My assignment:
Write a program that asks the user whether to read or write a file. If writing a file, it then asks for a line of text, and writes it to a file. If reading, it reads the files and outputs it to the screen. The text is written out in non-binary mode - which is different than the file reading and writing for project #2. Remember how we used fgets() with string input. There is also an fputs().
I am trying to code the write section, but I cannot figure out what my mistake is. Also I do not really know how to create the read section. Do I only have to open the file then?
My code is:
#include <stdio.h>
#define HOLD_SCREEN while(getchar()!='q')
int write() {
int num;
FILE*fptr;
fptr = fopen_s;
if (fptr == NULL) {
printf("ERROR");
exit(1);
}
printf("Enter num: ");
scanf_s("%d", &num);
fprintf(fptr, "%d", num);
fclose(fptr);
return(0);
}
int main(void) {
while (1) {
printf("\n\tEnter an option: (R)ead or (W)rite a file or enter a q if you want to quit: ");
char buf[50];
buf[0] = '\0';
scanf_s("%s", buf, 50);
switch (toupper(buf[0])) {
case 'W':
write();
break;
case 'R':
printInventory();
break;
case 'Q':
return(0);
}
}
HOLD_SCREEN;
return(0);
}
I would really appreciate your help.
For writing:
fopen_s requires 3 parameters,
errno_t fopen_s(FILE** pFile, const char *filename, const char *mode);
an example being (in write mode)
errno_t err;
FILE *fptr
err = fopen_s(&fptr, "text.txt", "w" );
I am also taking some liberties and guessing this is the microsoft stack by the use of fopen_s instead of fopen. Otherwise, you should be using fopen like
FILE *fptr
fptr= fopen("text.txt", "w");
For reading:
Similar except the 3rd parameter would be err = fopen_s(&fptr, "text.txt", "r" ); or fptr = fopen("text.txt, "r") and use fgets to read a line of input (lots of examples out there).
Related
I've used a ".txt" extension while reading and writing the file, also the file mode is corresponding to that of "text" type of file. The program runs fine, but instead of storing an ASCII character in the file, it is storing binary characters. I need some assistance here. Thank you.
int main(void)
{
FILE *fp;
int n2, n1;
printf("ENTER A NUMBER: ");
scanf("%d", &n1);
fp = fopen("hello.txt", "w");
if (fp == NULL)
{
printf("ERROR");
exit(1);
}
fprintf(fp, "%d", n1);
//fclose(fp);
//rewind(fp);
fp = fopen("hello.txt", "r");
if (fp == NULL)
{
printf("ERROR");
exit(1);
}
//n2 = getw(fp);
fscanf(fp, "%d", n1);
printf("%d", n1);
fclose(fp);
}
If you are going to close and reopen the file you don't need rewind. Or you can open the file to read and write, and then you can use rewind. Both work, here is a sample of the latter:
int main(void)
{
FILE *fp;
int n2, n1;
printf("ENTER A NUMBER: ");
if (scanf("%d", &n1) == 1) // checking if the input was correctly parsed
{
fp = fopen("hello.txt", "w+"); // open to read and write
if (fp == NULL)
{
printf("ERROR");
exit(1);
}
putw(n1, fp); // write to the file
rewind(fp); // go back to the beginning of the file
n2 = getw(fp); // get the data
printf("%d", n2);
fclose(fp);
}
else
{
puts("Bad input");
}
}
Live sample
There is still the matter of possible integer overflow when reading from stdin, if there is no requirement to guard against that, make sure to at least document the vulnerability, otherwise the advice is to use fgets to read input and strtol to convert the value.
You should send address of a variable in fscanf, like this:
fscanf(fp,"%d",&n1);
Hello I am writing a program that reads the contents of a binary file and prints them to the screen.
#include <stdio.h>
#include <stdlib.h> // For exit()
int main()
{
FILE *fptr;
char filename[100];
printf("Enter the filename to open \n");
scanf("%s", filename);
// Open file
fptr = fopen(filename, "rb");
if (fptr == NULL)
{
printf("Cannot open file \n");
exit(0);
}
// Read contents from file
fseek(fptr,0L,SEEK_END);
int fsize = ftell(fptr);
fseek(fptr,0L,SEEK_SET);
unsigned char *c = malloc(fsize);
fread(c,fsize,1,fptr);
fclose(fptr);
printf("%s",c);
return 0;
}
but it does not print anything.Can someone explain me why and how should I fix this problem.
