CS 31 Discussion: Week 6. Taylor Caulfield

Transcription

1 CS 31 Discussion: Week 6 Taylor Caulfield

2 Overview for this Week Arrays C-Strings

3 Arrays Arrays: collection fo variable of one type Get i'th index element of array A: A[i] range of valid indexes is 0 ~ (A's size -1) size of array must be declared in declaration (inside the brackets following variable name) must be a constant (either a constant variable or an integer) no size() method available to get # of elements use constant variables to easily keep track of size for loops, etc. sizeof() exists but not intuitive to use (it returns the size of array in bytes, not the number of elements) requires tinkering with to get array size so not recommended

4 Arrays 2-D Array: An array that carries arrays Get element j in array i of 2-D Array A: A[i][j] Get whole array i an A: A[i] Possible to layer arrays even more! (N-D Arrays) e.g. 5-D Array: int giantblob[1][2][6][24][120]

5 Arrays A 5x3 array of chars can be visualized as a table: A B C D E 1 F G H U J 2 K L M N Z Here, each column is a char array of size 3

6 Arrays A 5x3 array of chars can be visualized as a table: A B C D E 1 F G H U J 2 K L M N Z Let's name this 2D array A. What is the value(s) contained in: A[3][2] A[1] A[5][0]

7 Arrays A 5x3 array of chars can be visualized as a table: A B C D E 1 F G H U J 2 K L M N Z Let's name this 2D array A. What is the value(s) contained in: A[3][2] N A[1] BGL A[5][0] n/a (it's out of bounds)

8 Arrays Can pass array to a function specify array parameter by putting empty brackets after variable name e.g. int myfunc(int array[]) they must be empty, don't put anything inside them! technically always a pointer, but dereferenced like other basic variable types so you can just pass it into the function and use it like any other parameter

9 Arrays Can pass multi-dimensional arrays to functions too for all dimensions past the first, specify sizes (as constants) in brackets e.g. int myfunc(int array[][3]) notice that we can never define the first dimension, so this size value is lost thus it's wise to also pass in a variable denoting the size of the first dimension of the array you are passing in; otherwise you can't know array's size for the first dimension!

10 Arrays Spot the errors! int HEIGHT = 3; int LENGTH = 2; int eat_box(int box) { for (int x = 0; x < box.size(); x++) { if (box[x] > 50) return 1; cout << "I ate the " << box[x] << "rhinos in bento box " << x << endl; return 0; int main() { int boxes[height][length] = {{15, 25, 39,{1, 11, 12; for (int i = 0; i < height; i++) { int p = box[i]; if (eat_box(p) == 0) cout << "Ate box " << i << " successfully!" << endl; else cout << "Oh noes" << endl;

11 Arrays Spot the errors! int HEIGHT = 3; int LENGTH = 2; int eat_box(int box) { for (int x = 0; x < box.size(); x++) { if (box[x] > 50) return 1; cout << "I ate the " << box[x] << "rhinos in bento box " << x << endl; return 0; int main() { int boxes[height][length] = {{15, 25, 39,{1, 11, 12; for (int i = 0; i < height; i++) { int p = box[i]; if (eat_box(p) == 0) cout << "Ate box " << i << " successfully!" << endl; else cout << "Oh noes" << endl;

12 Arrays Fixed const int HEIGHT = 3; const int LENGTH = 2; int eat_box(int box[], int len) { for (int x = 0; x < len; x++) { if (box[x] > 50) return 1; cout << "I ate the " << box[x] << "rhinos in bento box " << x << endl; return 0; int main() { int boxes[height][length] = {{15, 25, 39,{1, 11, 12; for (int i = 0; i < HEIGHT; i++) { int p[height] = box[i]; if (eat_box(p,length) == 0) cout << "Ate box " << i << " successfully!" << endl; else cout << "Oh noes" << endl;

