Checking equality of a string in C and printing an answer [duplicate] - c

Question

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C Strings Comparison with Equal Sign
(5 answers)
Closed 9 years ago.
i've written some simple code as an SSCCE, I'm trying to check if string entered is equal to a string i've defined in a char pointer array, so it should point to the string and give me a result. I'm not getting any warnings or errors but I'm just not getting any result (either "true" or "false")
is there something else being scanned with the scanf? a termination symbol or something? i'm just not able to get it to print out either true or false
code:
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#define LENGTH 20
//typedef char boolean;
int main(void)
{
const char *temp[1];
temp[0] = "true\0";
temp[1] = "false\0";
char var[LENGTH];
printf("Enter either true or false.\n");
scanf("%s", var);
if(var == temp[0]) //compare contents of array
{
printf("\ntrue\n");
}
else if(var == temp[1]) //compare contents of array
{
printf("\nfalse\n");
}
}

Answer 1

const char *temp[1];
This defines tmp an array that can store 1 char* element.
temp[0] = "true\0";
Assigngs to the first element. This is okay.
temp[1] = "false\0";
Would assign to the second element, but temp can only store one. C doesn't check array boundaries for you.
Also not that you don't have to specify the terminating '\0' explicitly in string literals, so just "true" and "false" are sufficient.
if(var == temp[0])
This compares only the pointer values ("where the strings are stored"), not the contents. You need the strcmp() function (and read carefully, the returned value for equal strings might not be what you expect it to be).

Answer 2

Use strcmp for comparing strings:
#include <stdio.h>
int main(void)
{
const int LENGTH = 20;
char str[LENGTH];
printf("Type \"true\" or \"false\:\n");
if (scanf("%19s", str) != 1) {
printf("scanf failed.");
return -1;
}
if(strcmp(str, "true") == 0) {
printf("\"true\" has been typed.\n");
}
else if(strcmp(str, "false") == 0) {
printf("\"false\" has been typed.\n");
}
return 0;
}
and also note that:
string literals automatically contain null-terminating character ("true", not "true\0")
const int LENGTH is better than #define LENGTH since type safety comes with it
"%19s" ensures that no more than 19 characters (+ \0) will be stored in str
typedef char boolean; is not a good idea
unlikely, but still: scanf doesn't have to succeed
and there is no #include <iostream> in c :)

Answer 3

== checks for equality. Let's see what you're comparing.
The variable var is declared as a character array, so the expression var is really equivalent to &var[0] (the address of the first character in the var array).
Similarly, temp[0] is equivalent to &temp[0][0] (the address of the first character in the temp[0] array).
These addresses are obviously different (otherwise writing var would automatically write temp[0] as well), so == will always return 0 for your case.
strcmp, on the other hand, does not check for equality of its inputs, but for character-by-character equality of the arrays pointed to by its inputs (that is, it compares their members, not their addresses) and so you can use that for strings in C. It's worth noting that strcmp returns 0 (false) if the strings are equal.