C++ strings / arrays

c++_arrays_strings.md

Arrays

Strings

• Strings are represented by arrays of characters in this course
  • also with the String class, but not in this class
• size of array must be specified at time of creation (constant or literal)
  • must be known at compile time, can not be a variable brought in by cin, etc.
  • can copy elements into a bigger array
  • must settle on a max length (defined array size)
  • wherever the string ends, there will be a null character \0
  • must create an array that is +1 longer than desired max
• NULL / \0 character happens to coincide with 0 in the ASCII system
• ex: char str[5]
  • an array called str that holds 5 characters
  • on the stack, not in the heap, will get recycled when the current function exists
• can read in a string, but is dangerous
  • cin >> str
  • will skip leading whitespace
  • will continue reading until a whitespace is hit, then stops reading and enters \0
  • whitespace is left in the stream
• buffer overflow error
  • if we read in a string that is larger than our 5 character limit
  • will place the following overflowing characters in another place in memory
  • can possibly overwrite another variable
  • no error is generated!
• no array bounds checked
  • will let you check for things in memory beyond what you allocated for the array
  • str[7] may return something belonging to some other else!

cin

• use cin.get(arrayToPlaceInto, arraySize, characterToStopAt) instead
  • does not read in the characterToStopAt, have to manually eat it
  • will safely read in a certain number of characters to arraySize - 1 (so there is space for the null character \0)
  • ex: cin.get(str, 5, '\n')
    • will stop at and leave /n in the stream, will have to deal with that in the buffer
  • will assume \n if third parameter is left out, not required in function
  • cin.get() will get a single character and consume
    • equivalent to cin >>
• use cin.peek() to look at the next character in the buffer without consuming characters from buffer
• cin.get() is lower level, does not ignore whitespace (will stop at not consume) cin >> does (consumes)
• homework: cin.get() first character, cin.peek() at second character for '+' / '-'

flushing the stream

• while(cin.get() != '\n') {} will read in all characters until a newline is reached
  • but also reads in the newline (consumes)
• displaying strings is not as difficult
• cout << will print out characters continuously until a \0 is reached
  • if does not exist, will start to read out random memory
  • very dangerous!

string operators

• very few operations can be preformed on arrays of characters
• cannot compare
  • an array is the address is the first element in memory (a memory address)
• == or != simply be comparing memory locations
  • even in contents are the same
• strcmp(firstArray, secondArray) compares strings
  • 0 if equal
  • < 0 if first is less than second

  • 0 if first is greater than second

  • greater and less means total of character ASCII values
  • always compare in this manner: if (strcmp(srt1, str1) < 0)
• strcpy(str1, str2)
  • copies values in memory over, creates a new string in memory
  • will copy until the first \0 is hit

working with char arrays

• use for-loops as an alternative to cin.get()
• requires placing \0 manually
• must define a max character limit for the loop to not go past

I/O, conditionals, loops, arrays

conditionals

• may use any kind of value in the expression
  • will check for zero or nonzero (checking the numeric value)

helpers

• tolower() comes from the <ctype> library
• toupper() comes from the <std> library

loops

• prefix ++i instead of postfix i++