Input an arbitrary number of strings from the user

Question

I want to read in an arbitrary number of strings, one at a time, using <stdio.h> in C. I know that you can do this with integers using:

while (scanf("%d ", &integer))

but I cannot do:

while (scanf("%s", string))

How can I implement the above?

The input is on separate lines.

Don't ever use `scanf("%s")` - see http://stackoverflow.com/questions/4023895/how-to-read-string-entered-by-user-in-c/4023921#4023921 — paxdiablo, May 01 '12 at 05:46
You can use scanf if you bound the string size (per the answer you link to). — Eric J., May 01 '12 at 05:47
What do you want to do with each string after you've read it? Using `scanf("%s", ...)` is going to give you a new white space separated word on each iteration of the loop, but presumably you're going to do something with the value read before reading the next. That 'something' will dictate how you handle the storage of the next word. — Jonathan Leffler, May 01 '12 at 05:49
i want to create a binary tree from an input bitstring so scanf("%s"...) then creare binary tree for that string, then move on to the next string untill there are no more strings in the input — Wasiq Kashkari, May 01 '12 at 06:06
Don't test `scanf()` only for `false` (0) or `true`. `scanf()` will return `EOF` in case of failure by exhausting the input which your test won't catch. — pmg, May 01 '12 at 19:39
Possible duplicate of [How can I read an input string of unknown length?](https://stackoverflow.com/q/16870485/608639) — jww, Dec 08 '18 at 07:00

score 3 · Answer 1 · edited May 01 '12 at 06:09

3

You usually want to use fgets to read input as strings, especially when you want one line of input to end up as one string.

You can also use fscanf with a scanset conversion to read a line at a time, such as:

char line[100], newline;

fscanf("%99[^\n]%c", line, &newline);

Then you can check whether newline=='\n' to determine whether you've read the entire line successfully, or the line was larger than the buffer you provided.

When you're trying to read line-oriented input, you normally want to avoid "%s" (even with a specified length) though, as this reads white-space delimited tokens, not entire lines.

edited May 01 '12 at 06:09

Adam Rosenfield

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answered May 01 '12 at 05:51

Jerry Coffin

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fgets will not recognise hitting simply enter either :( – Wasiq Kashkari May 01 '12 at 06:02
Oh yes it will! `fgets()` will recognize a simple enter; it will be an 'empty' line, consisting of just the newline character. – Jonathan Leffler May 01 '12 at 06:06
@WasiqKashkari: What do you mean "won't recognize it"? It will read it as a string containin only a `new-line`. – Jerry Coffin May 01 '12 at 06:08
i dont know, if i hit just enter, the scanf doesnt return, so if i debug it, hitting enter wont let the debugger step to the next line of code – Wasiq Kashkari May 01 '12 at 06:18
@JerryCoffin [link](http://stackoverflow.com/questions/10394262/input-strings-into-c-in-a-loop) i think i asked it better here – Wasiq Kashkari May 01 '12 at 06:19
@WasiqKashkari: Like I said -- to read line-oriented input, you *usually* want to use `fgets`. – Jerry Coffin May 01 '12 at 06:21
@JerryCoffin so use while (fgets(...) == 1) ?? – Wasiq Kashkari May 01 '12 at 06:28
@WasiqKashkari: Usually you want `while (fgets(...) != NULL)`. – Jerry Coffin May 01 '12 at 13:18

juergen d · Answer 2 · 2012-05-01T05:44:32.740

0

Use a char array:

char charArray[100];
while (scanf("%s", &charArray))

edited May 01 '12 at 05:44

answered May 01 '12 at 05:43

juergen d

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4 is a little short, isn't it? And you don't want the `&` in front of `charArray`. I'd recommend `scanf("%99s", charArray)` to protect against buffer overflow. – Jonathan Leffler May 01 '12 at 05:44
2

Can you say "buffer overflow", little Juergen? :-) – paxdiablo May 01 '12 at 05:45
no using a char array doesnt work. the while loop never finishes – Wasiq Kashkari May 01 '12 at 05:48
i want it so if the user simply hits enter, the while loop finishes?? – Wasiq Kashkari May 01 '12 at 05:50
@WasiqKashkari If you want just one line of input, use `fgets()`. It's much simpler to deal with than `scanf()`. – Jonathan Leffler May 01 '12 at 05:51
nope nope == 1 wont work either, if i hit enter, scanf will not recognise that this is input, it still expects some kind of character – Wasiq Kashkari May 01 '12 at 05:59
@WasiqKashkari: Oh - yes...I remember now why I never use `scanf()`; it is just too damn hard to get it to work correctly. Especially if the input should be regarded as 'line based'; it is just the wrong tool for the job. Use `fgets()` or `getline()`. – Jonathan Leffler May 01 '12 at 06:00

score 0 · Answer 3 · answered May 01 '12 at 05:51

0

I guess your problem is to terminate the loop. scanf returns the number of successful scanned elements. In case of a string, also the empty string is successful scanned. Thus, you need another criterion, e.g.

 while(scanf("%s",string)  && (strlen(string)!=0))

answered May 01 '12 at 05:51

Matthias

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nope nope, the scanf doesnt recognise hitting enter as input. – Wasiq Kashkari May 01 '12 at 06:00

score 0 · Answer 4 · answered May 01 '12 at 19:35

I did not completely understand what you were trying to do from your original question. When you said you wanted to read in an arbitrary number of strings, I took that to mean, you wanted your program to be able to read 0 to n strings. Unfortunately in C, you will either have to cap off the maximum number of strings you would ever want to read in like #define MAX_NUMBER_OF_STRINGS_TO_READ 25, or get into some sophisticated memory allocation scheme to read a string in and then add it to dynamic memory (returned from malloc).

I took the cap the maximum number of strings approach and wrote the following snippet:

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

char charArray[5][25] = {0};

int main(int argc, char *argv[])
{
    int in_idx = 0;
    int out_idx = 0;

    printf("\n\n%s\n", "Enter no more than 5 strings, no more than 25 characters long.");

    while(fgets (charArray[in_idx], 25, stdin))
    {
        if('\n' == charArray[in_idx][0])
        {
            printf("%s\n", "Entry terminated with newline.");
            break;
        }

        in_idx++;
    }

    for(out_idx=0; out_idx < (in_idx + 1); out_idx++)
    {
        printf("%s", charArray[out_idx]);
    }

    printf("\n%s\n", "Program ended.");

    return 0;
}

I made the termination character a newline. If I only want two strings, I press Enter when I've entered the second string. I terminated fgets by looking for a '\n' in the first position of the character array.

Input an arbitrary number of strings from the user

4 Answers4