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Announcements. Mid-term exam II on 9 th October 0930 to 1030 Seating arrangement: Y4, Y5, Y7001 to Y7085 L1 OROS Y7092 to Y7233 L2 ERES Y7234 to Y7388 L16 OROS Y7391 to Y7518 L17 ERES Please take seats 15 minutes before the scheduled time. Strings. Instructor: Mainak Chaudhuri

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announcements
Announcements
  • Mid-term exam II on 9th October 0930 to 1030
  • Seating arrangement:

Y4, Y5, Y7001 to Y7085 L1 OROS

Y7092 to Y7233 L2 ERES

Y7234 to Y7388 L16 OROS

Y7391 to Y7518 L17 ERES

  • Please take seats 15 minutes before the scheduled time
strings

Strings

Instructor: Mainak Chaudhuri

mainakc@cse.iitk.ac.in

strings1
Strings
  • Can view as array of chartacters
    • Implemented as a class with its own methods
  • Simplest operation on strings is catenating two strings

String x = “Good”;

String y = “Morning!”;

String z = x + “ ” + y;

String w = x + ‘ ’ + y; // also works

  • Possible to concatenate strings to variables of other types e.g., int, float, etc.
    • Internally converted to string
methods of string class
Methods of string class
  • Determining the length of a string

String x = “abc”;

int lengthOfx = x.length();

  • Determining the character at a position

String x = “abc”;

char cAt2 = x.charAt(2);

char first = x.charAt(0);

char last = x.charAt(x.length()-1);

  • Case conversion

String x = “Mainak”;

String y = x.toUpperCase(); // y is “MAINAK”

String z = x.toLowerCase(); // z is “mainak”

more on concatenation
More on concatenation
  • Concatenating strings is different from concatenating characters

String x = “a”;

String y = “b”;

String z = x + y; // z is “ab”

char x1 = ‘a’;

char y1 = ‘b’;

char z1 = x1 + y1; // z is not “ab”

  • For concatenating two strings you can use the concat method also

String z = “to”.concat(“get”).concat(“her”);

extracting substrings
Extracting substrings
  • Can extract the substring starting at a position

String x = “together”;

String y = x.substring(2); // y is “gether”

String z = x.substring(3); // z is “ether”

String w = x.substring(0); // same as x

String u = x.substring(x.length()-1); // “r”

String v = x.substring(x.length()); // blank

  • Can extract substring between two positions

String y = x.substring(0, 5); // y is “toget”

String z = x.substring(5, x.length()); // “her”

String w = x.substring(5, 6); // w is “h”

String u = x.substring(5, 5); // u is “”

searching in a string
Searching in a string
  • Finding the first occurrence of a character or a substring within a string

String x = “abracadabra”;

int k = x.indexOf(‘a’); // k is 0

int p = x.indexOf(‘a’, 1); // p is 3; search

// begins from pos 1

int t = x.indexOf(‘e’); // t is -1

int q = x.indexOf(“ra”); // q is 2

int s = x.indexOf(“ra”, 3); // s is 9

  • Possible to find the last occurrence also

int p = x.lastIndexOf(‘r’); // p is 9

comparison of strings
Comparison of strings
  • Compares strings in dictionary order (also known as lexicographical order)
    • Returns zero if strings are equal

String x = “abc”;

String y = “abcd”;

String z = “ab”;

String w = “abd”;

int p = x.compareTo(y); // p is negative

int q = x.compareTo(x); // q is zero

int r = x.compareTo(z); // r is positive

int s = y.compareTo(w); // s is negative

  • This comparison is case sensitive
    • Use compareToIgnoreCase otherwise
some more useful methods
Some more useful methods
  • Removing leading and trailing whitespaces

String x = “ abc ”;

String y = x.trim(); // y is “abc”

  • Test for prefix

String x = “Mainak”;

boolean y = x.startsWith(“Main”); // y is true

boolean z = x.startsWith(“nak”, 3); // z is true

  • Test for suffix

String x = “Canada”;

boolean y = x.endsWith(“ada”); // y is true

some more useful methods1
Some more useful methods
  • Substitute all occurrences of a character with another character

String x = “deer”;

String y = x.replace(‘e’, ‘o’); // y is “door”

String z = x.replace(‘a’, ‘o’); // z is “deer”

  • Partial string match

String x = “abracadabra”;

boolean y = x.regionMatches(true, 2, “bracket”, 1, 3); // y is true

    • The first argument should be set to true if the match is intended to be case ignorant
converting string to integer
Converting string to integer

class StringToInt {

public static void main (String arg[]) {

// Assume arg[0] is the input string

int result = 0, pos;

int len = arg[0].length();

char c;

for (pos = 0; pos < len; pos++) {

c = arg[0].charAt(len-pos-1);

if ((c >= ‘0’) && (c <= ‘9’)) {

result += ((c-’0’)*Math.pow(10, pos));

}

// continued in next slide

converting string to integer1
Converting string to integer

else if ((c==‘-’) && (pos==len-1)) {

result = -result;

}

else {

System.out.println(“Invalid input: ” + arg[0]);

break;

}

} // end for

if (pos==len) {

System.out.println(“Integer value: ” + result);

}

}

}

reversing a string
Reversing a string

class StringReverse {

public static void main (String arg[]) {

// Assume that the input is arg[0]

String reversed = “”;

int pos;

for (pos=arg[0].length()-1; pos >= 0; pos--) {

reversed += arg[0].charAt(pos);

}

System.out.println (“Original: ” + arg[0] + “, Reversed: ” + reversed);

}

}

sorting a list of names
Sorting a list of names

class NaiveDictionarySort {

public static void main (String arg[]) {

// Assume that the names are in arg

int n = arg.length; // list size

String sortedList[] = new String[n];

boolean indexArray[] = new boolean[n];

int k, j, runningIndex=0, minIndex;

for (k=0; k<n; k++) {

indexArray[k] = false;

}

// continued on next slide

sorting a list of names1
Sorting a list of names

while (runningIndex < n) {

for (k=0; k<n; k++) {

if (!indexArray[k]) break;

}

sortedList[runningIndex] = arg[k];

minIndex = k;

for (j=k+1; j<n; j++) {

if (indexArray[j]) continue;

if (sortedList[runningIndex].compareTo(arg[j]) > 0) {

sortedList[runningIndex] = arg[j];

minIndex = j;

}

}

// continued on next slide

sorting a list of names2
Sorting a list of names

indexArray[minIndex] = true;

runningIndex++;

}

System.out.println (“Sorted list:”);

for (k=0; k<n; k++) {

System.out.println (sortedList[k]);

}

}

}