Can't get output reversed reading from a file C - c

Question

I want to do something pretty simple, but somehow it doesn't do what I want.
I have a file with numbers in it. I wanna read from that file and print those numbers but in reverse order.
so let's say we have the numers:
10
32
43
6
7
I want to have:
7
6
43
32
10
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 255
int main(int argc, char *argv[]) {
char userInput[MAX], target[MAX];
FILE *file = stdin;
if (argc > 2) {
fprintf(stderr, "Usage: %s[<file>]\n", argv[0]);
exit(EXIT_FAILURE);
}
if (argc == 2) {
file = fopen(argv[1], "r");
if (!file) {
perror(argv[0]);
exit(EXIT_FAILURE);
}
}
while(fgets(userInput, sizeof(userInput), file)) {
size_t len = strlen(userInput);
int i;
for (i = len-1; i >= 0; i--) {
if (userInput[i] == ' ') {
userInput[i] = '\0';
printf("%s ", &(userInput[i]) + 1);
}
}
printf("%s", userInput);
}
if (file != stdin) {
fclose(file);
}
exit(EXIT_SUCCESS);
}
Does anyone see the mistake?
This is the content of my .txt file:
11
34
45
3
78
43
3
4
9
34
23
43

Answer 1

Your program is designed to accept all of its input on a single line. Your input file contains multiple lines.

Answer 2

The following 1. solution is based on the example shown in the question.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 255
int main(int argc, char *argv[])
{
char userInput[MAX] = "";
char target[MAX] = "";
FILE *file = stdin;
if (argc > 2)
{
fprintf(stderr, "Usage: %s[<file>]\n", argv[0]);
exit(EXIT_FAILURE);
}
if (argc == 2)
{
file = fopen(argv[1], "r");
if (!file)
{
perror(argv[0]);
exit(EXIT_FAILURE);
}
}
/* Read all numbers from file and write it to target buffer */
while(fgets(userInput, sizeof(userInput), file))
{
/* Break loop if there is not enough space to store current number. */
if ((strlen(target) + strlen(userInput)) >= MAX)
{
break;
}
/* Add current number read from file to target buffer */
strcat(target, userInput);
/* Print current number read from file */
printf("%s", userInput);
}
if (file != stdin) {
fclose(file);
}
/*
* Run over target buffer in reverse order and replace current number
* split character '\n' by string end marker '\0'.
* After that print current number.
*/
size_t len = strlen(target);
if (len > 0)
{
for (size_t i = len-1; i != 0; i--)
{
if (target[i] == '\n')
{
target[i] = '\0';
printf("%s\n", &(target[i]) + 1);
}
}
/* Print first number */
printf("%s\n", &(target[0]));
}
exit(EXIT_SUCCESS);
}
However it might be better to store the numbers within an array of integer using sscanf() and after that print the content of the array in reverse order.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 255
int main(int argc, char *argv[])
{
char userInput[MAX] = "";
int nums[MAX];
int count = 0;
FILE *file = stdin;
if (argc > 2)
{
fprintf(stderr, "Usage: %s[<file>]\n", argv[0]);
exit(EXIT_FAILURE);
}
if (argc == 2)
{
file = fopen(argv[1], "r");
if (!file)
{
perror(argv[0]);
exit(EXIT_FAILURE);
}
}
/* Read all numbers from file and write it to target buffer */
while(fgets(userInput, sizeof(userInput), file))
{
sscanf(userInput, "%i", &nums[count]);
/* Break loop if there is not enough space to store current number. */
if (count >= MAX)
{
break;
}
count++;
/* Print current number read from file */
printf("%s", userInput);
}
if (file != stdin) {
fclose(file);
}
/* Print numbers stored in array in reverse order */
printf("\n");
for (int idx = count; idx != 0; idx--)
{
printf("%i\n", nums[idx-1]);
}
exit(EXIT_SUCCESS);
}

Answer 3

You can read your input file starting from the end using fseek
Here there's a possible implementation based on your code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 255
int main(int argc, char *argv[]) {
int i = 0, end = 0, cnt = 0;
char ch, cnt_flag = 0;
char userInput[MAX];
FILE *fp = stdin;
if (argc > 2) {
fprintf(stderr, "Usage: %s[<fp>]\n", argv[0]);
exit(EXIT_FAILURE);
}
if (argc == 2) {
fp = fopen(argv[1], "r");
if (!fp) {
perror(argv[0]);
exit(EXIT_FAILURE);
}
}
fseek(fp, 0, SEEK_END);
end = ftell(fp);
while (i < end) {
i++;
fseek(fp, -i, SEEK_END);
ch = fgetc(fp);
if (cnt_flag == 1) {
cnt++;
if (ch == '\n') {
/* skip empty lines */
if (cnt > 1) {
fgets(userInput, cnt, fp);
printf("%s\n", userInput);
}
cnt_flag = 0;
cnt = 0;
}
}
if (ch == '\n')
cnt_flag = 1;
}
/* read first line */
fseek(fp, 0, SEEK_SET);
fgets(userInput, cnt + 1, fp);
printf("%s\n", userInput);
if (fp != stdin) {
fclose(fp);
}
exit(EXIT_SUCCESS);
}

Answer 4

If u need to reverse strings in file, check this out. This is crossplatform solution without big chunk buffer usage. Only restriction that this is for file operations only. Advantages: file size independent solution.
This example based on your code.
#ifdef _WIN32
#define _CRT_SECURE_NO_WARNINGS
#define NEW_LINE 2 // Windows new line: 2 bytes (CR) + (LF)
#else
#define NEW_LINE 1 // Linux new line: 1 byte (LF)
#endif
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 255
// This function can be replaced with any other solution
// u need (write data into another file, or allocate data in the memory and etc)
void printLineFromFile(FILE *file) {
char buff[255];
// write line with \n to buff
fgets(buff, 255, file);
// total buff size
int buffLen = strlen(buff);
int printLen = buffLen;
// printLen equals buffLen if the last symbol isn't \n
if (buff[buffLen - 1] == '\n') printLen--;
// print specified amount of bytes from buff
fprintf(stdout, "%.*s\n", printLen, buff);
}
int main(int argc, char *argv[]) {
FILE *file = stdin;
if (argc > 2) {
fprintf(stderr, "Usage: %s[<file>]\n", argv[0]);
exit(EXIT_FAILURE);
}
if (argc == 2) {
file = fopen(argv[1], "r");
if (!file) {
perror(argv[0]);
exit(EXIT_FAILURE);
}
}
// set position to the end of file
fseek(file, 0, SEEK_END);
// endPosition - position of last byte in file. (not EOF)
long endPosition = ftell(file) - 1;
// currPosition - moving position across the file
long currPosition = endPosition;
// byte buffer to read into it
int ch;
while (currPosition >= 0) {
// moving file position to the currPosition
fseek(file, currPosition, SEEK_SET);
if (currPosition == 0) {
printLineFromFile(file);
currPosition -= NEW_LINE;
continue;
}
ch = fgetc(file);
if (ch == '\n') {
if (currPosition == endPosition) {
currPosition -= NEW_LINE;
fprintf(stdout, "\n");
}
else {
printLineFromFile(file);
currPosition -= NEW_LINE;
continue;
}
}
// move back to position before fgetc
currPosition--;
};
if (file != stdin) {
fclose(file);
}
exit(EXIT_SUCCESS);
}