Loop problem with switch case

Question

I reread the code following the guidelines given here. However, it has had the initial problem again... I choose the desired option, it performs the desired action but soon after it goes to the default of the switch case and prints "invalid information"

My code is like this:

#include <string.h>
#include <stdio.h>
#define TAM 20
#include <ctype.h>

int menu(){
    char opc;

    printf("| A | Ler string\n");
    printf("| B | Tamanho da string1\n");
    printf("| C | Comparar strings\n");
    printf("| D | Concatenar strings\n");
    printf("| E | Imprimir string1 invertida\n");
    printf("| F | Quantidade de ocorrencias de um caractere na string1\n");
    printf("| G | Subsituir o primeiro caractere1 pelo caractere2\n");
    printf("| H | Retornar substring da string1\n");
    printf("| I | Sair\n");
    scanf("%c", &opc);

    return tolower(opc);
}


void pedirstr1(char *str1){

        printf("Entre com a string1: ");
        fflush(stdin);
        fgets(str1, TAM, stdin);
        str1[strlen(str1)-1] = '\0';
}

void pedirstr2(char *str2){

        printf("Entre com a string1: ");
        fflush(stdin);
        fgets(str2, TAM, stdin);
        str2[strlen(str2)-1] = '\0';
}


int main(){
    char str1[TAM] = " NULL ", str2[TAM] = " NULL ", char1, char2, c, opc = 'f', sair=0, c2, junto[TAM];
    int inicio, fim, contchar = 0;


    do {
        opc = menu();
        printf("\n\n");

        switch(opc){

        case 'a':
                //system("CLS");

                pedirstr1(str1);

                break;

        case 'b':
                //system("CLS");

                if (strcmp(str1, " NULL ") == 0){
                    printf("Nao existe nenhum valor na string1.\n");
                    printf("\n\n");
                } else {
                    printf("O tamanho da string1 e: %d\n", strlen(str1));
                    printf("\n\n");
                }

                break;

        case 'c':
                //system("CLS");

                pedirstr1(str1);
                pedirstr2(str2);

                if (strcmpi(str1, str2) == 0){
                    printf("As strings 1 e 2 sao iguais.\n");
                    printf("\n\n");
                } else {
                    printf("As strings 1 e 2 sao diferentes. \n");
                    printf("\n\n");
                }

                break;

        case 'd':
                //system("CLS");

                pedirstr1(str1);

                pedirstr2(str2);

                printf("string1 + string2 = %s%s\n", str1, str2); //posso usar strcat mas da ruim pq fica armazenado na variavel
                printf("\n\n");

                break;

        case 'e':
                //system("CLS");

                pedirstr1(str1);

                printf("String1 invertida: ");
                for (c=strlen(str1); c>=0; c--){
                    printf("%c", str1[c]);
                }

                printf("\n\n");

                break;

        case 'f':
                //system("CLS");

                pedirstr1(str1);

                printf("Letra que deseja contar: ");
                scanf("%c", &char1);

                for (c=0; c<strlen(str1); c++){
                    if (str1[c] == char1){
                        contchar ++;
                    }
                }

                printf("A letra %c aparece %d vezes na string1.\n", char1, contchar);
                printf("\n\n");

                break;

        case 'g':
                //system("CLS");

                pedirstr1(str1);

                printf("Entre com o caractere que deseja substituir: [diferencie maiusculas de minusculas]");
                scanf("%c", &char1);

                printf("Qual caractere deseja colocar no lugar? ");
                scanf("%c", &char2);

                for (c=0; c<strlen(str1); c++){
                    if (str1[c] == char1){
                        str1[c] = char2;
                    }
                }

                printf("Nova string: %s", str1);
                printf("\n\n");

                break;

        case 'h':
                //system("CLS");

                pedirstr1(str1);

                printf("A partir de qual posicao? ");
                scanf("%d", &inicio);

                printf("Até onde? ");
                scanf("%d", &fim);

                while (inicio > fim){
                    printf("A posicao de inicio deve ser menor do que a do fim. \n");

                    printf("A partir de qual posicao? ");
                    scanf("%d", &inicio);

                    printf("Ate onde? ");
                    scanf("%d", &fim);
                }

                printf("Substring: ");
                for (c=inicio; c<fim; c++){
                    printf("%c", str1[c]);
                }

                printf("\n\n");

                break;
        case 'i':
                sair = 1;

                break;

        default:
                //system("CLS");

                printf("Informacao invalida, tente novamente. \n");
                printf("\n\n");

                break;
        }
    } while (!sair);

    return 0;
}

Answer 1

Add

getchar();

After

scanf("%c", &opc);

When you choose an option that is not read and key ENTER. He perform the operation you chose and understands the ENTER that you gave as an option you typed too, with this it enters the default.

Answer 2

You got the opc = menu(); out of your do while. In other words, your variable opc does not update after each cycle stays the same and keeps the first value entered.

Squeal:

do {
    opc = menu();
    switch(opc){

    //resto do codigo

    }
} while (!sair);