Computer Science 1620 Arrays. Problem: Given a list of 5 student grades, adjust the grades so that...

Computer Science 1620

Arrays

Problem: Given a list of 5 student grades, adjust the grades so that the average is 70%.

Program design:1. read in the grades

2. calculate the average A

3. add (70 – A) to each grade

4. re-output the new grades to one decimal place

1. Read in the grades 2. Calculate the average A

3. Add (70 – A) to each grade 4. Re-output the grades#include <iostream>#include <iomanip> using namespace std;

int main() {

double grade1, grade2, grade3, grade4, grade5;

cout << "Please enter the grades of the students" << endl; cin >> grade1 >> grade2 >> grade3 >> grade4 >> grade5;

double A = (grade1 + grade2 + grade3 + grade4 + grade5) / 5.0;

grade1 += (70 - A); grade2 += (70 - A); grade3 += (70 - A); grade4 += (70 - A); grade5 += (70 - A);

cout << fixed << setprecision(1);

cout << "New grades:" << endl; cout << grade1 << "%" << endl; cout << grade2 << "%" << endl; cout << grade3 << "%" << endl; cout << grade4 << "%" << endl; cout << grade5 << "%" << endl;

return 0;}



int main() {







return 0;}

4. re-output the new grades

1. Read in the grades





int main() {







return 0;}




Does the previous code work?Yes!

Problems:1) Repeated code!

adjusting each mark, and re-outputting each mark is basically the same for each mark



int main() {







return 0;}

Does the previous code work?Yes!

Problems:1) Repeated code!2) Scalability

what happens if the number of students increases to X, where X is large?

#include <iostream>#include <iomanip> using namespace std;

int main() {

double grade1, grade2, grade3, grade4, grade5; double grade6, grade7, grade8, grade9, grade10; double grade11, grade12, grade13, grade14, grade15; double grade16, grade17, grade18, grade19, grade20;

cout << "Please enter the grades of the students" << endl; cin >> grade1 >> grade2 >> grade3 >> grade4 >> grade5 >> grade6 >> grade7 >> grade8 >> grade9 >> grade10 >> grade11 >> grade12 >> grade13 >> grade14 >> grade15 >> grade16 >> grade17 >> grade18 >> grade19 >> grade20;

double A = (grade1 + grade2 + grade3 + grade4 + grade5 +grade6 + grade7 + grade8 + grade9 + grade10 +grade11 + grade12 + grade13 + grade14 + grade15 +grade16 + grade17 + grade18 + grade19 + grade20)/20;

grade1 += (70 – A); grade2 += (70 – A); grade3 += (70 – A); grade4 += (70 – A); grade5 += (70 – A); grade6 += (70 – A); grade7 += (70 – A); grade8 += (70 – A); grade9 += (70 – A); grade10 += (70 – A); grade11 += (70 – A); grade12 += (70 – A); grade13 += (70 – A); grade14 += (70 – A); grade15 += (70 – A); grade16 += (70 – A); grade17 += (70 - A); grade18 += (70 - A); grade19 += (70 - A); grade20 += (70 - A);

...

Arraysan aggregate data structure

vs. atomic data types (int, float, double, char, …)a group of variables of the same type

Declare 10 variables of type int:

int x1, x2, x3, x4, x5, x6, x7, x8, x9, x10;

int x[10];Without Arrays: With Arrays:

Array syntax: type name[size];

type can be any of the int or floating point types* name can be any valid variable name size can be any positive integral value, but MUST BE

CONSTANT*

How to access the elements in an arrayuse the subscript operator [ ] indices START AT 0, NOT 1value in the subscripts need not be a

constant

int x[3];

x[0] = 5; // sets the first element in the array to value 5x[1] = 6; // sets the second element in the array to value 6

int i = 2;x[i] = 7; // sets the third element in the array to value 7

The elements of an array are used exactly like a single variablecan assign a value to them:

x[i] = 5;can use them in an expression:

int sum = x[0] + x[1] + x[2];cin >> x[0];cout << "Second value: " << x[1];

