The Java Learning Project

I started learning Java programming and found a perfect website for practicing: Codingbat.com. CodingBat is a free site of live coding problems to build coding skill in Java and Python. CodingBat is a project by Nick Parlante, a computer science lecturer at Stanford.

I will upload my solutions here.

Given a string, if one or both of the first 2 chars is 'x', return the string without those 'x' chars, and otherwise return the string unchanged. This is a little harder than it looks.

withoutX2("xHi") → "Hi"
withoutX2("Hxi") → "Hi"
withoutX2("Hi") → "Hi"

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public String withoutX2(String str) {
  String result="";
  for (int i=0; i<=str.length()-1;i++){
    if ( !(i==0 || i==1) || !(str.substring(i,i+1).equals("x")) ){
      result=result+str.substring(i,i+1);
    }
  }return result;
}

Given a string of any length, return a new string where the last 2 chars, if present, are swapped, so "coding" yields "codign".

lastTwo("coding") → "codign"
lastTwo("cat") → "cta"
lastTwo("ab") → "ba"

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public String lastTwo(String str) {
  String result="";
  if (str.length()>=2){
    result=str.substring(0,str.length()-2)+str.substring(str.length()-1,str.length())+str.substring(str.length()-2,str.length()-1);
  }else{
    result=str;
  }return result;
}

Given two strings, append them together (known as "concatenation") and return the result. However, if the concatenation creates a double-char, then omit one of the chars, so "abc" and "cat" yields "abcat".

conCat("abc", "cat") → "abcat"
conCat("dog", "cat") → "dogcat"
conCat("abc", "") → "abc"

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public String conCat(String a, String b) {
  if ((a.length()!=0) && (b.length()!=0) && ((a.substring(a.length()-1,a.length())).equals(b.substring(0,1)))){
    return a+b.substring(1,b.length());
  }
  return a+b;
}

Given an array of ints, we'll say that a triple is a value appearing 3 times in a row in the array. Return true if the array does not contain any triples.

noTriples([1, 1, 2, 2, 1]) → true
noTriples([1, 1, 2, 2, 2, 1]) → false
noTriples([1, 1, 1, 2, 2, 2, 1]) → false

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public boolean noTriples(int[] nums) {
  boolean triples=true;
  for (int i=0;i<=nums.length;i++){
    if ((i<=nums.length-3) && (nums[i]==nums[i+1]) && (nums[i]==nums[i+2])){
      triples=false;
    } 
  }return triples;
}

Given an array of ints, return the number of times that two 6's are next to each other in the array. Also count instances where the second "6" is actually a 7. 

array667([6, 6, 2]) → 1
array667([6, 6, 2, 6]) → 1
array667([6, 7, 2, 6]) → 1

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public int array667(int[] nums) {
  int count=0;
  for (int i=0; i<nums.length-1;i++){
    if ((nums[i]==6) && ((nums[i+1]==6) || (nums[i+1]==7))){
      count++;
    }
  }return count;
}

Suppose the string "yak" is unlucky. Given a string, return a version where all the "yak" are removed, but the "a" can be any char. The "yak" strings will not overlap.

stringYak("yakpak") → "pak"
stringYak("pakyak") → "pak"
stringYak("yak123ya") → "123ya"

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public String stringYak(String str) {
         String result="";
        for (int i=0; i<=str.length()-1; i++){
            if ((i<=str.length()-3) && ((str.substring(i,i+1)).equals("y")) && ((str.substring(i+2,i+3)).equals("k"))) {
                i=i+2;
            }else{
                result=result+str.substring(i,i+1);
            }
        }return result;
}
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