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as a homework assignment for my computing 1 college course, my professor has given me the task of having the user input a string of characters into the terminal, taking that string, adding it into an array, then printing the array and printing the array backwards. I think that I know of a way to print the array backwards, however, I cannot come up with a way to read from the terminal and add the characters from the terminal to an array. I have tried doing the following:

char ch;
for (int i = 0; i <= 80 || str[i] == '\n'; ++i) {
scanf_s("%c", &str[i]);
}

I am wondering if someone could explain to me why this section of code does not operate as expected, and if someone could give me some other ideas to try. Thank you.

Jonathan Jerai
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    For starters it does not work because you don't have a main function. Please fix a [mre] – klutt Nov 14 '19 at 19:54
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    After leaving your loop` do you expect `str` to be a *nul-teminated* string? If so, how? What does `char ch;` have to do with the snippet you posted? Why use `scanf` in this case anyway? If you are reading a character at a time `str[i] = getchar();` will do the same thing. – David C. Rankin Nov 15 '19 at 04:34
  • One will find that `scanf` is not really suited for interactive input, see http://sekrit.de/webdocs/c/beginners-guide-away-from-scanf.html, and `scanf_s` is non-standard. – Neil Nov 15 '19 at 04:52
  • strongly suggest using `fgets()` to input lines of data. See the MAN page for `fgets()` for the details of that function. – user3629249 Nov 16 '19 at 01:08
  • how is `str[]` declared? – user3629249 Nov 16 '19 at 01:18
  • @user3121023, No, the loop increment has not been incremented before the call to `scanf()`. the 'step' parameter of a `for()` statement is applied at the end of the loop, not before – user3629249 Nov 16 '19 at 01:23
  • regarding: `for (int i = 0; i <= 80 || str[i] == '\n'; ++i) {` The expression: `str[i] == '\n'` will end the loop as soon as a NON '\n' character is entered. Suggest: `for (size_t i = 0; i < 80 && str[i] != '\n'; ++i) {` Note: both of the 'comparison' conditions must be TRUE to stay in the loop. Suggest using `size_t` rather than `int` as the index can never be less than 0 – user3629249 Nov 16 '19 at 01:26

3 Answers3

3

You are using scanf_s with %c specifier incorrectly.

Please take notice of compiler warnings, there is a size argument missing.

Microsoft's scanf_s is not a direct replacement for scanf.

Unlike scanf ... scanf_s ... requires the buffer size to be specified for all input parameters of type c, C, s, S, or string control sets that are enclosed in []. The buffer size in characters is passed as an additional parameter immediately following the pointer to the buffer or variable.

scanf_s("%c", &str[i], 1);

You might also want to filter out any newline which may have been left in the buffer, with

scanf_s(" %c", &str[i], 1);

notice the added space.

Weather Vane
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2

Why your code is showing this type of behaviour...

  1. use scanf instead of scanf_s

  2. the conditions you have provided in the for loop are wrong

    #include <stdio.h>
int main()
{
char ch;
char str[1000];
int i;
for (i = 0; i <= 80 ; i++)
{

    scanf("%c", &str[i]);
    if(str[i]=='\n')
    {
        str[i]='\0';
        break;
    }
}
printf(str);
}

I could show you the same task in simple manner. I have tried to answer your question in your way. That's why it may seem complicated.

Kazi Ziaul Hassan
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  • regarding: `for (i = -1; i <= 80 && str[i] != '\n' ; ) { ++i; scanf("%c", &str[i]);` This is a bad idea. Suggest: `for ( size_t i = 0; i < sizeof( str ) && str[i] != '\n' ; i++) { scanf("%c", &str[i]);` however, checking the contents of `str[]` before it has been initialized is undefined behavior as the `str[]` will contain what every trash in on the stack at the location(s) of `str[]` Therefore suggest declaring `str[]` via: `char str[1000] = {'\0'};` – user3629249 Nov 16 '19 at 01:14
  • regarding: `using namespace std;` This is a C++ language statement. in C, the code will fail to compile – user3629249 Nov 16 '19 at 01:20
  • I have edited @user3629249. By the way your suggested code is not going to work. I checked it. I made a mistake I ran my previous code on .cpp file. Thanks for mentioning the mistakes. – Kazi Ziaul Hassan Nov 16 '19 at 01:53
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#include <stdio.h>
#define MAX 25
int main()
{
    char buf[MAX];
    fgets(buf, MAX, stdin);
    printf("%s\n", buf);

    return 0;
}

fgets- Reads until new line character encountered or maximum limit of character array.

Anupama Karunarathna
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