Arrays and Loops the indexing feature of arrays makes it very easy to use them

in a loop very handy if you want to perform some operation on each

element in the array General template (using a for loop)

given an array x of size N begin integer index (call it i) at 0 loop up to (but not including) N increment one at a time perform operation on x[i] inside loop

int x[10];

for (int i = 0; i < 10; i++) { // do some operation on x[i]}

Example 1: given an array x of 10 ints, set each element in the array to equal 5

Example 2: given an array x of 10 ints, output the value of each element to

standard output

int x[10];

for (int i = 0; i < 10; i++) { x[i] = 5;}

int x[10];

for (int i = 0; i < 10; i++) { cout << x[i] << endl;}

Problem: Given a list of 5 student grades, adjust the grades so that the average is 70%.

Solution: read in the gradescalculate the average Aadd (70 – A) to each grade re-output the new grades



int main() {







return 0;}

Reprogram thisexample usingarrays!



int main() {







return 0;}

Convert theseto an array



int main() {

double grade[5];






return 0;}

Convert theseto an array



int main() {

double grade[5];






return 0;}

cin a value to eachelement in the array

1. begin integer index (call it i) at 0

2. loop up to (but not including) N

3. increment one at a time

4. perform operation on x[i] inside loop



int main() {

double grade[5];

cout << "Please enter the grades of the students" << endl; for (int i = 0; i < 5; i++) cin >> grade[i];





return 0;}

cin a value to eachelement in the array

1. begin integer index (call it i) at 0

2. loop up to (but not including) N

3. increment one at a time

4. perform operation on x[i] inside loop



int main() {

double grade[5];






return 0;}

Add the value of eachelement in the array toA.



int main() {

double grade[5];


double A = 0; for (int i = 0; i < 5; i++) A += grade[i];




return 0;}

Add the value of eachelement in the array toA.



int main() {

double grade[5];



A /= 5.0;




return 0;}

Divide the total by 5 to obtain the average.



int main() {

double grade[5];



A /= 5.0;




return 0;}

Subtract 70-A from each grade.



int main() {

double grade[5];



A /= 5.0;

for (int i = 0; i < 5; i++) grade[i] += (70 – A);



return 0;}

Subtract 70-A from each grade.



int main() {

double grade[5];



A /= 5.0;




return 0;}

Re-output each grade.



int main() {

double grade[5];



A /= 5.0;



cout << "New grades:" << endl; for (int i = 0; i < 5; i++) cout << grade[i] << "%" << endl;

return 0;}

Re-output each grade.



int main() {

double grade[5];



A /= 5.0;



cout << "New grades:" << endl; for (int i = 0; i < 5; i++) cout << grade[i] << "%" << endl;

return 0;}

We can save some spaceby combining these twoloops!



int main() {

double grade[5];



A /= 5.0;


cout << "New grades:" << endl; for (int i = 0; i < 5; i++) { grade[i] += (70 – A); cout << grade[i] << "%" << endl; }

return 0;}

We can save some spaceby combining these twoloops!



int main() {

double grade[5];



A /= 5.0;



return 0;}

Each of these statementsis only used once in the code!



int main() {

double grade[10];



A /= 10.0;



return 0;}

Adjusting the codeto accommodate 10students requires changingthe array size!

Good programming practice: try to avoid magic numbers in your code

numbers whose significance may not be obvioususe a const parameter instead this applies to the size of the arrays

this is exemplified by the last slideuse a const integral value for your array

sizes



int main() {

const int NUM_STUDENTS = 10; double grade[NUM_STUDENTS];

cout << "Please enter the grades of the students" << endl; for (int i = 0; i < NUM_STUDENTS; i++) cin >> grade[i];

double A = 0; for (int i = 0; i < NUM_STUDENTS; i++) A += grade[i];

A /= NUM_STUDENTS;


cout << "New grades:" << endl; for (int i = 0; i < NUM_STUDENTS; i++) { grade[i] += (70 – A); cout << grade[i] << "%" << endl; }

return 0;}



int main() {

const int NUM_STUDENTS = 1000; double grade[NUM_STUDENTS];

cout << "Please enter the grades of the students" << endl; for (int i = 0; i < NUM_STUDENTS; i++) cin >> grade[i];

double A = 0; for (int i = 0; i < NUM_STUDENTS; i++) A += grade[i];