13 C-Strings C doesn't have the easy-to-use string library of C++ Must use array of char instead Why use C instead of C++ faster, more control over memory can potentially mean better performance/memory usage but harder to use, not as many easy shortcuts, more room for error

14 C-Strings Initializing C-string no need to tediously define every individual char element like other arrays can just initialize it like a C++ string e.g. char mystring[10] = "hello"; but like all arrays, explicit maximum size of c-string must be specified and constant initialization value can be less than max size; free slots will not be used

15 C-Strings Initializing C-string one extra character appended after the end of the value in the c-string: '\0' '\0' is the null termination character (ASCII code is 0) everything to the right of it in the char array is ignored thus initialization with n chars actually has n+1 chars! so maximum size of initializations is actually (max size - 1) if initialization is of same size as max size or array, '\0' will be "appended" outside of the array, causing buffer overflow! convention is to stop reading a c-string when '\0' reached, ignoring '\0' e.g. "hello" is equivalent to {'h','e','l','l','o','\0' no need to explicitly specify the '\0'

16 C-Strings Useful functions size_t strlen(const char s[]): get number of characters in s stops before '\0', so gets number of chars coming before '\0', not the max array size technically, the type returned is not int but size_t, but value can be chucked into an int int strcmp(const char s1[], const char s2[]): compares s1 and s2 and returns negative, if s1 smaller lexicographically than s2 positive, if s1 larger lexicographically than s2 0 if equal strcat(const char s1[], const char s2[]): adds contents of s2 onto s1 strcpy(const char s1[], const char s2[]): replaces contents of s1 with s2 strncpy(const char s1[], const char s2[], size_t n): like strcpy but copies at most n characters

17 C-Strings One last note about string comparison mathematical operators can't be used to compare C-strings! you have to use strcmp

18 C-Strings Spot the errors! If we want BearyBearBear to demand only the first plate, what needs to be changed? int feast_on_chars(const char input1, const char input2) { int s = input1.size(); if (input2.size() > s) s = input2.size(); if (s <= 7) return 0; if (input1 > input2) return 1; else return 2; int main() { const char plate1[7] = "hellooo"; const char plate2[5] = "Goodbye"; int result = feast_on_chars(plate1,plate2); switch(result) { case 0: cout << "BearyBearBear hungry! BearyBearBear DEMAND MORE!" case 1: cout << "BearyBearBear demands first plate!" case 2: cout << "BearyBearBear demands second plate!" default: cout << "BearyBearBear slee--zzzzzzz"

19 C-Strings Spot the errors! If we want BearyBearBear to demand only the first plate, what needs to be changed? int feast_on_chars(const char input1, const char input2) { int s = input1.size(); if (input2.size() > s) s = input2.size(); if (s <= 7) return 0; if (input1 > input2) return 1; else return 2; int main() { const char plate1[7] = "hellooo"; const char plate2[5] = "Goodbye"; int result = feast_on_chars(plate1,plate2); switch(result) { case 0: cout << "BearyBearBear hungry! BearyBearBear DEMAND MORE!" case 1: cout << "BearyBearBear demands first plate!" case 2: cout << "BearyBearBear demands second plate!" default: cout << "BearyBearBear slee--zzzzzzz"

20 C-Strings int feast_on_chars(const char input1[], const char input2[]) { int s = input1.strlen(input1); if (strlen(input2) > s) s = strlen(input2); if (s <= 7) return 0; if (strcmp(input1,input2) > 0) return 1; else return 2; int main() { const char plate1[8] = "hellooo"; const char plate2[10] = "Goodbye"; int result = feast_on_chars(plate1,plate2); switch(result) { case 0: cout << "BearyBearBear hungry! BearyBearBear DEMAND MORE!" break; case 1: cout << "BearyBearBear demands first plate!" break; case 2: cout << "BearyBearBear demands second plate!" break; default: cout << "BearyBearBear slee--zzzzzzz"

21 Next Time What's coming up Project 5 will be using C-strings! Midterm next week (11/10) Future topics Pointers