Exercise 5

Transcription

Exercise 5

Exercise 5
Arrays and C-Strings
Informatik I für D-MAVT (FS 2016)
© M. Gross, ETH Zürich, 2016
Recap last exercise
• Question 1:
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int i; long int li; float f; double d; long double ld; char c; bool b;
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int x = f;
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short int x = li;
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long int x = f;
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float x = li;
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double x = c;
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int x = b;
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short int x = ld;
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char x = i;
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bool x = i;
2
Recap last exercise
• Question 1:
•
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int x = f;
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short int x = li;
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long int x = f;
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float x = li;
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double x = c;
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int x = b;
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short int x = ld;
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char x = i;
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bool x = i;
2
-> abschneiden
Recap last exercise
• Question 1:
•
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int x = f;
-> abschneiden
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short int x = li;
-> bits werden abgeschnitten!
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long int x = f;
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float x = li;
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double x = c;
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int x = b;
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short int x = ld;
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char x = i;
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bool x = i;
2
Recap last exercise
• Question 1:
•
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int x = f;
-> abschneiden
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short int x = li;
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long int x = f;
-> abschneiden
•
float x = li;
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double x = c;
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int x = b;
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short int x = ld;
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char x = i;
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bool x = i;
2
Recap last exercise
• Question 1:
•
•
int x = f;
-> abschneiden
•
short int x = li;
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long int x = f;
-> abschneiden
•
float x = li;
-> Info-verlust wenn li zu gross
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double x = c;
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int x = b;
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short int x = ld;
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char x = i;
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bool x = i;
2
Recap last exercise
• Question 1:
•
•
int x = f;
-> abschneiden
•
short int x = li;
•
long int x = f;
-> abschneiden
•
float x = li;
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double x = c;
-> ASCII Code von c
•
int x = b;
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short int x = ld;
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char x = i;
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bool x = i;
2
Recap last exercise
• Question 1:
•
•
int x = f;
-> abschneiden
•
short int x = li;
•
long int x = f;
-> abschneiden
•
float x = li;
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double x = c;
-> ASCII Code von c
•
int x = b;
-> false->0, true ->1
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short int x = ld;
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char x = i;
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bool x = i;
2
Recap last exercise
• Question 1:
•
•
int x = f;
-> abschneiden
•
short int x = li;
•
long int x = f;
-> abschneiden
•
float x = li;
•
double x = c;
-> ASCII Code von c
•
int x = b;
•
short int x = ld;
-> abschneiden
•
char x = i;
•
bool x = i;
2
Recap last exercise
• Question 1:
•
•
int x = f;
-> abschneiden
•
short int x = li;
•
long int x = f;
-> abschneiden
•
float x = li;
•
double x = c;
-> ASCII Code von c
•
int x = b;
•
short int x = ld;
-> abschneiden
•
char x = i;
•
bool x = i;
2
Recap last exercise
• Question 1:
•
•
int x = f;
-> abschneiden
•
short int x = li;
•
long int x = f;
-> abschneiden
•
float x = li;
•
double x = c;
-> ASCII Code von c
•
int x = b;
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short int x = ld;
-> abschneiden
•
char x = i;
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bool x = i;
-> x = (i != 0)
2
Recap last exercise
• Question 4:
3
Recap last exercise
• Question 4:
value = 2;
3
Recap last exercise
• Question 4:
value = 2;
result = 7;
3
Recap last exercise
• Question 4:
value = 2;
result = 7;
value = 3;
3
Recap last exercise
• Question 4:
value = 2;
result = 7;
value = 3;
return 7;
3
Recap last exercise
• Question 4:
value = 2;
result = 7;
value = 3;
return 7;
returnedValue = 7; value = 2;
3
Agenda
• Arrays
– 1-dimensional
– 2-dimensional
• C-Style strings
4
Arrays
• A datatype, storing multiple values of the same type
• Definition consists of three elements:
– Data type of elements 1
– Name of array 2
– Number of elements 3
1
2
3
int myArray[100]; // Allocate int array of size 100
5
Arrays: Definition
• Number of elements must be constant! int i=10; float myFloats[i]; // Invalid; must be sure about size of // array at compile time. const int j=10; float myFloats2[j]; // This is okay.
6
Arrays: Indexing
• Array elements are zero-indexed!
– Range is from 0 to (number of elements) – 1
int myArray[7];
myArray
7
Arrays: Initialisation and Assignment
• Initialisation in two ways:
– At definition: int a[5] = {0, 1, 2, 3, 4};
– Explicitly: int b[3]; b[0] = 7; b[1] = 3; b[2] = 3;
8
Arrays: Initialisation and Assignment
• Partial initialisation
int c[5] = {1, 2}; // Equals {1,2,0,0,0}
– Remaining elements are set to default value (0 for int)
• Let compiler determine array size
int d[] = {1, 2, 3}; // Equals int d[3]=…
– Array takes minimum size to store given values.
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Arrays: Out of Bounds
• Caution! Compiler doesn’t verify your indices.
• Exceeding array bounds results in runtime errors.
int e[3] = {1, 2, 3}; cout << e[0]; // Fine. cout << e[3]; // Out of bounds, but cout << e[9]; // compiler doesn’t complain
• Accessing uninitialised memory
– Results are undefined
10
Arrays: Example
• Finding the biggest value
• Better: use for/while loops
– See next exercise
const int dim = 3;
double values[dim] = {1.0, 8.0, -5.0};
double maxValue = values[0]; // Initialise
if (values[1] > maxValue)
maxValue = values[1];
if (values[2] > maxValue)
maxValue = values[2];
cout << "Maximum value: " <<
maxValue << endl; // 8.0
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Arrays: Example II
12
Arrays: Example II
• int a [ ] = {3, 5, 8, 1, 0};
12
Arrays: Example II
• int a [ ] = {3, 5, 8, 1, 0};
12
3
5
8
1
0
Arrays: Example II
• int a [ ] = {3, 5, 8, 1, 0};
3
• a[ 2 ] = 2;
12
5
8
1
0
Arrays: Example II
• int a [ ] = {3, 5, 8, 1, 0};
3
5
8
1
0
• a[ 2 ] = 2;
3
5
2
1
0
12
Arrays: Example II
• int a [ ] = {3, 5, 8, 1, 0};
3
5
8
1
0
• a[ 2 ] = 2;
3
5
2
1
0
• a [ a[3] ] = a [ 3 ] + 2;
12
Arrays: Example II
• int a [ ] = {3, 5, 8, 1, 0};
3
5
8
1
0
• a[ 2 ] = 2;
3
5
2
1
0
• a [ a[3] ] = a [ 3 ] + 2;
3
3
2
1
0
12
Arrays: Example II
• int a [ ] = {3, 5, 8, 1, 0};
3
5
8
1
0
• a[ 2 ] = 2;
3
5
2
1
0
• a [ a[3] ] = a [ 3 ] + 2;
3
3
2
1
0
• a[ a[4] ] = a[ a[3] + a[1] ];
12
Arrays: Example II
• int a [ ] = {3, 5, 8, 1, 0};
3
5
8
1
0
• a[ 2 ] = 2;
3
5
2
1
0
• a [ a[3] ] = a [ 3 ] + 2;
3
3
2
1
0
0
3
2
1
0
• a[ a[4] ] = a[ a[3] + a[1] ];
12
Multidimensional Arrays
• Arrays of arbitrary size can be allocated
• 2D array syntax: typename variableName [rows][columns];
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• Definition and initialisation:
int b[2][3] = {{1,2,3},{4,5,6}};
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• Definition and initialisation:
int b[2][3] = {{1,2,3},{4,5,6}};
• Assignment:
b[1][2] = 12;
int c = b[0][1];
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c == 2
12
Multidimensional Arrays: Example
int myArray[][4] = {
{236, 189, 189, 0},
{236, 80, 189, 189},
{236, 0, 189, 80},
{236, 189, 189, 80}};
myArray
• Representation of a simple greyscale image
– Array elements as brightness values
16
C-Strings
• A string is an array of characters.
– Represents a word, a sentence, etc.
• cstring library provides many useful functions.
#include <cstring>
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C-Strings
• C-style strings must end with ’\0’ (zero character)
– Valid:
char name[5] = {'H','a','n','s','\0'};
– Invalid: char name[4] = {'H','a','n','s'}; This is a character array, but not a string!
• Initialisation:
char name[5] = {'H','a','n','s','\0'}; char name2[] = "Hans"; // Much nicer!
18
C-Strings
• Beware your quotes – data type mismatch:
char a = 'x'; // Good. Single quotes denote a char char a = "x"; // Bad! Double quotes denote a string
• Reading in a string:
char name[100]; cin >> name;
• Reading in a line including spaces:
char sentence[200]; cin.getline(sentence, 200);
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C-Strings: Useful Functions
• toupper (from library <ctype.h>):
char str[] = "abc";
str[0] = toupper(str[0]); // str == Abc
• strcat and strcpy:
char str2[80];
strcpy(str2,str);
strcat(str2,"def");
// str2 == Abc
// str2 == Abcdef
• strlen:
int n;
n = strlen(str2);
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// n == 6
Exercise 5
•
•
•
•
•
Practice array declarations
Programming: create, use, and modify arrays
Programming: use C-style strings
Experiment with UNIX user rights
Hand in:
– Solutions to tasks 1 (arrays) and 5 (user rights)
– Source code for means2, temperature, names
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Exercise 5

Transcription

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