Print an empty line every other line when reading from file

Question

In my .txt file I have follow data.

Name: name
Score: 0.00%
Name: name
Score: 0.00%
Name: name
Score:6.67%

and I tried using this code

    if (userAnswer1 == 3){
        printf("Test History\n\n");
        FILE * fPointer;
        fPointer = fopen("history.txt", "r");
        char singleLine[50];
        while(!feof(fPointer)) {
            fgets(singleLine, 50, fPointer);
            puts(singleLine);
            int i;
            for(i=0; i>=0; i++){
                if(i%2==0){
                    printf("\n");
                }
                break;
            }
        }
        fclose(fPointer);
    }

to get this output:

Name: name
Score: 0.00%

Name: name
Score: 6.67%

Instead of that I got below output.

Test History

Name: name

Score: 0.00%

Name: name

Score: 0.00%

Name: name

Score: 6.67%

This is impossible to reproduce. There is no `andre` anywhere. Please create [mcve]. — user694733, Dec 12 '17 at 12:14
Please take some time to read [Why is “while ( !feof (file) )” always wrong?](https://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong) — Some programmer dude, Dec 12 '17 at 12:14
'for(i=0; i>=0; i++)' ?? is that condition not always true, (at least until wrap)? — Martin James, Dec 12 '17 at 12:16
@MartinJames Even if it wraps, it's [undefined behavior](https://stackoverflow.com/q/18195715/5958455). — iBug, Dec 12 '17 at 12:22
May be unrelated to your question, but `fopen` can fail. You need to check this. — Jabberwocky, Dec 12 '17 at 12:37

Some programmer dude · Answer 1 · 2017-12-12T12:19:09.773

1

Two things: The puts function writes a newline; And the fgets functions may include the newline in the buffer.

That means when you print singleLine first there is a newline (because fgets put it there), then puts writes its own newline.

Either remove the newline that fgets writes into the buffer, or don't use puts.

edited Dec 12 '17 at 12:19

answered Dec 12 '17 at 12:13

Some programmer dude

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35
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iBug · Answer 2 · 2017-12-12T14:14:22.147

1

while(!feof(fPointer)){
    fgets(singleLine, 50, fPointer);
    puts(singleLine);
    int i;
    for(i=0; i>=0; i++){
        if(i%2==0){
            printf("\n");
        }
        break;
    }
}

There are a few things to note:

fgets() reads until a newline, and includes it to the buffer. So on the following puts() call, you've already output a newline after that, and puts() even adds a newline on its own.
Consider this code:
```
int i;
for(i=0; i>=0; i++){
    if(i%2==0){
        printf("\n");
    }
    break;
}
```
You're actually printing one newline, then break the loop. So it makes no difference from a plain
```
printf("\n");
```

Counting both things above, you're actually printing 3 newlines every while loop: One is what fgets() writes to the buffer, one from puts(), one manually.

To fix them, you need to do two jobs:

Strip the newline from whatever fgets() gives you:
```
singleLine[strlen(singleLine) - 1] = '\0';
```

Make the loop work better:

int i = 0; // Must initialize!
while (condition) {
    other_statements;

    i++;
    if (i%2==0) {
        printf("\n");
    }
    // Do nothing if 'i' is odd
}

Stop using while (!feof()). Check the return value of fgets instead:
```
char *ret = fgets( ... );
if (ret == NULL)
    break;
```

edited Dec 12 '17 at 14:14

answered Dec 12 '17 at 12:16

iBug

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Ok, so i tried this, but it will not print anything now except empty lines. I guess singleLine[strlen(singleLine) - 1] = '\0'; seem to remove it all – andre2012 Dec 12 '17 at 13:55
So i came a little closer to a solution, int i=0; while(!feof(fPointer)){ fgets(singleLine, 50, fPointer); printf("%s", singleLine); i++; if(i%2==0){ printf("\n"); } this seem to work except that it prints out the last score again; "Score: 6.67%" – andre2012 Dec 12 '17 at 14:11
@andre2012 Don't use `while(!feof())`. Check the return value of `fgets` instead. – iBug Dec 12 '17 at 14:16
thanks for the help, didnt really understand but checket tutorial on youtube and got it right, will post code in a reply. – andre2012 Dec 14 '17 at 15:00
@ander2012 Which part don't you understand? – iBug Dec 14 '17 at 16:22
kinda new to programming, so barely anything with checking return value of fgets, i know it can return NULL or something else. I guess that if it returns NULL it is supposed to stop checking in the file – andre2012 Dec 14 '17 at 16:34

score 0 · Accepted Answer · answered Dec 14 '17 at 15:01

    printf("Test History\n\n");

    FILE *pToFile = fopen("history.txt", "r");

    char input[50];
    int i=0;
    while(fgets(input, 50, pToFile)){
        printf("%s", input);
        i++;
        if(i%2==0){
            printf("\n");
        }
    }
    fclose(pToFile);

score -2 · Answer 4 · answered Dec 12 '17 at 12:17

the trouble is with your logic - you have the 'if' inside the 'while' so it loops and counts on each 'while' (line from your file).

I'm not c expert, so no code, but it seems you get the commands. The logic I would use is to look a the text in the line and add the '\n' from that.

i.e.

if singleline (contains - however you do that in c) "%"

add the \n

Print an empty line every other line when reading from file

4 Answers4