters #include <cctype>
Some functions in this library:
int isalnum(int c); // alphanumeric
int isdigit(int c); // decimal digit
int islower(int c); // lowercase letter
int isupper(int c); // uppercase letter
int isspace(int c); // white-space
int toupper(int c); // convert to uppercase
int tolower(int c); // convert to lowercase
Don’t memorize them!

31. #include <cctype> example

32. Worksheet 2
Let’s team up and work on worksheet 2