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Gator Engineering

Google Code Jam 2015

Gator Engineering

Rules

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Sample questions

•P1: Reverse words

–Given a list of space separated words, reverse the orderof the words. Each line of text contains L lettersand W words. A line will only consist of letters andspace characters. There will be exactly one spacecharacter between each pair of consecutive words.

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P1: Reverse Words

•Input

–The first line of input gives the number of cases, N.N test cases follow. For each test case there will a lineof letters and space characters indicating a list of spaceseparated words. Spaces will not appear at the start orend of a line.

•Output

–For each test case, output one line containing "Case #x:" followed by the list of words in reverse order.

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P1: Reverse Words

•Limits

–Small dataset

–N = 51 ≤ L ≤ 25

•Large dataset

–N = 1001 ≤ L ≤ 1000

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P1: Reverse Words

•Sample

Input

Output

3this is a testfoobarall your base

Case #1: test a is thisCase #2: foobarCase #3: base your all

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P2: T9 Spelling

•Problem

–The Latin alphabet contains 26 characters andtelephones only have ten digits on the keypad. Wewould like to make it easier to write a message toyour friend using a sequence of keypresses toindicate the desired characters. The letters aremapped onto the digits as shown below. To insertthe character B for instance, the program wouldpress 22. In order to insert two characters insequence from the same key, the user must pausebefore pressing the key a second time. The spacecharacter ' ' should be printed to indicate a pause. Forexample, 2 2 indicates AA whereas 22 indicates B.

https://code.google.com/codejam/contest/images/?image=keypad.png&p=379101&c=351101

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P2: T9 Spelling

•Input

–The first line of input gives the number of cases, N. Ntest cases follow. Each case is a line of text formattedas desired_message

–Each message will consist of only lowercase charactersa-z and space characters ' '. Pressing zero emits a space.

•Output

–For each test case, output one line containing "Case #x:" followed by the message translated into the sequenceof keypresses.

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P2: T9 Spelling

•Limits

–1 ≤ N ≤ 100.

•Small dataset

–1 ≤ length of message in characters ≤ 15.

•Large dataset

–1 ≤ length of message in characters ≤ 1000.

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P2: T9 Spelling

•Sample

Input

Output

4hiyesfoo barhello world

Case #1: 44 444Case #2: 999337777Case #3: 333666 6660 022 2777Case #4: 4433555555666096667775553

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Chapter 12

Pointers and Arrays (Continued)

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Using an Array Name as a Pointer

•Pointer arithmetic is one way in which arrays andpointers are related.

•Another key relationship:

The name of an array can be used as a pointer tothe first element in the array.

•This relationship simplifies pointer arithmetic andmakes both arrays and pointers more versatile.

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Using an Array Name as a Pointer

•Suppose that a is declared as follows:

int a[10];

•Examples of using a as a pointer:

*a = 7; /* stores 7 in a[0] */

*(a+1) = 12; /* stores 12 in a[1] */

•In general, a + i is the same as &a[i].

–Both represent a pointer to element i of a.

•Also, *(a+i) is equivalent to a[i].

–Both represent element i itself.

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Using an Array Name as a Pointer

•The fact that an array name can serve as a pointermakes it easier to write loops that step through anarray.

•Original loop:

for (p = &a[0]; p < &a[N]; p++)

sum += *p;

•Simplified version:

for (p = a; p < a + N; p++)

sum += *p;

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Using an Array Name as a Pointer

•Although an array name can be used as a pointer,it’s not possible to assign it a new value.

•Attempting to make it point elsewhere is an error:

while (*a != 0)

a++; /*** WRONG ***/

•This is no great loss; we can always copy a into apointer variable, then change the pointer variable:

p = a;

while (*p != 0)

p++;

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Array Arguments (Revisited)

•When passed to a function, an array name is treated as a pointer.

•Example:

int find_largest(int a[], int n)

{

int i, max;

max = a[0];

for (i = 1; i < n; i++)

if (a[i] > max)

max = a[i];

return max;

}

•A call of find_largest:

largest = find_largest(b, N);

This call causes a pointer to the first element of b to be assignedto a; the array itself isn’t copied.

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Array Arguments (Revisited)

•The fact that an array argument is treated as apointer has some important consequences.

•Consequence 1: When an ordinary variable ispassed to a function, its value is copied; anychanges to the corresponding parameter don’taffect the variable.

•In contrast, an array used as an argument isn’tprotected against change.

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Array Arguments (Revisited)

•For example, the following function modifies anarray by storing zero into each of its elements:

void store_zeros(int a[], int n)

{

int i;

for (i = 0; i < n; i++)

a[i] = 0;

}

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Array Arguments (Revisited)

•To indicate that an array parameter won’t bechanged, we can include the word const in itsdeclaration:

int find_largest(const int a[], int n)

{

…

}

•If const is present, the compiler will check thatno assignment to an element of a appears in thebody of find_largest.

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Array Arguments (Revisited)

•Consequence 2: The time required to pass an arrayto a function doesn’t depend on the size of thearray.

•There’s no penalty for passing a large array, sinceno copy of the array is made.

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Array Arguments (Revisited)

•Consequence 3: An array parameter can bedeclared as a pointer if desired.

•find_largest could be defined as follows:

int find_largest(int *a, int n)

{

…

}

•Declaring a to be a pointer is equivalent todeclaring it to be an array; the compiler treats thedeclarations as though they were identical.

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Array Arguments (Revisited)

•Although declaring a parameter to be an array isthe same as declaring it to be a pointer, the sameisn’t true for a variable.

•The following declaration causes the compiler toset aside space for 10 integers:

int a[10];

•The following declaration causes the compiler toallocate space for a pointer variable:

int *a;

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Array Arguments (Revisited)

•In the latter case, a is not an array; attempting touse it as an array can have disastrous results.

•For example, the assignment

*a = 0; /*** WRONG ***/

will store 0 where a is pointing.

•Since we don’t know where a is pointing, theeffect on the program is undefined.

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Array Arguments (Revisited)

•Consequence 4: A function with an arrayparameter can be passed an array “slice”—asequence of consecutive elements.

•An example that applies find_largest toelements 5 through 14 of an array b:

largest = find_largest(&b[5], 10);

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Using a Pointer as an Array Name

•C allows us to subscript a pointer as though itwere an array name:

#define N 10

…

int a[N], i, sum = 0, *p = a;

…

for (i = 0; i < N; i++)

sum += p[i];

The compiler treats p[i] as *(p+i).