Strings

Member functions

Member function name	Description
`s.append(str)`	add text to the end of a string
`s.compare(str)`	return <0, 0, or >0 depending on relative ordering
`s.erase(index, length)`	delete text from a string starting at given `index`
`s.find(str), s.rfind(str)`	first or last index where the start of str appears in this string (returns string::npos if not found).
`s.insert(index, str)`	add text into a string at a given index.
`s.length() or s.size()`	number of characters in this string.
`s.replace(index, len, str)`	replaces len chars at given index with new text.
`s.substr(start, length) or s.substr(start)`	the next `length` characters beginning at start (inclusive); if length omitted, graps till end of string.

Example:

string name = "Thomas Edison";
if (name.find("Edi") != string::npos) {
  name.erase(7, 6); //"Thomas" remains
}

Operators

Concatenate using + or +=:

string s1 = "Nik";
s1 += "ola"; //Nikola

Compare using relational operators (ASCII ordering):

string s2 = "Tesla";  // ==, !=, <, <=, >, >=
if (s1 < s2 && s2 != "abc") { //true
  ...
}

Strings are "mutable", i.e., they can be changed!

s1.append(" Tesla"); //"Nikola Tesla"
s1.erase(3, 3);  //"Nik Tesla"
s1[8] = '@';  //"Nik Tesl@"

Stanford library for strings

function name	Description
`endsWith(str, suffix), startsWith(str, suffix)`	true if str ends or begins with the given text suffix
`integerToString(int), realToString(double), stringToInteger(str), stringToReal(str)`	convert between numbers and strings
`equalsIgnoreCase(s1, s2)`	true if s1 and s2 have same chars, ignoring casing
`toLowerCase(str), toUpperCase(str)`	returns an upper/lowercase version of a string.
`trim(str)`	return string with surrounding whitespace removed.

Example:

if (startsWith(name, "Mr. ")) {
  name += integerToString(age) + " years old";
}

Exercise: implement endsWith() using rfind() and length().

Exercise: implement endsWith() using substr() and length().

Exercise: what's the output?

void mystery(string a, string &b)
{
  a.erase(0, 1);   // erase 1 from index 0
  b += a[0];
  b.insert(3, "FOO");  //insert at index 3
}

int main()
{
  string a = "nikola";
  string b = "tesla";
  mystery(a, b);
  cout << a << " " << b << endl;
  return 0;
}

What is printed?

nikola tesFOOlai

String exercise: write a function nameDiamond that accepts a string parameter and prints its letters in a "diamond" format as shown below. For example, nameDiamond("TESLA") should print:

T
TE
TES
TESL
TESLA
 ESLA
  SLA
   LA
    A

Solution

void nameDiamond(string s)
{
  int len = s.length();
  //print top half of diamond
  for (int i = 1; i <= len; i++) {
    cout << s.substr(0, i) << endl;
  }
  //print bottom half of diamond
  for (int i = 1; i < len; i++) {
    for (int j = 0; j < i; j++) { //indent
      cout << " "; // with spaces
    }
    cout << s.substr(i, len - i) << endl;
  }
}

Another solution

void nameDiamond(string s)
{
  int len = s.length();
  //print top half of diamond
  for (int i = 1; i <= len; i++) {
    cout << s.substr(0, i) << endl;
  }
  //print bottom half of diamond
  for (int i = 1; i < len; i++) {
    s.replace(i, 1, " ");
    cout << s.substr(i, len - i) << endl;
  }
}

Question: Will this solution work if the parameter s was pass-by-reference? What if s was a const reference parameter?

C vs. C++ strings

C++ has two kinds of strings: C strings (char arrays) and C++ strings (String objects).
A string literal such as "hi there" is a C string!
- C strings don't include any methods/behaviour shown previously. e.g., no member functions like length, find, or operators.
Converting between the two types:
- string("text") : C string to C++ string
- string.c_str() : C++ string to C string

C string bugs

string s = "hi" + "there"; //C-string + C-string
string s = "hi" + '?'; //C-string + char
string s = "hi" + 41; //C-string + int

C strings can't be concatenated with +. C-string + char/int produces garbage, not "hi?" or "hi41". The bug usually manifests in print statements and you will see partial strings.

string s = "hi"; s += 41;
- Adds character with ASCII value 41 ')', doesn't produce "hit41"
int n = (int) "42"; //n = 0x7ffdcb08
- Bug; sets n to the memory address of the C string "42"!!

C string bugs fixed

string s = string("hi") + "there"; //C-string + C-string

or:

string s = "hi";  //convert to C++ string
s += "there";

These both compile and work properly.

Similarly:

string s = "hi";  //convert to C++ string
s += '?';

Works because of auto-conversion from C string to C++ string in string a = ....

For appending integers to strings:

s += integerToString(41); //"hi?41"

Another example:

int n = stringToInteger("42"); //42

Explicit string-int conversion using Stanford library.