Tisknuti retezcu a zmnena pismenek z v na m, C (vc)

Language: Layout:

#include  <stdio.h>
#define N 20
#define _CRT_SECURE_NO_WARNING

int main(void)
{
    char str[N + 1] = { '\0'}; // +1 pro zapsání ukončujícího znaku \0 pro "" se zapisují automaticky
    printf("Zadej retezec: \n"); 
    //scanf("%s", str);    znaky mezera a enter ukončují printf
    for(size_t i = 0; i < N; ++i)
        {
        int ch = getchar();
        if(ch == EOF || ch == '\n')//EOF - end of file
            {
            str[i] = '\0';
            break;
            }
        str[i] = (char) ch;
        }
    printf("Retezes: %s \n", str); //%s tiskne string
    
    size_t pocet_vp = 0;
    for(size_t i = 0; str[i] != '\0' && (i < N); ++i)
        {
        //if(str[i] >= 'A' && str[i] <= 'Z') ++pocet_vp;
        if(isupper(str[i]) != 0)
            {
                //str[i] += 'a' - 'A'; //pricte rozdil velkych a a malip písmen
                str[i] = tolower(str[i]);
                ++pocet_vp;
            }
        }
    printf("Pocet velkych pismen je: %zu \n", pocet_vp);
    printf("Opraveny retezec: %s\n", str);
    
    return 0;
}

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Compilation time: 1,97 sec, absolute running time: 0,22 sec, absolute service time: 2,21 sec