A /= NUM_STUDENTS;


cout << "New grades:" << endl; for (int i = 0; i < NUM_STUDENTS; i++) { grade[i] += (70 – A); cout << grade[i] << "%" << endl; }

return 0;}

Adjusting the codeto accommodate 10000students requires changingone variable value• minimal effect on code length

Arrays: Common Programming Errorsoff by 1 (run-time error)

using indices beginning at 1

int x[10];

for (int i = 1; i <= 10; i++) { cout << x[i];}

Arrays: Common Programming Errorsusing a non const value for size (compiler

error)

int size = 6;

int x[size];

for (int i = 0; i < size; i++) { cout << x[i];}

Arrays: Common Programming Errorsgoing out of bounds (runtime error)

C++ does not do bounds checking

int x[10];

for (int i = 0; i <= 10; i++) { cout << x[i];}

Array Initializationwhen declaring an atomic variable, we had

the option to initialize to a value: int x = 10;

we can do the same with arrays, using braces

int x[5] = {1, 2, 3, 4, 5};

x[0] x[1] x[2] x[3] x[4]

Array Initializers if you do not include enough initializers, fills the rest

with 0 int x[5] = {1,2,3}; // x[3] and x[4] get value 0

if you include too many initializers, compiler error int x[5] = {1,2,3,4,5,6}; // won't compile

when using initializers, can exclude the size of the array altogether

C++ uses size of initializer list int x[ ] = {1,2,3,4,5}; // same as x[5] = {1,2,3,4,5};

Arrays and Operatorsas mentioned, the elements of an array can

be used wherever a variable canint i;

int a[10];

cout << i << " " << a[4] << endl;

cout << 3 + i << " " << 3 + a[4] << endl;

Arrays and Operators the array however, cannot be used just like

a variablesome common errors:

1) You cannot output all the elements of an array with one redirect*

int a[3];a[0] = a[1] = a[2] = 4;cout << a << endl;



2) You cannot input all the elements of an array with one redirect*

int a[3];cin >> a;cout << a[0] << endl;



3) You cannot assign the elements of one array to another array with one assignment

int a[3];a[0] = a[1] = a[2] = 4;int b[3];b = a;cout << b[0] << endl;



4) You cannot compare two arrays with one == operator

int a[3] = {4, 4, 4};int b[3] = {4, 4, 4};

if (a == b) cout << "True" << endl;else cout << "False" << endl;

Arrays and Operators as a rule, you typically have to manually

perform operations on each element of an array*

1) To output all values:

int a[3];a[0] = a[1] = a[2] = 4;cout << a[0] << endl;cout << a[1] << endl;cout << a[2] << endl;

or

int a[3];a[0] = a[1] = a[2] = 4;for (int i = 0; i < 3; i++) cout << a[i] << endl;

Arrays and Operatorsas a rule, you typically have to manually

perform operations on each element of an array*

2) To input all values:

int a[3];cin >> a[0] >> a[1] >> a[2];

or int a[3];for (int i = 0; i < 3; i++) cin >> a[i];

Arrays and Sizes once you declare the size of an array, you cannot resize it

this can be inconvenient, especially if you don't know how much data you need

when the amount of incoming data is not known, an array is typically declared to be larger than necessary

this typically requires a second integer, for storing the number of values in the array

example: write a program to read in a list of numbers from a user, and print out these numbers in reverse. The user will indicate the end of the numbers by entering a 0.

int a[3];

// a has three integers in it // for the rest of its lifetime


3. Add (70 – A) to each grade 4. Re-output the grades#include <iostream> using namespace std;

int main() {

const int MAX_SIZE = 512;

int numbers[MAX_SIZE]; int length = 0;

int input; cout << "Please enter up to 512 numbers, 0 to stop: "; cin >> input;

while ( (length < 512) && (input != 0)) { numbers[length] = input; length++; cin >> input; }

for (int i = length-1; i >= 0; i--) cout << numbers[i] << endl;

return 0;

