slides-09-02.qmd

---
title: "files and strings (slides)"
format: revealjs
slide-number: true
df-print: kable
---

# CSc 110 - files and strings


## String methods

-   `string.strip(chars)` -- removes any of the characters in chars from the beginning or end of string, returns a string
-   `string.split(chars)` -- splits string at the chars, returns a list
-   `string.lower(string)` -- forces all characters to lowercase, returns a string

## Write a function

Its name is `sum_all` that takes a string `filename` as argument.
It reads the file with `filename` and iterates over each line in [the file](data/numbers.txt), stripping the line breaks, casting each line as integer (assume each line is a single number), summing each number to a total variable. Return the total.

Name your file `sum_numbers.py` and submit to Gradescope.


```{python}
#| echo: true
#| eval: false
assert sum_all("numbers.txt") == 15
```

## Write a function -- solution

```{python}
#| echo: true
#| eval: true
def sum_all(file_name):
  total = 0
  for line in open(file_name, "r"):
    number = int(line.strip("\n"))
    total += number
  return total

def main():
  assert sum_all("numbers.txt") == 15

main()
```


## Modes

-   To read a file:

    -   Use **'r'** for reading the contents of a file

-   To write to a file: use

    -   Use **'a'** to append to the existing file content

    -   Use **'w'** to write to a file, and replace existing content

## Writing to a file

-   After you have opened the file in either **'a'** or **'w'** mode

    ```         
    a_file = open(file_name, mode)
    ```

-   Use the **write** function to write text content to the file

    ```         
    a_file.write('put this content in a file')
    ```

-   When finished writing, **close** the file **a_file.close()**

## Create a file

words.txt

```{html}
Let us start a story.
```

## Evaluate the code

```{python}
#| echo: true
#| eval: true
words = open('words.txt', 'w')
words.write('The slow wolf')
words.write('jumped over')
words.write('the bear')
words.close()
```

## Evaluate the code

```{python}
#| echo: true
#| eval: true
words = open('words.txt', 'w')
words.write('The slow wolf')
words.write('jumped over')
words.write('the bear')
words.close()
```

words.txt

```{html}
The slow wolfjumped overthe bear
```

## Evaluate the code

```{python}
#| echo: true
#| eval: true
words = open('words.txt', 'w')
words.write('The slow wolf\n')
words.write('jumped over\n')
words.write('the bear')
words.close()
```

## Evaluate the code

```{python}
#| echo: true
#| eval: true
words = open('words.txt', 'w')
words.write('The slow wolf\n')
words.write('jumped over\n')
words.write('the bear')
words.close()
```

words.txt

```{html}
The slow wolf
jumped over
the bear
```

## Evaluate the code -- continued

```{python}
#| echo: true
#| eval: true
words = open('words.txt', 'a')
words.write(' The quick fox\n')
words.write('jumped over\n')
words.write('the bear')
words.close()
```

## Evaluate the code -- continued

```{python}
#| echo: true
#| eval: true
words = open('words.txt', 'a')
words.write(' The quick fox\n')
words.write('jumped over\n')
words.write('the bear')
words.close()
```

words.txt

```{html}
The slow wolf
jumped over
the bear The quick fox
jumped over
the bear
```

## Combining dictionary with write

```{python}
#| eval: false
#| echo: true
info = {"a": 2, "b": 5, "c": 0}
output = open("count.txt", "w")

for key, value in info.items():
  output.write(key + ", " + str(value) + "\n")
output.write("end")

output.close()
```

count.txt

```{html}
a, 2
b, 5
c, 0
end   
```

## Write two functions

*   Both functions takes a string `file_name` as argument: 
    *  `count_words` count frequency of each lowercase word, return a dictionary.
    *  `write_word_count` call `count_words`, iterate it and write a comma separated file ("out_" + `file_name`). 
*   [Download test file](data/alien.txt)

Name file as `read_and_write.py`, submit for attendance.

Test case:

```{python}
#| eval: false
#| echo: true
write_word_count("alien.txt")
# writes out out_alien.txt with word counts
```

## Write two functions -- solution

```{python}
#| eval: true
#| echo: true
def count_words(file_name):
  f = open(file_name, "r")
  counts = {}
  for line in f:
    words = line.strip("\n").split(" ")
    for w in words:
      lower_case_w = w.lower()
      if lower_case_w not in counts:
        counts[lower_case_w] = 1
      else:
        counts[lower_case_w] += 1
  return counts

def write_word_count(file_name):
  count_dict = count_words(file_name)
  output_file = open("out_" + file_name, "w")
  for key, value in count_dict.items():
    output_file.write(key + "," + str(value) + "\n")
  output_file.close()
  
def main(): 
  write_word_count("alien.txt")
  
main()
```