# Compute the Number of Times a Pattern Appears in a Text solved by 1955

Sept. 9, 2015, 3:38 a.m. by Rosalind Team

This is the first problem in a collection of "code challenges" to accompany Bioinformatics Algorithms: An Active-Learning Approach by Phillip Compeau & Pavel Pevzner.

A k-mer is a string of length k. We define Count(Text, Pattern) as the number of times that a k-mer Pattern appears as a substring of Text. For example,

$\textit{Count}(\text{ACA}\color{green}\textbf{ACTAT}\color{black}\text{GCAT}\color{green}\textbf{ACTAT}\color{black}\text{CGGGA}\color{green}\textbf{ACTAT}\color{black}\text{CCT}, {\color{green}\textbf{ACTAT}}) = 3$.

We note that Count($\text{CG}\color{green}\textbf{ATATA}\color{black}\text{TCC}\color{green}\textbf{ATA}\color{black}\text{G}$, $\color{green}\textbf{ATA}$) is equal to 3 (not 2) since we should account for overlapping occurrences of Pattern in Text.

To compute Count(Text, Pattern), our plan is to “slide a window” down Text, checking whether each k-mer substring of Text matches Pattern. We will therefore refer to the k-mer starting at position i of Text as Text(i, k). Throughout this book, we will often use 0-based indexing, meaning that we count starting at 0 instead of 1. In this case, Text begins at position 0 and ends at position |Text| − 1 (|Text| denotes the number of symbols in Text). For example, if Text = GACCATACTG, then Text(4, 3) = ATA. Note that the last k-mer of Text begins at position |Text| − k, e.g., the last 3-mer of GACCATACTG starts at position 10 − 3 = 7. This discussion results in the following pseudocode for computing Count(Text, Pattern).

    PatternCount(Text, Pattern)
count ← 0
for i ← 0 to |Text| − |Pattern|
if Text(i, |Pattern|) = Pattern
count ← count + 1        return count


## Implement PatternCount

Given: {DNA strings}} Text and Pattern.

Return: Count(Text, Pattern).

## Sample Dataset

GCGCG
GCG


## Sample Output

2