}

Arrays as Function Arguments

Array elements can be passed by valueonly a copy is sent

void swap(int a, int b) { int temp = a; a = b; b = temp;}

int main() { int x[3] = {0, 1, 2}; swap(x[0], x[1]); cout << x[0] << ","; cout << x[1] << ","; cout << x[2] << endl;}

Output: 0,1,2

Arrays as Function Arguments

Array elements can be passed to reference parameters

void swap(int &a, int &b) { int temp = a; a = b; b = temp;}

int main() { int x[3] = {0, 1, 2}; swap(x[0], x[1]); cout << x[0] << ","; cout << x[1] << ","; cout << x[2] << endl;}

Output: 1,0,2

Passing Arrays as Parametersan entire array can be passed as a

parameterExample: Write a program with a function

called sum that takes an array of 5 integers, and returns the sum of those integers

#include <iostream> using namespace std;

int sum(int a[5]) {

int result = 0;for (int i = 0; i < 5; i++) {

result += a[i];}return result;

}

int main() {

int x[5] = {1,2,3,4,5};cout << "Sum = " << sum(x) << endl;

int y[5] = {5, 10, 15, 20, 25};cout << "Sum = " << sum(y) << endl;return 0;

}

Write a program with a function called sum that takes an array of 5 integers, and returns the sum of those integers


int sum(int a[5]) {



}

int main() {



}

Write a program with a function called sum that takes an array of 5 integers, and returns the sum of those integers.


int sum(int a[5]) {



}

int main() {



}


Our summation code goes here:Add each element in the array to result.


int sum(int a[5]) {



}

int main() {



}



int sum(int a[5]) {



}

int main() {



}


Note: When sending the array variable as an argument, no parentheses are needed!

Size element in array parameter the size element in the array parameter has

no consequence on the code


int sum(int a[5]) {



}

int main() {

int x[6] = {1,2,3,4,5,6};cout << "Sum = " << sum(x) << endl;

int y[7] = {5, 10, 15, 20, 25, 30, 35};cout << "Sum = " << sum(y) << endl;return 0;

}


Note: The size of these arrays are not 5, but 6 and 7.

Size element in array parameter the size element in the array parameter has

no consequence on the code in fact, the size of the array can be omitted

completely


int sum(int a[]) {



}

int main() {

int x[6] = {1,2,3,4,5,6};cout << "Sum = " << sum(x) << endl;

int y[7] = {5, 10, 15, 20, 25, 30, 35};cout << "Sum = " << sum(y) << endl;return 0;

}


Note: Size of array not specified!

The last example works fine, but is slightly limitedsuppose we want to add the first n numbers

in our array, where n != 5Solution:

build more functions? inefficient

send a second parameter!


int sum(int a[], int size) {

int result = 0;for (int i = 0; i < size; i++) {


}

int main() {

int x[6] = {1,2,3,4,5,6};cout << "Sum = " << sum(x, 6) << endl;

int y[7] = {5, 10, 15, 20, 25, 30, 35};cout << "Sum = " << sum(y, 7) << endl;return 0;

}


Note: Size of array specified as second parameter.

In C/C++, when a function takes an array as input, it very often takes in the size of that array as well exception: strings

Hence, the format of your functions with array parameters should typically look like previous example: int sum(int a[], int size) {



}

Arrays and Memoryarrays are passed BY REFERENCE

no copy of the array is madeonly the address of the array is passed no reference indicator (&) is required on the

parameterconsequently, all changes to the array made

in the function are permanent, not localExample: write a function that adds 5 to

each value of an array


void add5(int a[], int size) {

for (int i = 0; i < size; i++) { a[i] += 5;

}

}

int main() {

int x[3] = {0, 3, 6}; cout << x[0] << "," << x[1] << "," << x[2] << endl;

add5(x, 3); cout << x[0] << "," << x[1] << "," << x[2] << endl;

return 0;}

Example: write a function that adds 5 to each value of an array

Const Parameterssuppose I have an array of important

informatione.g. a list of student grades

suppose that I want to calculate the sum of all of those grades

e.g. for computing an averagewhat if I use the sum function?





