Strcmp doesn't work and I don't seem to understand why - transformation to ASCII code makes the program work as expected though

Question

First of all, please don't critique the way the program is written, because this is what we study in my country.

I know it is a mixture of C and C++ and the things I use are outdated, but that's how the things are here.

So I have to make a program which gets as an input n words. Then I have to print the words that have the last one as a prefix.

e.g.

input: 
n=6 
raita grai raid raion straie rai
output:
raita raid raion

This is my program. It works as expected:

#include <iostream>
#include <string.h>

using namespace std;

int main()
{
    int n;
    char a[100][100];
    bool ok=1;
    cin >> n;
    cin.get();
    for (int i = 0; i < n; i++)
    {
        cin.get(a[i], 100);
        cin.get();
    }
    int p = strlen(a[n - 1]);
    for (int i = 0; i < n - 1; i++)
    {
        for(int j = 0; j < p; j++)
        {
            ok = 1;
            if ((unsigned int)a[i][j] != (unsigned int)a[n-1][j])
            {
                ok = 0;
                break;
            }
        }
        if (ok == 1)
        {
            cout << a[i] << " ";
        }
    }
}

But initially, it looked like this:

/* strstr example */
#include <iostream>
#include <string.h>

using namespace std;

int main()
{
    int n;
    char a[100][100];
    bool ok=1;
    cin >> n;
    cin.get();
    for (int i = 0; i < n; i++)
    {
        cin.get(a[i], 100);
        cin.get();
    }
    int p = strlen(a[n - 1]);
    for (int i = 0; i < n - 1; i++)
    {
        for(int j = 0; j < p; j++)
        {
            ok = 1;
            if (strcmp(a[i][j], a[n-1][j]) != 0)
            {
                ok = 0;
                break;
            }
        }
        if (ok == 1)
        {
            cout << a[i] << " ";
        }
    }
}

and it throws some errors:

Severity    Code    Description Project File    Line    Suppression State
Error (active)  E0167   argument of type "char" is incompatible with parameter of type "const char *"   ConsoleApplication1 25  

Severity    Code    Description Project File    Line    Suppression State
Error   C2664   'int strcmp(const char *,const char *)': cannot convert argument 1 from 'char' to 'const char *'    ConsoleApplication1     25

I cannot seem to understand why this happens. Can any of you help me understand? Also, should I be using conversion to (unsigned int) or just strcmp?

Thanks.

You should not call `strcmp` on each single character of the string. It already compares strings after all. — Gerhardh, Apr 29 '20 at 15:11
@user3121023 I'm comparing each letter. a[i]'s first letter with a[n-1]'s first letter, and so on. — Octavian Niculescu, Apr 29 '20 at 15:19
@Gerhardh well it seems more like C to me tbh. The only C++ thing I'm using is iostream with cin/cout. Excepting that, everything is C-style, that's what I tagged it as C. Also, I didn't know that I can't use strcmp to compare each character. — Octavian Niculescu, Apr 29 '20 at 15:20
@JohnSmith Then `strcmp` is the wrong tool for the job, it compares strings not characters. — john, Apr 29 '20 at 15:20
@john Yeah. I didn't know that I can't use it to compare characters. Thanks. — Octavian Niculescu, Apr 29 '20 at 15:24
Just remove the `j` loop completely and use `if (strcmp(a[i], a[n-1]) != 0)`. — David C. Rankin, Apr 29 '20 at 15:24
@DavidC.Rankin -- the requirement here is to find strings for which the final string is a **prefix**. It's not searching for identical strings, so `strcmp` won't work. — Pete Becker, Apr 29 '20 at 16:01

score 1 · Accepted Answer · answered Apr 29 '20 at 15:21

 int strcmp(const char *s1, const char *s2);

strcmp used to compare string and string. But in your code, you compare char and char (a[i][j] and a[n-1][j]).

In your case, you can use strncmp that compares only the first (at most) n bytes (in your case, n is strlen(a[n-1])) of two strings:

int strncmp(const char *s1, const char *s2, size_t n);

So, your program becomes as below:

    for (int i = 0; i < n - 1; i++)
    {
        ok = 1;
        if (strncmp(a[i], a[n-1], p) != 0)
        {
            ok = 0;
        }
        if (ok == 1)
        {
            cout << a[i] << " ";
        }
    }

Thanks. I didn't even know strncmp existed. – Octavian Niculescu Apr 29 '20 at 15:56 — Octavian Niculescu, Apr 29 '20 at 15:56

score 1 · Answer 2 · answered Apr 29 '20 at 15:32

1

In this statement

if (strcmp(a[i][j], a[n-1][j]) != 0)

the both expressions a[i][j] and a[n-1][j] have the type char while the function strcmp expects two pointers to strings of the type char *.

So the compiler issues an error.

You could simplify your first program using the standard function strncmp. For example

size_t p = strlen(a[n - 1]);
for (int i = 0; i < n - 1; i++)
{
    if ( strncmp( a[i], a[n-1], p ) == 0 ) cout << a[i] << " ";
}

Pay attention to that you should use the header <cstring> instead of the header <string.h>,

answered Apr 29 '20 at 15:32

Vlad from Moscow

301,070
26
186
335

Even if your answer is a bit better, I've chosen the other one as the correct one because it was faster. Can you elaborate on using "cstring" instead of "string.h"? As far as I know, cstring should be avoided (because it's obsolete). – Octavian Niculescu Apr 29 '20 at 15:58
@JohnSmith It is a requirement of the C++ Standard. – Vlad from Moscow Apr 29 '20 at 15:59
@JohnSmith -- `` puts the names of the C functions that are declared in `` into the namespace `std::`. It is not obsolete, and there is no reason to avoid it. There were some folks early on who thought the the entire C library should be tolerated until it could be removed. That effort quickly failed. – Pete Becker Apr 29 '20 at 16:08

score 0 · Answer 3 · answered Apr 29 '20 at 16:09

0

The simplest change to the code in the question is to change the test from if (strcmp(a[i][j], a[n-1][j]) != 0) to if (a[i][j] != a[n-1][j]).

answered Apr 29 '20 at 16:09

Pete Becker

74,985
8
76
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Strcmp doesn't work and I don't seem to understand why - transformation to ASCII code makes the program work as expected though

3 Answers3