程序仅在C ++中打印数组中的前三个字符

Question

My program is to read characters into an array and display it to the console. But I don't know why it only reads first 3 characters.

#include <iostream>

using namespace std;
int main() {    
    int length=0,i;
    char str[10];
    cout<<"Enter a string"<<"\n";

    for(i=0; str[i] != '\0'; i++) {
        cin>>str[i];
    }

    for(int i=0; str[i]!='\0'; i++) {
        cout<<str[i];
        length++;
    }
    cout<<"\n"<<"Length of the string="<<""<<length<<"\n";
}

The output looks like:

Answer 1

Here are some of the issues I noticed:

1. You're checking the contents of str before you even initialize it.
2. You're assuming the string you're fetching from the stream will be null-terminated, but that's actually not the case.
3. You aren't checking whether cin is working correctly
4. You don't check to make sure the string is 10 characters or less, this could cause you to overflow past the end of the buffer
5. When you're checking the length of the array, again you assume the string is null-terminated

If you want to fix these issues and still use a char buffer, see user4581301's comprehensive answer. However, I'd suggest simply switching to std::string. For example:

#include<iostream>

using namespace std;

int main()
{
    string str;
    cout<<"Enter a string"<<"\n";
    if (cin >> str) {
        cout << str << endl;
        cout << "Length of the string = " << str.length() << endl;
    }
}

Answer 2

TL;DR version

std::string str; //use std::string. practically no reasons not to in C++
if (cin >> str) // read into string and test that read succeeded.
{
    std::cout << str << '\n'
              << "Length of the string=" << str.length() << endl;
}

Explaining and salvaging Asker's version

for(i=0; str[i] != '\0'; i++){ // str[i] tested here
    cin>>str[i]; // but it has no assigned value until here.

uses str[i] before it is assigned a value. The program likely found a null character in the block of memory allocated for str and stopped prematurely, but technically anything can happen if you use an uninitialized variable. For example, you got the expected result 3 times before finding the null. The program could never found a null and run forever. It could have rained unicorns. Anything.

int i = 0;
do {
    cin>>str[i];
} while (str[i++] != '\0');

reads then tests. But lines of data in a stream are not C-style strings and are not terminated with null.

int i = 0;
do {
    cin>>str[i];
} while (!std::isspace(str[i++]));

exits when whitespace is found, typically signalling the end of a word, rather than null.

But what if cin>>str[i]; failed for some reason?

int i = 0;
do {
    cin>>str[i];
} while (cin && !std::isspace(str[i++]));

Adds a test to ensure something was read. But what if there are more than 10 characters and char str[10]; is overflowed?

int i = 0;
do {
    cin>>str[i];
} while (cin && !std::isspace(str[i]) && ++i < sizeof(str));

unless I am reading That is legal unless I'm reading [expr.cond] and [expr.log.and] wrong, sequencing of when ++i occurs in !std::isspace(str[i]) && ++i < sizeof(str)) is guaranteed to not affect !std::isspace(str[i])

But what if you run out of space before find the null? str is unterminated and not a string! This ruins the for loop later in the program.

int i = 0;
do {
    cin>>str[i];
} while (cin && !std::isspace(str[i]) && ++i < sizeof(str));
if (i == sizeof(str))
{
    str[sizeof(str)-1] = '\0';
}

I think that covers everything you're likely to run into here.

Answer 3

You read in character by character and store it in str[i] ; but then you increment i++ before checking str[i]!='\\0' .

There are two issues with this approach: First, you check a value at a position that has not been written at that point in time. Second, cin>>str[i] will never write the string termination character - it just reads in valid characters, and if input is terminated (eg by EOF), nothing is written.

So you are approaching this the wrong way.

If you want to read at most 10 characters up to a new line (ie when the user presses enter), use fgets . Or - and this is the preferred option - use cin and write into an std::string -object.

int main()
{
    std::string str;
    cin >> str;
    std::cout << str << std::endl;
}

Answer 4

str [i]！='\\ 0'检查存储在str [i]的现有数据，而不是用户输入的值。

Answer 5

Add string initialization:

char str[10] = {'\0'};

and change the reading with:

char c;
for(int i = 0; cin>> c && c!=/*add your termination cond. here*/ && i < 10;++i)
    str[i] = c;

So you can ensure that the string is filled with correct values and terminated appropriately.

But better solution would be to use std::string . In that case you dont have to check the sizes, because the string grows by itself.For example:

    std::string str;
    for(char c; cin>>c && c!=/*add your termination cond. here*/;) 
        str += c;