What you have attempted is not at all what you wanted to achieve.
Remember printf() formats the data it prints. To be printed properly with the %s formatting, the binary data values must be ASCII values but , of course, they are not.
You should probably attempt to printf() with %d.
I am trying to write a simple C program which will read data from a csv file and perform some calculations on this data.
Unfortunately I have a problem where a file pointer of mine, fptr , is not being assigned a value after calling fopen(). I know this is the case after stepping through VS 2017's debugger. Yet I do not know why this is the case. This is a huge problem and means my program will throw some very nasty exceptions any time I try to read data from the file or close the file.
My code is below:
main.c
#include<stdio.h>
#include <stdlib.h> // For exit() function
#include"constants.h" //For access to all project constants
/***************************************************************************************************************
To keep the terminal from automatically closing
Only useful for debugging/testing purposes
***************************************************************************************************************/
void preventTerminalClosure() {
//flushes the standard input
//(clears the input buffer)
while ((getchar()) != '\n');
printf("\n\nPress the ENTER key to close the terminal...\n");
getchar();
}
/***************************************************************************************************************
Read the given input file
***************************************************************************************************************/
void readInputFile(char fileName[]) {
FILE *fptr;
char output[255];
//open the file
if (fptr = fopen(fileName, "r") != NULL) { //read file if file exists
//fscanf(fptr, "%[^\n]", output);
//printf("Data from the file:\n%s", output);
printf("<--Here-->");
}else {
printf("\nERROR 1: File %s not found\n", fileName);
preventTerminalClosure();
exit(1);
}
fclose(fptr); //close the file
}
/***************************************************************************************************************
* * * Main * * *
***************************************************************************************************************/
void main() {
char testName[MAX_NAME_SIZE];
printf("Hello World!\n");
printf("Please enter your name: ");
scanf("%s", testName);
printf("It's nice to meet you %s!", testName);
readInputFile("dummy.txt");
preventTerminalClosure(); //Debug only
}
I have made sure that my fake file does indeed exist and is located in the correct location. Otherwise my code would hit the else block inside of readInputFile(). That is something I have thoroughly tested.
There is clearly something basic that I am missing which explains this pointer behavior; but what that is, I am not sure. Any help would be appreciated! :)
Use parenthesis to enforce order, so that fptr is compared against NULL after it has been assigned value returned by fopen:
FILE *fptr;
char output[255];
//open the file
if ( (fptr = fopen(fileName, "r")) != NULL)
I am trying to repeatedly read a string from the command line and print it to a file. This is my code:
int main ()
{
FILE* fp=fopen("test.txt","w");
char* tofile[10];
while(1){
printf("cat: ");
scanf("%s",tofile);
fprintf(fp,"%s\n",tofile);
}
return 0;
}
It works just fine outside the loop. But inside, it just doesn't print.
The fprintf function returns the correct amount of characters it has to print.
Note: I know there's a similar question out there, but it hasn't been answered yet, and I hope my code can help in this matter since it's simpler.
Well first it doesn't seem that what you want is reading on the command line.
The command line what you write right when you execute your program such as:
./main things that are on the command line
What it seems you want to do is to read on the standard input.
What you should consider is to use the fgets function, as it has a limit of characters to be read, so that you can store them "safely" into a buffer, like your tofile.
As you want to read on the standard input you can use the stdin stream (which is a FILE* that is automatically created for every program)
The line goes
fgets(tofile, 10, stdin);
Your loop becoming :
while (fgets(tofile, 10, stdin) != NULL) {
printf("cat: ");
fprintf(fp, "%s\n", tofile);
}
meaning: as long as we can read on the standard input, print "cat :" and store what we just read in the file controlled by the stream pointer fp.