}

int main() {

int marks = {87, 62, 49, 71, 55};

cout << "Average = " << sum(marks, 5) / 5.0 << endl;

return 0;}


Previous example works finebut what if you couldn't see the code for

sum?


/* this function computes the sum of the first size elements in the array a */int sum(int a[], int size);int main() {

int marks = {87, 62, 49, 71, 55};


return 0;}

// the code for sum is somewhere down here


Problem with the previous code remember that arrays are passed by reference this means that the function can change the values

in the array is this safe?

for information that you do not want changed (such as a list of grades), not really

how do we safeguard against this? Solution 1: check the source code of the called function


/* this function computes the sum of the first size elements in the array a */int sum(int a[], int size);int main() {

int marks = {87, 62, 49, 71, 55};


return 0;}

// the code for sum is somewhere down hereint sum(int a[], int size) {



}


Programmer has to manually check the sum code to make sure no changes to a occur.

Problems with this approach time-consuming

what if the function had 500 lines of codeerror-prone

what if programmer overlooked a linedifficult

sometimes, code is stored in a totally separate header file

Solution: const parameters prepend an array parameter declaration with the

keyword const

when an array parameter is declared const, then the function is forbidden from changing the values in that array

int sum(const int a[], int size) {



}

What happens if the function tries to change a value in a const array?

int sum(const int a[], int size) {


result += a[i]++;}return result;

}


/* this function computes the sum of the first size elements in the array a */int sum(const int a[], int size);int main() {

int marks = {87, 62, 49, 71, 55};


return 0;}

// the code for sum is somewhere down here


Programmer can call sum with total confidence that the array will not be altered, without even knowing what the source code of sum looks like.

When should you declare an array parameter const?whenever the function is not going to

change any values of the array two purposes:

1) guarantees other programmers using your function that no changes will occur

2) good error check for yourself if you accidentally write code that changes the array,

compiler will alert you

Example:write a function that returns the maximum

value of a set of integers


int biggest = a[0];

for (int i = 1; i < size; i++) if (a[i] > biggest) biggest = a[i];

return biggest;}

does this function change any values in a? no therefore, we should declare it

const

Example:write a function that returns the maximum

value of a set of integers

int maximum(const int a[], int size) {

int biggest = a[0];

for (int i = 1; i < size; i++) if (a[i] > biggest) biggest = a[i];

return biggest;}

does this function change any values in a? no therefore, we should declare it

const

Example:write a function that sets all negative values

to positive in a function

void sum(int a[], int size) {

for (int i = 0; i < size; i++) a[i] = abs(a[i]);

}

does this function change any values in a? yes therefore, we should leave it

as non-const

Const Reference Parameterswe can also declare const reference

parameters this guarantees that we will not make any

changes to the parameters in the calling function

this is useful when we want to send data to the function, but do not want to make copies

e.g. for strings

#include <iostream>#include <string> using namespace std;

void bio(string &name, string &city, string &province);

int main() {

string name = "Kevin"; string city = "Lethbridge"; string province = "Alberta";

bio(name, city, province);

return 0;

}

void bio(string &name, string &city, string &province) { cout << name << " currently lives in " << city << ", " << province << endl;}

write a function that takes a name, city, and province as inputs, and writes a short bio of the person.

No guarantee function will not alter strings, unless function code is checked.

#include <iostream>#include <string> using namespace std;

void bio(const string &name, const string &city, const string &province);

int main() {

string name = "Kevin"; string city = "Lethbridge"; string province = "Alberta";

bio(name, city, province);

return 0;

}

void bio(const string &name, const string &city, const string &province) { cout << name << " currently lives in " << city << ", " << province << endl;}

write a function that takes a name, city, and province as inputs, and writes a short bio of the person.

Function is not allowed to change the const values.

When should you declare reference parameters const?whenever the function is not going to

change their valuesconst reference parameters are often used

for large data typesstringsstructures/classes (stay tuned!!)

void swap(int &a, int &b) { int temp = b; b = a; a = temp;} does this function change

its reference parameters? yes therefore, they cannot be

const

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