CS253 DRY

Staying DRY with references

☔

Using references

As we all know, a reference creates an alias for another thing. For example:

int a = 42;
int &r = a;
cout << "r=" << r << " a=" << a << '\n';
a = 99;
cout << "r=" << r << " a=" << a << '\n';
r = 56;
cout << "r=" << r << " a=" << a << '\n';

r=42 a=42
r=99 a=99
r=56 a=56

Loops

Let’s say that we want to change all the fives in this vector to negative fives:

vector<int> values = {3,1,4,1,5,9,2,6,5,3,5};
for (size_t i=0; i<values.size(); i++)
    if (values[i] == 5)
        values[i] = -5;
for (size_t i=0; i<values.size(); i++)
    cout << values[i] << ' ';

3 1 4 1 -5 9 2 6 -5 3 -5

That works, but it’s a lot of repetition—quite a few instances of values[i].

Better loops

Let’s use a more modern loop to write out the numbers:

vector<int> values = {3,1,4,1,5,9,2,6,5,3,5};
for (size_t i=0; i<values.size(); i++)
    if (values[i] == 5)
        values[i] = -5;
for (int v : values)
    cout << v << ' ';

3 1 4 1 -5 9 2 6 -5 3 -5

Hooray! One fewer values[i]!

Betterer loops

Let’s use that same technique to change 5 to −5:

vector<int> values = {3,1,4,1,5,9,2,6,5,3,5};
for (int v : values)
    if (v == 5)
        v = -5;
for (int v : values)
    cout << v << ' ';

3 1 4 1 5 9 2 6 5 3 5

That didn’t work. Why?

Best loops

REFERENCES!

vector<int> values = {3,1,4,1,5,9,2,6,5,3,5};
for (int &v : values)
    if (v == 5)
        v = -5;
for (int v : values)
    cout << v << ' ';

3 1 4 1 -5 9 2 6 -5 3 -5

I need &v in the first loop, because I’m changing the number. I’m not changing anything in the second loop, so a reference isn’t required.

Ducks

Consider this file:

% cat ~cs253/pub/ducks
Huey (red)
Dewey (blue)
Louie (green)

I want code that reads that file into a vector<string>, and converts the vowels to asterisks, so it looks like censorship.

Attempt #1

ifstream in("/s/bach/a/class/cs253/pub/ducks");
vector<string> ducks;
string line;
while (getline(in, line))
    ducks.push_back(line);
for (size_t i=0; i<ducks.size(); i++)
    for (size_t j=0; j<ducks[i].size(); j++)
        if (ducks[i][j]=='a' || ducks[i][j]=='e' ||
            ducks[i][j]=='i' || ducks[i][j]=='o' ||
            ducks[i][j]=='u')
            ducks[i][j] = '*';
for (size_t i=0; i<ducks.size(); i++)
    cout << ducks[i] << '\n';

H**y (r*d)
D*w*y (bl**)
L**** (gr**n)

That was horrible. So soon, we’ve forgotten all that we’ve learned.

Attempt #2

ifstream in("/s/bach/a/class/cs253/pub/ducks");
vector<string> ducks;
string line;
while (getline(in, line))
    ducks.push_back(line);
for (string &s : ducks)
    for (size_t j=0; j<s.size(); j++)
        if (s[j]=='a' || s[j]=='e' ||
            s[j]=='i' || s[j]=='o' ||
            s[j]=='u')
            s[j] = '*';
for (string s : ducks)
    cout << s << '\n';

H**y (r*d)
D*w*y (bl**)
L**** (gr**n)

Better, but still awful.

Attempt #3

ifstream in("/s/bach/a/class/cs253/pub/ducks");
vector<string> ducks;
string line;
while (getline(in, line))
    ducks.push_back(line);
for (string &s : ducks)
    for (char &c : s)
        if (c=='a' || c=='e' || c=='i' || c=='o' || c=='u')
            c = '*';
for (string s : ducks)
    cout << s << '\n';

H**y (r*d)
D*w*y (bl**)
L**** (gr**n)

Finally, it’s DRY! Still, we’re copying strings in the last loop.

Attempt #4

ifstream in("/s/bach/a/class/cs253/pub/ducks");
vector<string> ducks;
string line;
while (getline(in, line))
    ducks.push_back(line);
for (string &s : ducks)
    for (char &c : s)
        if (c=='a' || c=='e' || c=='i' || c=='o' || c=='u')
            c = '*';
for (const string &s : ducks)
    cout << s << '\n';

H**y (r*d)
D*w*y (bl**)
L**** (gr**n)

DRY and efficient.

Attempt #5

ifstream in("/s/bach/a/class/cs253/pub/ducks");
vector<string> ducks;
string line;
while (getline(in, line))
    ducks.push_back(line);
for (string &s : ducks)
    for (size_t p=0; (p = s.find_first_of("aeiou",p)) != s.npos; p++)
        s[p] = '*';
for (const string &s : ducks)
    cout << s << '\n';

H**y (r*d)
D*w*y (bl**)
L**** (gr**n)

A different approach.

CS253: Software Development with C++

Fall 2018

DRY