1. CS31 Discussion 1H Fall18: week 6
TA: Behnam Shahbazi
Credit to former TAs: Chelsea Ju and Bo-Jhang Ho, Ricky Lee, Sepideh Mazrouee

2. Overview Review: Multi-dimensional Arrays C-Strings
Pseudocode & Test Cases
Project 5
Multi-dimensional Arrays
C-Strings
Midterm 2 - Practice Problems

3. Pass a 2D array to a function
Can we pass a 2-dimensional array to a function?
Yes, but with restriction
Have to provide the size of the array
int fillInArray(int params[3][5]) { for (int i = 0; i < 3; i++) for (int j = 0; j < 5; j++) params[i][j] = i * 10 + j; } int main() { int arr[3][5]; fillInArray(arr); return 0; }
Cannot omit the two numbers like one-dimensional array
Have to match the caller

4. Pass an array to a function – Option 2
Can we pass a 2-dimensional array to a function?
Yes, but with restriction
int fillInArray(int params[][5], int row_num) { for (int i = 0; i < row_num; i++) for (int j = 0; j < 5; j++) params[i][j] = i * 10 + j; } int main() { int arr[3][5]; fillInArray(arr); return 0; }
You can leave off the first bound, but you must supply the  second bound as a compile-time constant expression

5. Pass a 2D array to a function
main() {
const int NUM_SECTIONS = 4;
const int MAX_STUDENTS = 5;
int scores[NUM_SECTIONS][MAX_STUDENTS];
scores[0][0] = 97;
scores[1][3] = 83;
scores[3][4] = 69; } 1 2 3 97 83 69

6. 2D Arrays
void main(void) {
int table[5][5];
int r,c;
for (r=0;r<5;r++)
for (c=0;c<5;c++)
table[r][c] = (r+1) * (c+1);
cout << table[4][3] << endl;
cout << table[2][1] << endl; }
When you use two nested loops as above, the outer loop iterates over the rows, and the inner loop iterates over the columns. 1 2 3 4 1 2 2 6 3 12 20 4 20

7. Why learn C Strings? If you ever need to program in C
Ex: embedded firmware
Many legacy libraries are written in C, so there is no #include <string> library.
Better performance.

8. What makes C strings different than C++ strings?
C strings can be viewed as an array of chars.
C strings have a null-terminating character at the end.

9. Null-terminated character\0
Allows us to know when the end of the string is.

10. Declaring a C string Template: char <name>[size];
char myString[10] = {‘h’,’e’,’l’,’l’,’o’,’\0’};
char myString[10] = “hello”; //does same as above
char myString[10]; // same as char myString[10] = { '\0' }
char myString[] = “hhheeellllooooooo”;

11. Example use of utilizing the null character
While I is not \0, enter loop

12. Will this compile?
No, no room for the null character that compiler will automatically put in

13. Will it compile? What if we try to put a null character at the end?
No, with double quotes, compiler will put in a null character, making it too big.

14. Undefined, need the null character if you wont put the number in the braces

15. Now the compiler knows when the end is

16. Will this compile? If so what is the output?
-first print will read until null terminating char
-second print will access 3rd char directly

17. Will it compile?
-cant reassign C strings like this after initializing it as empty.

18. One element at a time

19. Does this program not compile or have undefined behavior
Does this program not compile or have undefined behavior? If not, what does this program output?
i = 0, j = 0
Do{
print s2[0], increment 1
} while j < 5 && s2[j] isnt null && s2[j] == s1[j]

20. C String Library #include <cstring>
strlen “string length” returns the number of characters before the null character.
strcpy “string copy” copies the second argument into the first argument
strcat “string concatenate” adds the second argument to the first argument
strcmp “string compare” compares two strings.

21. strlen
-syntax is a little different instead of the .length() in c++

22. Write the strlen() function
Const means you wont modify it in this function

23. Would this program compile?
They don’t work on c++ strings

24. strcpy -copies all characters including null character
-when source array is larger than destination, undefined behavior. It tries copying past the allocated memory of the destination.

25. strcat -appends source to destination
-destination’s null character is replaced by the first character of source
-The behavior is undefined if the destination array is not large enough for the contents of both src and dest and the terminating null character.
Output: C strings are C strings are

26. strcmp
-the # is determined by the difference of the values of the stopping point
<0 the first character that does not match has a lower value in ptr1 than in ptr2
0 the contents of both strings are equal
>0 the first character that does not match has a greater value in ptr1 than in ptr2

27. strcmp
Prints 0

28. Will it compile? If so, what is the output?3

29. Will it compile? If so, what is the output?
No, cant do line 7

30. Will it compile? If so, what is the output?
lol
LOL

31. Will it compile? If so, what is the output? 10

32. What is the output? 10
01234! 6
012! 4
01! 3 0! 2

33. Summary Are stored in standard char arrays.
C strings:
Are stored in standard char arrays.
An array of size n may hold n-1 characters.
Must contain an ASCII 0 character at the end of the string to be valid.
You may have an empty string too. It has an ASCII 0 in the first slot.
Void main(void) {
char name[5];
// holds up to a 4 char
name[0] = ‘H’;
name[1] = ‘i’;
name[2] = 0; // or ‘\0’ }

34. Summary (Cont’d) C strings:
5. Always make sure when you copy one C string into another that there’s enough room!
6. It’s a good idea to initialize each string immediately before using it!
7. Remember, an uninitialized array contains a random, possibly invalid string!
void main(void) {
char name[5];
strcpy ( name , ”Adele” );
name[0] = ‘\0’;
strcat(name,”Hi”); }

35. In class exercise
Please write two versions of a function that replaces the existing all chars in a C string with the char ‘X'.
1. Uses the strlen function
2. Can’t use strlen
void fillX(char s[]) {
for (int i = 0; i < strlen(s); i++)
s[i] = 'X';
//2nd version: replace for loop with
// for (int i = 0; s[i] != '\0'; i++) }

36. - s and t represent strings.
Don't use C++ functions on C strings!
- s and t represent strings.

37. Array of char arrays Row 1 (a[0]) Row 2 (a[1]) Row m (a[m]) A matrix
That means, an array of char arrays!
Row 1 (a[0])
Row 2 (a[1])
Row m (a[m])
A matrix

38. Array of char arrays char a[10][20];
Expression with only one subscript, like a[k], can be treated like a C string.
(assuming that row has a zero byte marking the end) -- strlen(a[k]) is  fine, for example.
Strlen(a[k])
strcpy(a[0],"hello”)
strcmp(a[0],a[1]) …