I write this code and declare the c variable in if chains, but the compiler gives me an error says you did not declare this variable. `

#include <stdio.h>
#include <cs50.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>


int main(int argc, char** argv)
{

    int k = atoi(argv[1]);
    printf ("%i\n", k);

    string p = get_string ("Plaintext: \n");

    for (int i = 0; i < strlen(p); i  )
    {
        if (isalpha(p[i]))
        {
            if (islower(p[i]))
            {
                char c = (((p[i] - 97)   k) % 26)   97;
                return 0;
            }
            else if (isupper(p[i]))
            {
                char c = (((p[i] - 65)   k) % 26)   65;
                return 0;
            }
            else{
                char c = (((p[i] - 65)   k) % 26)   65;
            }

            p[i] = c;
        }
    }
    printf ("%s\n", p);
    return 0;
}

` test2.c: In function ‘main’: test2.c:34:20: error: ‘c’ undeclared (first use in this function) 34 | p[i] = c; | ^ test2.c:34:20: note: each undeclared identifier is reported only once for each function it appears in

CodePudding user response：

'If' block itself is a scope, and you define a scope and the variables are only accessible inside the scope, not outside, and not at the boundary either.

For sure, the c variable should be declared outside the For loop:

char c = ' ';
for ...

CodePudding user response：

The error would occur at following line:

        p[i] = c;

It is because c is local scope at the following line:

        if (islower(p[i]))
        {
            char c = (((p[i] - 97)   k) % 26)   97;
            return 0;
        }

and

        {
            char c = (((p[i] - 65)   k) % 26)   65;
            return 0;
        }

and

        else{
            char c = (((p[i] - 65)   k) % 26)   65;
        }

Local scope means that it will be released after exit the }.

Additionally, after first 2 condition of if, else if, there is also return after define value to c. If so, the API will return immediate and c is not used at any line because the assignment p[i] = c; will not execute.