Some important stuff
When you try to open a stream it may fail and you should test it:
char filename[] = "test.txt";
FILE *fp = fopen(filename, "w");
if (fp == NULL) {
fprintf(stderr, "Failed to open the file of name : %s", filename);
return EXIT_FAILURE;
}
Right before exiting your main, you should also close the file and check if it has succeeded, like that for example:
if (fclose(fp) != 0) {
fprintf(stderr, "Failed to close the file of name : %s", filename);
return EXIT_FAILURE;
}
The whole thing becomes:
#include <stdio.h>
#include <stdlib.h>
int main (void) {
char filename[] = "test.txt";
FILE *fp = fopen(filename, "w");
if (fp == NULL) {
fprintf(stderr, "Failed to open the file of name : %s", filename);
return EXIT_FAILURE;
}
char tofile[10];
printf("cat: ");
while (fgets(tofile, 10, stdin) != NULL) {
printf("cat: ");
fprintf(fp, "%s\n", tofile);
}
if (fclose(fp) != 0) {
fprintf(stderr, "Failed to close the file of name : %s", filename);
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}
Improvements
I don't know if it is just a little program or it aspires to become a greater program.
In the last case you should consider using defines and not a magical number such as
#define BUFFER_MAX_SIZE 10
char tofile[BUFFER_MAX_SIZE];
while (fgets(tofile, BUFFER_MAX_SIZE, stdin) != NULL) { ... }
This helps for readability and makes the program less apt to debug when modifying such a size. Because with the define all the part of the code needing the size will still be fully functional without modifying them.
Please also keep in mind that your tofile acts as a buffer, and it's really a small buffer that can easily be overflowed.
This will work. fgets() returns the string it reads from the specified file pointer. If this string returns only a newline ("\n"), that means nothing was entered at stdin.
#include <stdio.h>
#include <string.h>
int main(void)
{
FILE *fp = fopen("test.txt","w");
// always check if fopen() == null
if (!fp) {
fprintf(stderr, "Could not write to file\n");
return 1;
}
char tofile[30];
printf("cat: ");
while (fgets(tofile, 30, stdin)) {
if (strcmp(tofile, "\n") == 0)
break;
fprintf(fp, "%s", tofile);
printf("cat: ");
}
// always fclose()
fclose(fp);
return 0;
}
Edited code.
So i've been given an exercise to work on: Have the user input a number and the program will display the line of text associated with that line for example
Password
abcdefg
Star_wars
jedi
Weapon
Planet
long
nail
car
fast
cover
machine
My_little
Alone
Love
Ghast
Input 3: Output: Star_wars
Now i have been given a program to solve this, however it uses the function getline() , which doesn't complie on DEV C++.
#include <stdio.h>
int main(void)
{
int end = 1, bytes = 512, loop = 0, line = 0;
char *str = NULL;
FILE *fd = fopen("Student passwords.txt", "r");
if (fd == NULL) {
printf("Failed to open file\n");
return -1;
}
printf("Enter the line number to read : ");
scanf("%d", &line);
do {
getline(&str, &bytes, fd);
loop++;
if (loop == line)
end = 0;
}while(end);
printf("\nLine-%d: %s\n", line, str);
fclose(fd);
}
All i need is to know how to do this, in a simple program without the use of getline()
Thanks
Edit: I also don't want to download software to make this work
use fgets instead of getline.
#include <stdio.h>
int main(void){
int end, loop, line;
char str[512];
FILE *fd = fopen("data.txt", "r");
if (fd == NULL) {
printf("Failed to open file\n");
return -1;
}
printf("Enter the line number to read : ");
scanf("%d", &line);
for(end = loop = 0;loop<line;++loop){
if(0==fgets(str, sizeof(str), fd)){//include '\n'
end = 1;//can't input (EOF)
break;
}
}
if(!end)
printf("\nLine-%d: %s\n", line, str);
fclose(fd);
return 0;
}
You have wrote:
char *str = NULL;
and you used it without initializing:
getline(&str, &bytes, fd);
first you must initialize it:
char *str=(char*)malloc(SIZEOFSTR);
you can add this part in your program instead of your do-while loop. You will be using fscanf() whose arguments are the file pointer, specifier of data type and the variable you want to store.
printf("Enter the line number to read : ");
scanf("%d", &line);
while(line--) {
fscanf(fd,"%s",str);
}
printf("\nLine-%d:%s\n",line,str);