reveal.js

# C Wrap Up

---

CS 130 // 2024-12-05

# Review

## struct and typedef

```c
#include <stdio.h>
#include <math.h>

typedef struct {
    double x;
    double y;
} POINT;

double distance(POINT p1, POINT p2)
{
    double xdiff = p1.x - p2.x;
    double ydiff = p1.y - p2.y;
    return sqrt(xdiff * xdiff + ydiff * ydiff);
}

int main()
{
    POINT p1 = {.x=4,.y=5};
    POINT p2 = {.x=0,.y=0};
    printf("Distance: %f\n",distance(p1,p2));
    return 0;
}
```

## Pointers to Structs
- Pointers to structs are so common that there is a special operator to dereference and extract a field simultaneously
    ```c
    POINT p = {.x = 4, .y = 5};
    POINT *pp = &p;
    int x = pp->x;   // same as (*pp).x
    int y = pp->y;   // same as (*pp).y
    ```

## Exercise from last time
- Suppose we'd like to parse a table of integers into a 2D array in C
- 
 Let's assume the file format is as follows:
    ```text
    4/4
    0,1,2,3
    4,5,6,7
    8,9,10,11
    12,13,14,15
    ```
- 
 First two numbers are the width/height of the table

## Progress from 11:00 class

This only works if there are always 4 columns

```c
#include <stdio.h>
  
int main()
{
    FILE *fp = fopen("my_file.txt", "r");
    char buffer[80];
    int rows = 0;
    int cols = 0;

int total = 0;

fgets(buffer, 80, fp);
    sscanf(buffer,"%d/%d",&rows,&cols);
    printf("%d %d\n",rows,cols);

while (fgets(buffer, 80, fp) != NULL) {
        // buffer contains the current line here
        //printf("%s",buffer);

int w, x, y, z;
        sscanf(buffer,"%d,%d,%d,%d",&w,&x,&y,&z);
        total += w+x+y+z;
    }
    fclose(fp);

printf("Total: %d\n",total);

return 0;
}
```

## Progress from 12:30 class

This reads the first item from each row four times
* it doesn't consume characters from the buffer as you scan

```c
#include <stdio.h>

int main()
{
    FILE *fp = fopen("my_file.txt", "r");
    char buffer[80];
    int rows = 0;
    int cols = 0;
    int total = 0;

fgets(buffer, 80, fp);
    sscanf(buffer,"%d/%d",&rows,&cols);
    printf("%d rows and %d columns\n",rows,cols);

while (fgets(buffer, 80, fp) != NULL) {
        // buffer contains the current line here
        //printf("%s",buffer);

for(int c = 0; c < cols; c++)
        {
            int curr_int;
            sscanf(buffer,"%d,",&curr_int);

total += curr_int;
        }
    }
    fclose(fp);

printf("Total: %d\n",total);

return 0;
}
```

## Exercise Solution

```c
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main()
{
    FILE *fp = fopen("twoDarray.txt", "r");
    char buffer[80];
    int rows = 0;
    int cols = 0;
    int **array_from_file; //2D array for data from the file

//read the first line from the file
    fgets(buffer, 80, fp);
    sscanf(buffer,"%d/%d",&rows,&cols);

//allocate outer array of 2D array
    array_from_file = malloc(rows*sizeof(int *));

for(int i = 0; i < rows; i++)
    {
        //allocate this inner array of the 2D array
        array_from_file[i] = malloc(cols*sizeof(int));
        fgets(buffer, 80, fp);

//following approach from https://cplusplus.com/reference/cstring/strtok/
        char *token = strtok(buffer,",\n");
        int j = 0;
        while(token != NULL)
        {
            int intvaloftoken;
            sscanf(token,"%d",&intvaloftoken);
            array_from_file[i][j] = intvaloftoken;
            j++;
            //printf("%s\n",token); //for debugging
            token = strtok(NULL,",\n");
        }
    }

//print out the data from the table
    for( int i = 0; i < rows; i++)
    {
        for( int j = 0; j < cols; j++)
        {
            printf("%d ",array_from_file[i][j]);
        }
        printf("\n");
    }
    fclose(fp); //close the file

return 0;
}
```

## Continuing the exercise

Can you find a function from `<string.h>` that would allow us to iterate through portions of a string?

https://cplusplus.com/reference/cstring/

Look for sample on the documentation - how would this look for our problem?

# Writing to Files

## Writing to a File
- To open a file for writing you use:
    ```c
    FILE *fp = fopen("input.txt", "w");
    ```
    + 
 If `input.txt` already exists, it is deleted
    + 
 If it doesn't exit, it is created
- 
 You can use `fprintf` to print to a file:
    ```c
    fprintf(fp, "%d %d %d\n", 100, 200, 300);
    ```

## Closing Files
- It is important to **close** your files to inform your OS that you're done with the file
- 
 You can do this with:
    ```c
    fclose(fp);
    ```

# Structs and Files Exercise

## Exercise
- Suppose we have have some 2D points in a file
- 
 We could encode them in the following way:
    ```text
    4
    0 0
    1 1
    0 5
    3.14 2.71
    ```
- 
 First line tells you how many points are in the file and then every other line is of the form `x y`

## Exercise (cont.)
- Write a function called `read_points` with the following signature:
    ```c
    POINT *read_points(char *filename)
    ```
- 
 Should parse the file into an array of `POINT` objects
- 
 Remember to use `malloc` to allocate the memory on the heap!